Manufacturing Method of Alloy Steel For Cold-Forging, Alloy Steel For Cold-Forging, and Gear For Cold-Forging

Abstract

A manufacturing method of an alloy steel for cold-forging according to an embodiment of the present disclosure comprises homogenizing a steel material, furnace-cooling the steel material, and segmentation-annealing the steel material, in which the segmentation-annealing may be performed at least once and is optionally repeated one or more times.An alloy steel for cold-forging according to an embodiment of the present disclosure may comprise segmented cementite.A gear for cold-forging according to an embodiment of the present disclosure may have a gear surface grain boundary oxide layer of 19 μm or less.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This present application claims the benefit of priority to Korean Patent Application No. 10-2024-0094863, entitled “Manufacturing method of alloy steel for cold-forging, alloy steel for cold-forging and gear for cold-forging” filed on Jul. 18, 2024, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.FIELD

[0002] The present disclosure relates to a manufacturing method of an alloy steel for cold-forging, an alloy steel for cold-forging, and a gear for cold-forging.BACKGROUND

[0003] For a gear for cold-forging, cold-forging after cutting materials supplied from a steelmaking company and then performing spheroidizing annealing (S.A) is performed, and normalizing annealing for homogenizing the residual stress and an agglomerated structure generated during cold-forging is performed. Thereafter, after the shaping of the gear, carburizing annealing is performed to harden the surface and secure the physical properties and precision machining is performed to ensure the dimensional accuracy of the gear to manufacture a final product.

[0004] In such methods the spheroidizing annealing, which is performed to prevent forged cracks and bursts during cold-forging, is typically performed for a long time of about 30 hours to cause a lot of carbon emissions. There remains a need for improving cold-forging methods, the alloy steel for cold-forging, and gears for cold-forging.SUMMARY

[0005] An object of the present disclosure is to provide a manufacturing method of an alloy steel for cold-forging with a reduced annealing time. Another object of the present disclosure is to provide an alloy steel for cold-forging and a gear for cold-forging with excellent quality. An aspect of the present disclosure provides a manufacturing method of an alloy steel for cold-forging including homogenizing a steel material, furnace-cooling the steel material, and segmentation-annealing the steel material, in which the segmentation-annealing may be performed at least once and is optionally repeated one or more times.

[0006] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the homogenizing may be performed at or above an austenitizing temperature of the steel material.

[0007] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the homogenizing may be maintained at a temperature of 800° C. to 900° C. for 2 hours to 3 hours.

[0008] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the furnace-cooling may be performed at a temperature of 550° C. to 700° C.

[0009] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the segmentation-annealing may be performed by segmenting cementite.

[0010] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the segmentation-annealing may comprise an annealing at a temperature of 740° C. to 760° C.; and stabilizing at a temperature of 680° C. to 700° C.

[0011] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the segmentation-annealing may be performed one to three times.

[0012] In the manufacturing method of the alloy steel for cold-forging according to an embodiment of the present disclosure, the homogenizing, the furnace-cooling, and the segmentation-annealing comprises an annealing time of 8 hours or less.

[0013] Another aspect of the present disclosure provides an alloy steel for cold-forging including segmented cementite.

[0014] In the alloy steel for cold-forging according to an embodiment of the present disclosure, the segmented cementite may have a size of 0.1 μm to 3 μm.

[0015] The alloy steel for cold-forging according to an embodiment of the present disclosure may have hardness from 100 HV to 200 HV.

[0016] The alloy steel for cold-forging according to an embodiment of the present disclosure may comprise carbon (C): 0.17 to 0.23 wt %, silicon (Si): more than 0 to 0.7 wt % or less, manganese (Mn): 0.45 to 0.9 wt %, phosphorus (P): more than 0 to 0.020 wt % or less, sulfur (S): more than 0 to 0.030 wt % or less, chromium (Cr): 0.85 to 2.25 wt %, nickel (Ni): more than 0 to 0.25 wt % or less, and niobium (Nb): 0.015 to 0.035 wt %.

[0017] Another aspect of the present disclosure provides a gear for cold-forging comprising a gear surface grain boundary oxide layer of 19 μm or less.

[0018] According to the present disclosure, in the manufacturing method of the alloy steel for cold-forging, it is possible to omit a low annealing (LA) process and a spheroidizing annealing (SA) process. The manufacturing method of the alloy steel for cold-forging of the present disclosure may comprise a cementite segmentation (CS) annealing process instead of the LA process. In the manufacturing method of the alloy steel for cold-forging of the present disclosure, it is possible to reduce the process and reduce an annealing time by omitting the SA process that is typically associated with such process methods.

[0019] In the manufacturing method of the alloy steel for cold-forging of the present disclosure, the total annealing time may be within 8 hours. In the manufacturing method of the alloy steel for cold-forging of the present disclosure, the annealing time may be reduced by 80% or more compared to the methods currently employed in the art. Accordingly, the methods provided herein provide for the reduction of carbon emissions generated by the annealing process.

[0020] In the manufacturing method of the alloy steel for cold-forging of the present disclosure, it is possible to improve the cold-forging property by homogenizing the alloy structure and segmenting cementite with only one CS annealing process to secure a low hardness value. In the manufacturing method of the alloy steel for cold-forging of the present disclosure, it is possible to produce an alloy having a hardness value equivalent to the hardness value of a material after the SA annealing process known in the art.

[0021] The hardness of the alloy steel for cold-forging according to various embodiments of the present disclosure may have a low hardness value of 100 HV to 200 HV.

[0022] The gear for cold-forging according to various embodiments of the present disclosure may have a grain boundary oxide layer of 19 μm or less on a gear surface. That is, it is possible to reduce the grain boundary oxide layer on the gear surface that can be caused by an excessive annealing time associated with known methods. Therefore, the methods in accordance with the disclosure can improve the durability of the gear for cold-forging.

[0023] The effects of the present disclosure are not limited to the aforementioned effects, and other effects, which are not mentioned above, will be apparent to those skilled in the art from the following description.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and other aspects, features, and advantages of the present disclosure will become apparent from the detailed description of the following aspects in conjunction with the accompanying drawings, in which:

[0025] FIG. 1 is a process flowchart of a manufacturing method of an alloy steel for cold-forging according to various embodiments of the present disclosure;

[0026] FIG. 2 is a detailed process flowchart of a segmentation-annealing step;

[0027] FIG. 3 is a graph showing a manufacturing method of an alloy steel for cold-forging according to various embodiments of the present disclosure;

[0028] FIG. 4 is a photograph comparing a general pearlite structure and a segmented iron carbide structure;

[0029] FIG. 5 is a process flowchart for comparing the related art and an embodiment art;

[0030] FIG. 6A is a result of observing an SEM image of an alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0031] FIG. 6B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0032] FIG. 6C is a graph showing a hardness distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0033] FIG. 7A is a result of observing an SEM image of an alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0034] FIG. 7B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0035] FIG. 7C is a graph showing a hardness distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0036] FIG. 8A is a result of observing an SEM image of an alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0037] FIG. 8B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0038] FIG. 8C is a graph showing a hardness distribution in the alloy steel for cold-forging manufactured according to illustrative embodiments in accordance with the disclosure;

[0039] FIG. 9 is a graph showing comparing cementite segmentation sizes of illustrative embodiments in accordance with the disclosure;

[0040] FIG. 10 is a graph showing comparing hardnesses of illustrative embodiments in accordance with the disclosure;

[0041] FIG. 11A is a hardness map of a gear for cold-forging according to an illustrative Comparative Example;

[0042] FIG. 11B is an abnormal grain occurrence temperature prediction map of the gear for cold-forging according to an illustrative Comparative Example;

[0043] FIG. 11C is a result of observing a microstructure of the gear for cold-forging according to an illustrative Comparative Example;

[0044] FIG. 12A is a hardness map of a gear for cold-forging according to illustrative embodiments in accordance with the disclosure;

[0045] FIG. 12B is an abnormal grain occurrence temperature prediction map of the gear for cold-forging according to illustrative embodiments in accordance with the disclosure;

[0046] FIG. 12C is a result of observing a microstructure of the gear for cold-forging according to illustrative embodiments in accordance with the disclosure;

[0047] FIG. 13 is a photograph of observing a grain boundary oxide layer of the gear for cold-forging according to an illustrative Comparative Example; and

[0048] FIG. 14 is a photograph of observing a grain boundary oxide layer of the gear for cold-forging according to illustrative embodiments in accordance with the disclosure.DETAILED DESCRIPTION

[0049] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Like reference numerals designate like elements throughout the specification, and the detailed description may be omitted.

[0050] As used herein, the terms “comprising”, “including”, and “having” specify the presence of features, integers, steps, operations, elements, components, or combinations thereof disclosed on the present disclosure, but do not preclude any one of features, integers, steps, operations, elements, components, and / or combinations thereof.

[0051] Various embodiments of the present disclosure relate to a manufacturing method of an alloy steel for cold-forging.

[0052] FIG. 1 is a process flowchart of a manufacturing method of an alloy steel for cold-forging according to various embodiments of the present disclosure. FIG. 2 is a detailed process flowchart of a segmentation-annealing step. FIG. 3 is a graph showing a manufacturing method of an alloy steel for cold-forging according to various embodiments of the present disclosure.

[0053] Particularly, referring to FIGS. 1 to 3, a manufacturing method 10 of an alloy steel for cold-forging according to various embodiments of the present disclosure may comprise a homogenizing step (S100), a furnace-cooling step (S200), and a segmentation-annealing step (S300). The homogenizing step (S100), the furnace-cooling step (S200), and the segmentation-annealing step (S300) may be named a cementite segmentation (CS) annealing process.

[0054] The homogenizing step (S100) may be performed by homogenizing a steel material for cold-forging. The steel material for cold-forging may comprise carbon (C): 0.17 to 0.23 wt %, silicon (Si): more than 0 to 0.7 wt % or less, manganese (Mn): 0.45 to 0.9 wt %, phosphorus (P): more than 0 to 0.020 wt % or less, sulfur(S): more than 0 to 0.030 wt % or less, chromium (Cr): 0.85 to 2.25 wt %, nickel (Ni): more than 0 to 0.25 wt % or less, and niobium (Nb): 0.015 to 0.035 wt %. The steel material for cold-forging may further comprise molybdenum (Mo): 0.33 to 0.65 wt %. The steel material for cold-forging may further comprise nitrogen (N): 100 to 160 ppm, oxygen (O): more than 0 to 15 ppm, and the remainder iron (Fe) and other unavoidable impurities. The steel material for cold-forging may further comprise boron (B): 10 to 30 ppm.

[0055] Referring to FIG. 3, the homogenizing step (S100) may be performed at an austenitizing (γ-Fe) temperature (A3) or higher of the steel material for cold-forging. The homogenizing step (S100) may be performed at a heating rate of 1° C. / min or more. The homogenizing step (S100) may be performed by maintaining the temperature at 800° C. to 900° C. for 2 to 3 hours. When the heating temperature is less than 800° C., austenite homogenization of the steel material for cold-forging may be difficult. When the heating temperature is more than 900° C., the temperature is too high and a lot of energy is wasted, which may be economically disadvantageous. If the maintenance time is less than 2 hours, homogenization of the entire material may not be performed. If the maintenance time is more than 3 hours, it may be economically disadvantageous due to a lot of energy waste.

[0056] In the furnace-cooling step (S200), the steel material for cold-forging subjected to the homogenizing step (S100) may be furnace-cooled. The furnace-cooling step (S200) may be performed by cooling the steel material for cold-forging subjected to austenitization. In the furnace-cooling step (S200), the austenite structure of the steel material for cold-forging may be transformed into ferrite and cementite (Fe3C) structures. In the furnace-cooling step (S200), a coarse pearlite structure with a wide interlamellar spacing may occur in the steel material for cold-forging.

[0057] The furnace-cooling step (S200) may be performed by cooling the temperature to 550° C. to 700° C. If the cooling temperature is less than 550° C., the cooling time may be increased and thus productivity may decrease. If the cooling temperature is more than 700° C., the ferrite and cementite structures may not be formed homogeneously.

[0058] The furnace-cooling step (S200) may be performed at a cooling rate of 0.04° C. / s to 0.08° C. / s. If the cooling rate is less than 0.04° C. / s, the furnace-cooling step (S200) may become too long, which may reduce productivity. If the cooling rate is more than 0.08° C. / s, the stress generated by phase transformation during cooling may increase. Alternatively, initial ferrite may not be formed and the pearlite structure may be excessively formed. In addition, a fine pearlite structure with a narrow interlamellar spacing may be generated. Accordingly, the process time of the segmentation-annealing step (S300) for segmenting the cementite thereafter may be increased, which may be economically disadvantageous.

[0059] The segmentation-annealing step (S300) may be performed by annealing the steel material for cold-forging subjected to the furnace-cooling step (S200). In the segmentation-annealing step (S300), the cementite structure within the pearlite generated in the furnace-cooling step (S200) may be segmented to break the lamellar structure and homogenized. In the segmentation-annealing step (S300), cementite may be dissolved into a matrix structure and then slowly cooled to precipitate again as stabilized fine cementite.

[0060] The segmentation-annealing step (S300) may comprise an annealing step at a temperature of 740° C. to 760° C.; and a stabilizing step at a temperature of 680° C. to 700° C.

[0061] Referring to FIG. 3, the segmentation-annealing step (S300) may be performed at a temperature of Al or higher. For example, Al may be 740° C. to 760° C. That is, the segmentation-annealing step (S300) may be performed at a temperature of 740° C. to 760° C. When annealing at a temperature of less than 740° C., cementite may not be finely segmented and it may take a long time. When annealing at a temperature of more than 760° C., an austenite structure is additionally generated and an austenite fraction increases, and then the austenite structure may be transformed into a pearlite structure during the subsequent stabilization step. Accordingly, a lamellar cementite structure may be generated within the pearlite structure to lose the segmentation effect.

[0062] The segmentation-annealing step (S300) may be performed for 60 minutes to 90 minutes. If the heating time is less than 60 minutes, the cementite segmentation effect may be reduced. If the heating time is more than 90 minutes, it may be economically disadvantageous due to a lot of energy waste.

[0063] The segmentation-annealing step (S300) may further comprise a stabilizing step. The stabilizing process may be performed by cooling to 680° C. to 700° C. and maintaining the temperature for 30 minutes to 1 hour. At less than 680° C., the cementite structure may be excessively precipitated. At more than 700° C., the cementite structure is not stabilized, and thus cementite may be additionally precipitated during subsequent processes.

[0064] Referring to FIG. 2, the segmentation-annealing step (S300) may comprise a first segmentation-annealing step (S310), a second segmentation-annealing step (S320), and a third segmentation-annealing step (S330). That is, the first segmentation-annealing step (S310), the second segmentation-annealing step (S320), and the third segmentation-annealing step (S330) may be processes equal or similar to the segmentation-annealing step (S300) described above.

[0065] As the number of times of the segmentation-annealing step (S300) increases, the segmented cementite size may become fine. For example, depending on the number of times of the segmentation-annealing step (S300), the segmented cementite may have a size of 0.1 μm to 3 μm in a longitudinal direction. Preferably, through the segmentation-annealing step (S300), a material having a segmented cementite size of 0.2 μm to 2 μm in the longitudinal direction may be manufactured. More preferably, through the segmentation-annealing step (S300), a material having a segmented cementite size of 0.4 μm to 1.1 μm in the longitudinal direction may be manufactured. Meanwhile, the width of the segmented cementite may be 0.1 μm to 1 μm.

[0066] As the number of times of the segmentation-annealing step (S300) increases, the hardness of the manufactured material may be lowered. For example, depending on the number of times of the segmentation-annealing step (S300), a material having the hardness of 100 HV to 200 HV may be manufactured. Preferably, through the segmentation-annealing step (S300), a material having the hardness of 170 HV to 143 HV may be manufactured. Accordingly, the number of times of the segmentation-annealing step (S300) may be controlled depending on a desired or required hardness of the final material. For example, when manufacturing a component with a complex cold-forging shape and a high deformation rate, the number of times of the segmentation-annealing step (S300) may be performed in order to manufacture a material with lowered or the lowest hardness. As a result, it is possible to prevent forged cracks and bursts by reducing the load of cold-forging. The annealing time required in the homogenizing step (S100), the furnace-cooling step (S200), and the segmentation-annealing step (S300) may be within total 8 hours. The homogenizing step (S100), the furnace-cooling step (S200), and the segmentation-annealing step (S300) may be performed in a continuous annealing furnace.

[0067] FIG. 4 is a photograph comparing a general pearlite structure and a segmented iron carbide structure. Referring to FIG. 4, it can be seen that a general pearlite structure has a lamellar structure of cementite (Fe3C). Meanwhile, it can be seen that after the segmentation-annealing step (S300), the pearlite structure has a segmented iron carbide (cementite, Fe3C) structure. That is, after the segmentation-annealing step (S300), the alloy steel for cold-forging may comprise a cementite (Fe3C) segmented structure.

[0068] FIG. 5 is a process flowchart for comparing the related art method (Comparative Example) and an embodiment of a method in accordance with the disclosure. Specifically, FIG. 5 illustrates an overall process for manufacturing a gear for cold-forging.

[0069] Referring to FIG. 5, the related art may comprise steelmaking / casting, rolling, low annealing (LA), BAR straightening, peeling, cutting, spheroidizing annealing (SA), cold-forging, normalizing, gear machining, carburizing annealing, and precision machining processes. That is, the related art comprises an LA process and an SA process. The LA process is a process for controlling the hardness of raw materials and removing residual stress, and may be performed for about 6 hours. The SA process is a process for homogenizing a structure as a process for adjusting the hardness of a spheroidizing structure and forged pieces for cold-forging. Typically, the SA process is an essential annealing process for cold-forging and long-term annealing of about 30 hours is required to prevent forged cracks and bursts during cold-forging.

[0070] Meanwhile, the illustrative embodiment in accordance with the methods of the disclosure may omit an LA process and an SA process. Specifically, the illustrative example embodiment may comprise the cementite segmentation (CS) annealing process described above instead of the LA process. In addition, the SA process of the related art is deleted to reduce the process and reduce the annealing time.

[0071] According to the illustrative example embodiment, the number of processes may be reduced by omitting twice annealing processes of the LA process and the SA process in the related art and performing the CS annealing process only once. The total annealing time of the illustrative example embodiment may be 8 hours or less. The total annealing time of the related art is about 36 hours, and the annealing time may be reduced by 80% or more compared to the related art through the illustrative example embodiment of the method disclosed herein. Accordingly, it is possible to reduce carbon emissions by the annealing process.

[0072] In addition, according to the illustrative example embodiment, cold-forging properties maybe improved by homogenizing the structure and segmenting cementite with only one CS annealing process, thereby securing a low hardness value. That is, it is possible to secure a hardness value equivalent to the hardness value of the material after the SA process of the related art. In addition, the gear for cold-forging manufactured by the illustrative example embodiment may have a grain boundary oxide layer of 19 μm or less. Through the illustrative example embodiment, it is possible to reduce the grain boundary oxide layer on the gear surface caused by an excessive annealing time. Therefore, it is possible to improve the durability of the gear for cold-forging.

[0073] The alloy steel for cold-forging according to various embodiments of the present disclosure may be manufactured by the above-described manufacturing method. The alloy steel for cold-forging according to various embodiments of the present disclosure may be manufactured by the above-described CS annealing process.

[0074] The alloy steel for cold-forging according to various embodiments of the present disclosure may comprise carbon (C): 0.17 to 0.23 wt %, silicon (Si): more than 0 to 0.7 wt % or less, manganese (Mn): 0.45 to 0.9 wt %, phosphorus (P): more than 0 to 0.020 wt % or less, sulfur (S): more than 0 to 0.030 wt % or less, chromium (Cr): 0.85 to 2.25 wt %, nickel (Ni): more than 0 to 0.25 wt % or less, and niobium (Nb): 0.015 to 0.035 wt %. The alloy steel for cold-forging may further comprise molybdenum (Mo): 0.33 to 0.65 wt %. The alloy steel for cold-forging may further comprise nitrogen (N): 100 to 160 ppm, oxygen (O): more than 0 to 15 ppm, and the remainder iron (Fe) and other unavoidable impurities. The alloy steel for cold-forging may further comprise boron (B): 10 to 30 ppm. For example, the alloy steel for cold-forging may be a SCr420HB, SCM920HvSi or SCM820PRH material having the same composition as Table 1 below.TABLE 1ClassificationC (wt %)Si (wt %)Mn (wt %)P (wt %)S(wt %)Cr (wt %)Ni (wt %)Mo (wt %)Nb (wt %)SCr420HB0.17~0.230.15~0.350.55~0.90~0.020~0.0200.85~1.25~0.25—0.015~0.035SCM920HvSi0.17~0.21~0.150.60~0.85~0.020~0.0201.25~1.45~0.250.55~0.650.015~0.035SCM820PRH0.17~0.230.50~0.700.45~0.75~0.020~0.0301.95~2.25~0.250.33~0.430.015~0.035

[0075] The alloy steel for cold-forging according to various embodiments of the present disclosure may have hardness of 100 HV to 200 HV. Preferably, the hardness may be 170 HV to 143 HV. The alloy steel for cold-forging according to various embodiments of the present disclosure may comprise ferrite and segmented cementite.

[0076] The segmented cementite may have a size of 0.1 μm to 3 μm in a longitudinal direction. Preferably, the segmented cementite may have a size of 0.2 μm to 2 μm in a longitudinal direction. More preferably, the segmented cementite may have a size of 0.4 μm to 1.1 μm in a longitudinal direction. Meanwhile, the width of the segmented cementite may be 0.1 μm to 1 μm.

[0077] The alloy steel for cold-forging according to various embodiments of the present disclosure may be used as a material for manufacturing the gear for cold-forging. The alloy steel for cold-forging may be manufactured into a gear for cold-forging as a final product, through BAR straightening, peeling, cutting, cold-forging, normalizing, gear machining, carburizing annealing, and precision machining. The gear for cold-forging may have a grain boundary oxide layer of 19 μm or less on a gear surface. Preferably, the gear for cold-forging may have a grain boundary oxide layer of 16 μm or less on the gear surface. Therefore, it is possible to improve the durability of the gear for cold-forging.

[0078] Hereinafter, the present disclosure will be described in more detail by the following Examples. However, the following Examples and Experimental Examples are only intended to describe the present disclosure in more illustrative detail for purposes of additional clarity and explanation. It will be understood that the scope of the present disclosure and appended claims are not limited by the following illustrative Examples and Experimental Examples.Example 1

[0079] For a steel material for cold-forging having a composition shown in Table 1 below, a homogenizing step (S100) was performed at a temperature of 850° C. for 2.5 hours. Next, a furnace-cooling step (S200) was performed by cooling the temperature to 600° C. at a cooling rate of 0.06° C. / s. Next, a segmentation-annealing step (S310) including an annealing step at a temperature of 750° C. for 70 minutes and stabilizing step for 50 minutes by cooling the temperature to 690° C. was performed.TABLE 2Material nameCSiMnPSCrNiMoNbSCr420HB0.17~0.230.15~0.350.55~0.900.020 ↓0.020 ↓0.85~1.250.25 ↓—0.015~0.035Example 2

[0080] After the segmentation-annealing step (S310) in Example 1, a segmentation-annealing step (S320) was further performed under the same process conditions. That is, the segmentation-annealing step was performed twice.Example 3

[0081] After the segmentation-annealing step (S320) in Example 2, a segmentation-annealing step (S330) was further performed under the same process conditions. That is, the segmentation-annealing step was performed three times.Experiment Example 1: Observation of Segmented Cementite

[0082] With respect to the alloy steels for cold-forging manufactured according to Examples 1 to 3, SEM image observation, cementite size distribution, and hardness were identified. FIG. 6A is a result of observing an SEM image of the alloy steel for cold-forging manufactured according to Example 1. Referring to FIG. 6A, it can be seen that segmented shapes are observed in elongated cementite.

[0083] FIG. 6B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to Example 1. Referring to FIG. 6B, it can be seen that cementite in the alloy steel for cold-forging manufactured according to Example 1 has a mean size of 1.1 μm in the longitudinal direction, a standard deviation of 0.73, and a minimum to maximum distribution of 0.07 μm to 5.02 μm.

[0084] FIG. 6C is a graph showing a hardness distribution in the alloy steel for cold-forging manufactured according to Example 1. Referring to FIG. 6C, it can be seen that the alloy steel for cold-forging manufactured according to Example 1 has a mean hardness of 170 HV, a standard deviation of 6.1, and a minimum to maximum distribution of 159 HV to 184 HV.

[0085] FIG. 7A is a result of observing an SEM image of the alloy steel for cold-forging manufactured according to Example 2. Referring to FIG. 7A, it can be seen that finely segmented cementite of less than 1 μm is observed.

[0086] FIG. 7B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to Example 2. Referring to FIG. 7B, it can be seen that cementite in the alloy steel for cold-forging manufactured according to Example 2 has a mean size of 0.63 μm in the longitudinal direction, a standard deviation of 0.35, and a minimum to maximum distribution of 0.07 μm to 2.14 μm.

[0087] FIG. 7C is a graph showing a hardness distribution in the alloy steel for cold-forging manufactured according to Example 2. Referring to FIG. 7C, it can be seen that the alloy steel for cold-forging manufactured according to Example 2 has a mean hardness of 152 HV, a standard deviation of 3.0, and a minimum to maximum distribution of 147 HV to 157 HV. FIG. 8A is a result of observing an SEM image of the alloy steel for cold-forging manufactured according to Example 3. Referring to FIG. 8A, it can be seen that finely segmented cementite of less than 1 μm is observed.

[0088] FIG. 8B is a graph showing a cementite size distribution in the alloy steel for cold-forging manufactured according to Example 3. Referring to FIG. 8B, it can be seen that cementite in the alloy steel for cold-forging manufactured according to Example 3 has a mean size of 0.40 μm in the longitudinal direction, a standard deviation of 0.16, and a minimum to maximum distribution of 0.10 μm to 1.09 μm.

[0089] FIG. 8C is a graph showing a hardness distribution in an alloy steel for cold-forging manufactured according to Example 3. Referring to FIG. 8C, it can be seen that the alloy steel for cold-forging manufactured according to Example 3 has a mean hardness of 143 HV, a standard deviation of 6.3, and a minimum to maximum distribution of 131 HV to 157 HV. FIG. 9 is a graph showing comparing cementite segmentation sizes of Examples 1 to 3. FIG. 10 is a graph showing comparing hardnesses of Examples 1 to 3.

[0090] Referring to FIG. 9, it can be confirmed that as the number of times of the segmentation-annealing step increases, the mean segment sizes of cementite in the longitudinal direction decrease to 1.1 μm, 0.6 μm, and 0.4 μm, respectively. Referring to FIG. 10, it can be confirmed that as the number of times of the segmentation-annealing step increases, the hardness decreases to 170 HV, 152 HV and 143 HV, respectively.Experiment Example 2: Identification of Abnormal Grains

[0091] As illustrated in FIG. 5, the gear for cold-forging as a final product was manufactured by performing both a steelmaking company process and a component company process according to the related art (Comparative Example) and embodiment example (Example 3). That is, as Comparative Example, a gear for cold-forging manufactured according to the related art of FIG. 5 was prepared. In addition, after the CS annealing process of Example 3 was performed, a gear for cold-forging manufactured according to the illustrative embodiment of the methods described herein (e.g., of FIG. 5) was also prepared. A hardness map and an abnormal grain (pearlite agglomeration) occurrence temperature prediction map for the gear for cold-forging as the final product were prepared. FIG. 11A is a hardness map of a gear for cold-forging according to Comparative Example. FIG. 11B is an abnormal grain occurrence temperature prediction map of the gear for cold-forging according to Comparative Example. FIG. 11C is a result of observing a microstructure of the gear for cold-forging according to Comparative Example.

[0092] Referring to a box mark in FIG. 11A, high hardness of 280 HV to 300 HV was formed at a gear tooth bottom portion.

[0093] Referring to a box mark of FIG. 11B, when converting an abnormal grain occurrence temperature according to hardness, it can be seen that in the case of Comparative Example, abnormal grains occur when the temperature of the tooth bottom portion is 920° C. or higher. Referring to FIG. 11C, a lot of pearlite agglomeration was observed.

[0094] FIG. 12A is a hardness map of a gear for cold-forging according to Example 3. FIG. 12B is an abnormal grain occurrence temperature prediction map of the gear for cold-forging according to Example 3. FIG. 12C is a result of observing a microstructure of the gear for cold-forging according to Example 3.

[0095] Referring to a box mark in FIG. 12A, high hardness of 280 HV to 300 HV was formed at a gear tooth bottom portion similarly to Comparative Example.

[0096] Referring to a box mark of FIG. 12B, when converting an abnormal grain occurrence temperature according to hardness, it can be seen that in the case of Example 3, abnormal grains occur when the temperature of the tooth bottom portion is 940° C. or higher. That is, it can be seen that the abnormal grains occur at a temperature higher than the abnormal grain occurrence temperature of Comparative Example.

[0097] Referring to FIG. 12C, it can be confirmed that, unlike Comparative Example, there is no pearlite agglomeration phenomenon.Experiment Example 3: Observation of Grain Boundary Oxide Layer

[0098] Grain boundary oxide layers of the gears for cold-forging manufactured according to Comparative Example and Example 3 were observed. The grain boundary oxide layer needs to be managed because it acts as a starting point for fatigue failure of the gear, and the smaller the grain boundary oxide layer, the longer the durability.

[0099] FIG. 13 is a photograph of observing a grain boundary oxide layer of the gear for cold-forging according to Comparative Example. FIG. 14 is a photograph of observing a grain boundary oxide layer of the gear for cold-forging according to Example 3.

[0100] Referring to FIG. 13, it was confirmed that the grain boundary oxide layer on the surface of the gear for cold-forging according to Comparative Example was a maximum of 20.0 μm. On the other hand, referring to FIG. 14, it was confirmed that the grain boundary oxide layer on the surface of the gear for cold-forging according to Example 3 was a maximum of 15.5 μm, which was reduced by 4.5 μm compared to Comparative Example. That is, in the case of the process in Examples, it can be seen that the grain boundary oxide layer on the gear surface decreases as the final annealing time decreases.

[0101] Hereinabove, the embodiments of the present disclosure have been described together with the drawings. The embodiments are illustrative, and the present disclosure is not limited to the above-described embodiments and the contents of the drawings.

[0102] It will be apparent to those skilled in the art that modifications of the present disclosure may be made within the scope of the disclosed technical idea. The described embodiments should be considered as part of the present disclosure, and the scope of the present disclosure should not be limited only to the described embodiments.

[0103] The scope of the present disclosure should be judged by the technical ideas set forth in the appended claims. In addition, even if the actions or effects according to the configuration are not explicitly described while explaining the embodiments of the present disclosure, it is obvious that the actions or effects that can be predicted by the configuration should also be recognized as the present disclosure.

Claims

1. A manufacturing method of an alloy steel for cold-forging, the manufacturing method comprising:homogenizing a steel material;furnace-cooling the steel material; andsegmentation-annealing the steel material,wherein the segmentation-annealing is performed at least once and is optionally repeated one or more times.

2. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the homogenizing is performed at or above an austenitizing temperature of the steel material.

3. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the homogenizing is maintained at a temperature of 800° C. to 900° C. for 2 hours to 3 hours.

4. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the furnace-cooling is performed at a temperature of 550° C. to 700° C.

5. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the segmentation-annealing is performed by segmenting cementite.

6. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the segmentation-annealing comprisesannealing at a temperature of 740° C. to 760° C.; andstabilizing at a temperature of 680° C. to 700° C.

7. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the segmentation-annealing is performed one to three times.

8. The manufacturing method of the alloy steel for cold-forging of claim 1, wherein the homogenizing, the furnace-cooling, and the segmentation-annealing comprises an annealing time of 8 hours or less.

9. An alloy steel for cold-forging comprising segmented cementite.

10. The alloy steel for cold-forging of claim 9, wherein the segmented cementite has a size of 0.1 μm to 3 μm in a longitudinal direction.

11. The alloy steel for cold-forging of claim 9, wherein the alloy steel comprises a hardness from 100 HV to 200 HV.

12. The alloy steel for cold-forging of claim 9, wherein the alloy steel comprises carbon (C): 0.17 to 0.23 wt %, silicon (Si): more than 0 to 0.7 wt % or less, manganese (Mn): 0.45 to 0.9 wt %, phosphorus (P): more than 0 to 0.020 wt % or less, sulfur (S): more than 0 to 0.030 wt % or less, chromium (Cr): 0.85 to 2.25 wt %, nickel (Ni): more than 0 to 0.25 wt % or less, and niobium (Nb): 0.015 to 0.035 wt %.

13. A gear for cold-forging comprising a gear surface grain boundary oxide layer of 19 μm or less.

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