Alloy steel for transmission gear and method of manufacturing the same

Abstract

The present invention provides an alloy steel for transmission gears and a method for manufacturing the same. According to one embodiment of the present invention, the alloy steel for transmission gears comprises, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): 0.01% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mo): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to 15 ppm or less, and the remainder being iron (Fe) and other unavoidable impurities, and satisfies a hardness of 241 HB or less.

Description

Technology Field

[0001] The technical concept of the present invention relates to steel materials and methods for manufacturing the same, and more specifically, to alloy steel for transmission gears and methods for manufacturing the same. Background Technology

[0002] Generally, alloy steel for transmission gears is manufactured by hot or cold forging the material, followed by pre-processing heat treatment and precision machining, and then carburizing heat treatment and post-processing. Since the material takes on a rough shape through forging and a precise shape through precision machining, it must have excellent forgeability and easy machinability.

[0003] Recently, there has been a trend toward increasing material hardness for alloy steel used in transmission gears, as high strength and durability characteristics are increasingly required for the purpose of component weight reduction. Additionally, during the gear component manufacturing process, material cutting is performed prior to forging; at this stage, customers requiring cutting services demand that the material's hardness be maintained at 241 HB or lower to extend tool life and prevent cracking. Generally, steel manufacturers supply materials with reduced hardness achieved through low-temperature heat treatment to ensure machinability; however, since this heat treatment takes at least five hours, the issue of reduced productivity is constantly being raised. Prior art literature

[0004] Korean Registered Patent No. 10-0848784 The problem to be solved

[0005] The technical problem that the technical concept of the present invention aims to solve is to increase productivity and provide an alloy steel for transmission gears having high strength and high durability, as well as a method for manufacturing the same. However, this problem is exemplary, and the technical concept of the present invention is not limited thereto. means of solving the problem

[0006] According to one aspect of the present invention, an alloy steel for a transmission gear and a method for manufacturing the same are provided.

[0007] According to one embodiment of the present invention, the alloy steel for a transmission gear comprises, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): 0.01% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mo): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to 15 ppm or less, and the remainder being iron (Fe) and other unavoidable impurities, and may satisfy a hardness of 241 HB or less.

[0008] According to one embodiment of the present invention, the alloy steel may further include boron (B): 10 ppm to 35 ppm.

[0009] According to one embodiment of the present invention, the alloy steel has a microstructure including bainite and ferrite, the area fraction of the bainite is 36% to 99%, and the area fraction of the ferrite may be the remainder.

[0010] According to one embodiment of the present invention, a method for manufacturing the alloy steel for a transmission gear comprises the step of hot rolling a steel material comprising, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): 0.01% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mo): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to less than or equal to 15 ppm, and the remainder being iron (Fe) and other unavoidable impurities; To control the time for the temperature of the hot-rolled steel to reach a temperature below the A1 transformation point, the method may include: a step of performing a first heat treatment on the hot-rolled steel at a temperature above the A3 transformation point; and a step of performing a second heat treatment on the first heat-treated steel at a temperature below the A1 transformation point.

[0011] According to one embodiment of the present invention, in the first heat treatment step, the steel material can be heat-treated at a temperature of 850°C to 900°C.

[0012] According to one embodiment of the present invention, in the second heat treatment step, the steel material can be heat-treated at a temperature of 700°C to 750°C.

[0013] According to one embodiment of the present invention, in the first heat treatment step and the second heat treatment step, the heat treatment time may be 3 hours or less.

[0014] According to one embodiment of the present invention, the steel produced after performing the second heat treatment step satisfies a hardness of 241 HB or less and has a microstructure including bainite and ferrite, wherein the area fraction of bainite is 36% to 99% and the area fraction of ferrite is the remainder.

[0015] According to one embodiment of the present invention, the steel may further include boron (B): 10 ppm to 35 ppm. Effects of the invention

[0016] According to the technical concept of the present invention, it is possible to provide an alloy steel for transmission gears and a method for manufacturing the same, which can satisfy the raw material hardness required by the customer and reduce the heat treatment time by 40% compared to conventional methods by using a variable temperature heat treatment process. The effects of the present invention described above are illustrative and the scope of the present invention is not limited by these effects. Brief explanation of the drawing

[0017] Figure 1 is a graph illustrating the heat treatment process of a method for manufacturing alloy steel for a transmission gear according to an embodiment of the present invention. FIG. 2 is a graph illustrating the heat treatment process of a method for manufacturing alloy steel for a transmission gear according to a comparative example of the present invention. Figure 3 is a graph showing the time taken to reach the target temperature of the material as the initial temperature increases in the method for manufacturing alloy steel for transmission gears according to the experimental example of the present invention. Figures 4 to 6 are photographs of the microstructure of a steel material according to an experimental example of the present invention analyzed using an optical microscope. Specific details for implementing the invention

[0018] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical concept of the present invention to those skilled in the art, and the following embodiments may be modified in various different forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical concept of the present invention to those skilled in the art. In this specification, the same reference numerals denote the same elements throughout. Furthermore, various elements and areas in the drawings are depicted schematically. Accordingly, the technical concept of the present invention is not limited by the relative sizes or spacing depicted in the attached drawings.

[0019] The technical concept of the present invention satisfies the required physical properties in alloy steel for transmission parts and proposes heat treatment conditions that reduce the time required for conventional heat treatment by about 40% in a continuous heat treatment furnace, rather than a conventional batch heat treatment furnace, in order to maximize productivity, thereby providing improved part productivity and effective hardness reduction effects.

[0020] Hereinafter, an alloy steel for a transmission gear, which is one aspect of the present invention, will be described.

[0021] Alloy steel for transmission gears

[0022] An alloy steel for a transmission gear, which is one aspect of the present invention, comprises, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): greater than 0% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mon): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to 15 ppm or less, and the remainder being iron (Fe) and other unavoidable impurities. Alternatively, the alloy steel further comprises boron (B): 10 ppm to 35 ppm.

[0023] The role and content of each component included in the alloy steel for transmission gears according to the present invention are described below. In this case, the content of the constituent elements refers to weight percent.

[0024] Carbon (C): 0.17% ~ 0.25%

[0025] Since carbon (C) can secure strength by improving the strength of steel and forming precipitates, yield strength and tensile strength may increase as the carbon content increases. If the carbon content is less than 0.17%, it may be difficult to secure strength. If the carbon content exceeds 0.25%, toughness may decrease. Therefore, it is desirable to add carbon in an amount of 0.17% to 0.25% of the total weight of the steel.

[0026] Silicon (Si): 0.01% ~ 0.70%

[0027] Silicon (Si) is added as a deoxidizer to remove oxygen from steel and is effective in securing strength as a solid solution strengthening element. If the silicon content is less than 0.01%, the effect of the addition is negligible. If the silicon content exceeds 0.70%, workability such as cold forging may be reduced, or it may be difficult to secure the target strength. Therefore, it is desirable to add silicon in an amount of 0.01% to 0.70% of the total weight of the steel.

[0028] Manganese (Mn): 0.45% ~ 1.10%

[0029] Manganese (Mn) is an element that improves hardenability and strength. If the manganese content is less than 0.45%, the effect of its addition is negligible. If the manganese content exceeds 1.10%, the amount of MnS-based inclusions increases, which may lead to increased brittleness or decreased toughness during forging. Therefore, it is desirable to add manganese in an amount of 0.45% to 1.10% of the total weight of the steel.

[0030] Chrome (Cr): 0.95% ~ 2.25%

[0031] Chromium (Cr) is an element that improves fatigue strength, has a cementite stabilizing effect, increases hardenability, and enhances strength. When the chromium content is less than 0.95%, the effect of its addition is negligible. When the chromium content exceeds 2.25%, toughness or workability may decrease. Therefore, it is desirable to add chromium in an amount of 0.95% to 2.25% of the total weight of the steel.

[0032] Molybdenum (Mo): 0.07% ~ 0.65%

[0033] Molybdenum (Mo) is an element that improves the hardenability of steel for cold forging. When the molybdenum content is less than 0.07%, the effect of its addition is negligible. When the molybdenum content exceeds 0.62%, ductility may decrease. Therefore, it is desirable to add molybdenum in an amount of 0.07% to 0.65% of the total weight of the steel.

[0034] Niobium (Nb): 0.015% ~ 0.045%

[0035] Niobium (Nb) refines the austenite grain size by forming fine carbides, nitrides, and carbonitrides, thereby increasing the cold forgeability and fatigue strength of steel. If the niobium content is less than 0.015%, the effect of its addition is negligible. If the niobium content exceeds 0.045%, the aforementioned effect becomes saturated and the machinability of the steel may decrease. Therefore, it is desirable to add niobium in an amount of 0.015% to 0.045% of the total weight of the steel.

[0036] Nitrogen (N): 100 ppm ~ 160 ppm

[0037] Nitrogen (N) reacts with niobium to form nitrides or carbonitrides, thereby refining the austenite grain size and increasing the cold forgeability and fatigue strength of the steel. If the nitrogen content is less than 100 ppm, the effect of its addition is negligible. If the nitrogen content exceeds 160 ppm, the nitrogen dissolved in the alloy steel increases the deformation resistance during cold forging. Furthermore, if the nitrogen content is high when the alloy steel contains boron, BN is formed, which can reduce the effect of boron addition (improvement in hardenability). Therefore, it is desirable to add nitrogen at a concentration of 100 ppm to 160 ppm of the total weight of the steel.

[0038] Oxygen (O): >0 ppm ~ 15 ppm or less

[0039] Oxygen (O) exists as an oxide inclusion in steel and is an element that reduces fatigue strength, so it is desirable to have a lower amount, but up to 15 ppm is acceptable. Typically, it is difficult to make the oxygen content 0 weight%, but it is desirable to minimize it as much as possible.

[0040] Boron (B): 10 ppm ~ 35 ppm

[0041] Boron (B) has an excellent effect on improving hardenability and significantly affects hardenability even when added in small amounts. Therefore, to ensure cold workability, instead of reducing the manganese content, boron can be added to offset the reduction in hardenability caused by the reduction in manganese. In addition, boron is an element that improves quenching properties. If the boron content is less than 10 ppm, the effect of its addition is negligible. If the boron content exceeds 35 ppm, it degrades toughness and increases production costs. Therefore, it is desirable to add boron at a concentration of 10 ppm to 35 ppm of the total weight of the steel.

[0042] The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0043] The alloy steel for transmission gears, manufactured by controlling the specific components and content ranges of the alloy composition described above and the steel manufacturing method described below, may have a microstructure including bainite and ferrite. The fraction of bainite may be, for example, in the range of 36% to 99%, and the area fraction of ferrite may be included as the remaining fraction, for example, in the range of 1% to 64%. In addition, in addition to the ferrite microstructure, it may further include a pearlite microstructure of less than 1% (greater than 0%).

[0044] The above fraction refers to the area ratio derived from the microstructure photograph of the steel using an image analyzer. The microstructure photographs are shown in FIGS. 4 to 6 and refer to the microstructure of steel sample 1 (Fig. 4), the microstructure of steel sample 2 (Fig. 5), and the microstructure of steel sample 3 (Fig. 6) listed in Table 1 as experimental examples to be described later.

[0045] Another aspect of the present invention provides a method for manufacturing alloy steel for transmission gears.

[0046] The steel described above may undergo a first heat treatment at a temperature above the A3 transformation point, and the steel that has undergone the first heat treatment may undergo a second heat treatment at a temperature below the A1 transformation point.

[0047] For example, the first heat treatment may be performed at a temperature of 850°C to 900°C, and the second heat treatment may be performed at a temperature of 700°C to 750°C. In the first heat treatment step and the second heat treatment step, the heat treatment time may be 3 hours or less.

[0048] The steel produced after performing the second heat treatment step satisfies a hardness of 241 HB or less and has a microstructure including bainite and ferrite, wherein the area fraction of bainite may be 36% to 99% and the area fraction of ferrite may be included as the remainder.

[0049] A method for manufacturing alloy steel for transmission gears according to the present invention will be described below with reference to the attached drawings.

[0050] Method for manufacturing alloy steel for transmission gears

[0051] In the method for manufacturing steel according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a bloom or a billet. The bloom or billet in the semi-finished product state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0052] The above steel comprises, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): 0.01% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mo): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to less than or equal to 15 ppm, and the remainder being iron (Fe) and other unavoidable impurities. Alternatively, the above alloy steel further comprises boron (B): 10 ppm to 35 ppm.

[0053] A method for manufacturing alloy steel for transmission gears according to an embodiment of the present invention includes a step of reheating and a step of hot rolling. After reheating the alloy steel described above in a range of 1100°C to 1300°C, hot rolling can be performed continuously.

[0054] In order to control the time for the temperature of the hot-rolled steel to reach a temperature below the A1 transformation point, a step of performing a first heat treatment of the hot-rolled steel at a temperature above the A3 transformation point and a step of performing a second heat treatment of the first heat-treated steel at a temperature below the A1 transformation point may be performed.

[0055] Hereinafter, the first heat treatment step and the second heat treatment step will be described in detail with reference to the drawings.

[0056] Specifically, in the method for manufacturing alloy steel for transmission gears according to the present invention, the cutability of the alloy steel can be secured and the hardness reduced through heat treatment of the steel material.

[0057] FIG. 1 is a graph illustrating the heat treatment process of a method for manufacturing alloy steel for a transmission gear according to an embodiment of the present invention, and FIG. 2 is a graph illustrating the heat treatment process of a method for manufacturing alloy steel for a transmission gear according to a comparative example of the present invention.

[0058] First, referring to FIG. 2, as a heat treatment process for the method of manufacturing alloy steel for transmission gears according to a comparative example of the present invention, a heat treatment was performed by heating to a temperature just below A1 in a batch type furnace, maintaining it at a temperature just below A1 for a certain period of time, and then cooling. However, at this time, since it takes a long time for the material to reach the target temperature, the total heat treatment time is extended, resulting in problems of increased production speed and cost.

[0059] To solve this, in the present invention, as shown in FIG. 1, the material is heated to a temperature exceeding the A3 transformation point at the beginning of the heat treatment in a continuous heat treatment furnace, but the temperature of the material is designed to reach only the target temperature below the phase transformation within a certain time, thereby accelerating the heating of the material and performing a first heat treatment for a predetermined time. The temperature exceeding the A3 transformation point may be, for example, a temperature of 850°C to 900°C, preferably 870°C to 900°C, and more preferably 890°C to 900°C. In addition, the predetermined time may be, for example, 15 minutes to 25 minutes, and preferably 18 minutes to 22 minutes.

[0060] If the first heat treatment temperature is less than 850°C, the temperature of the steel may not reach the temperature of the heat treatment furnace, and thus the target hardness value may not be satisfied, which may result in insufficient reduction of the total heat treatment time. If the first heat treatment temperature exceeds 900°C, it may cause problems such as reduced productivity due to deterioration of the durability of the production equipment and damage. Therefore, in the present invention, the first heat treatment must be controlled to be performed at a temperature of 850°C to 900°C.

[0061] After performing a first heat treatment at a temperature exceeding the A3 transformation point, a second heat treatment may be performed by cooling the steel to a temperature below the A1 transformation point and maintaining it for a predetermined time. The temperature below the A1 transformation point may be, for example, a temperature of 700°C to 750°C, preferably 720°C to 750°C, and more preferably 730°C to 740°C. In addition, the predetermined time may be, for example, 1.5 hours to 2.5 hours, and preferably 2 hours to 2.5 hours. After the second heat treatment, the steel is cooled to room temperature.

[0062] The first heat treatment and the second heat treatment can be performed for a period of time of 3 hours or less, including a process of changing the temperature once in the middle from a temperature above A3 to a temperature below A1.

[0063] The steel produced after performing the second heat treatment step satisfies a hardness of 241 HB or less and has a microstructure including bainite and ferrite, wherein the area fraction of bainite may be 36% to 99% and the area fraction of ferrite may be included as the remainder.

[0064] <Experimental Example>

[0065] Preferred experimental examples are presented below to aid in understanding the present invention. However, the following experimental examples are intended only to aid in understanding the present invention and do not limit the present invention. Details not described herein can be sufficiently technically inferred by those skilled in the art, so their description is omitted.

[0066] Table 1 shows the composition of alloy steel samples for transmission gears. The content of each component is in weight%.

[0067] furtherance wt% ppm C Si Mn Cr Mo Nb N O B Steel 1 0.20 0.61 0.58 2.04 0.35 0.021 130 9 19 Steel 2 0.19 0.10 0.70 1.29 0.58 0.019 119 10 19 Steel 3 0.21 0.26 0.94 1.29 0.14 0.020 123 9 19

[0068] In order to compare the effects of the heat treatment process according to the composition of the steel materials listed in Table 1 above, example samples were prepared according to the heat treatment process conditions as shown in Table 2, and for comparison, comparative example samples were prepared by performing isothermal heat treatment at a temperature below A1 for each of the materials shown in Table 2.

[0069] Figure 3 shows the difference in the time it takes for the temperature of the steel samples to reach the target temperature during the initial heat treatment.

[0070]

[0071] For example, based on the composition of Steel 1, a first heat treatment was performed at 900°C, and a second heat treatment was performed after controlling the temperature to 750°C, which is the temperature directly below A1, to produce a sample of Example 1. At this time, the first and second heat treatments were performed continuously and for a total of 3 hours, including the time for temperature change. Based on the composition of Steel 1, an isothermal heat treatment was performed at 750°C, which is the temperature directly below A1, for 5 hours to produce a sample of Comparative Example 1. In the same manner, heat treatments were performed for the same amount of time within the temperature range listed in Table 1 to produce Examples 2, 3, Comparative Example 2, and 3, respectively.

[0072] The hardness values ​​derived according to the heat treatment conditions of the above examples and comparative samples are shown in Table 3.

[0073]

[0074] Referring to Table 3, it was confirmed that all samples from Examples 1 to 3 satisfied the target hardness value of 241 HB or less after undergoing variable temperature heat treatment for 3 hours. On the other hand, it was confirmed that all samples from Comparative Examples 1 to 3 reached the target hardness value only after undergoing isothermal heat treatment for 5 hours. As such, the total heat treatment time can be reduced depending on whether the material temperature reaches the target temperature quickly during the initial heat treatment.

[0075] Figure 3 is a graph showing the time taken to reach the target temperature of the material as the initial temperature increases in the method for manufacturing alloy steel for transmission gears according to the experimental example of the present invention.

[0076] Referring to FIG. 3, it was confirmed that through variable temperature heat treatment according to an embodiment of the present invention, the time it takes for the material temperature to reach the target temperature (e.g., 750°C) at the beginning of the heat treatment is faster for the material in the example sample (indicated as 900°C in FIG. 3) than for the material in the comparative example sample (indicated as 750°C and 850°C in FIG. 3). Through this, the present invention was able to control the hardness reduction effect of the material after heat treatment more quickly. However, the above target temperature may vary depending on the steel grade, and based on the above results, the present invention sets 750°C as the target temperature and shows the results of experiments conducted under process conditions in the form of Table 2 as an example in Table 3.

[0077] It will be obvious to those skilled in the art that the technical concept of the present invention described above is not limited to the aforementioned embodiments and attached drawings, and that various substitutions, modifications, and changes are possible within the scope of the technical concept of the present invention.

Claims

Claim 1 An alloy steel for transmission gears, comprising, in weight percent, carbon (C): 0.17% to 0.25%, silicon (Si): 0.01% to 0.70%, manganese (Mn): 0.45% to 1.10%, chromium (Cr): 0.95% to 2.25%, molybdenum (Mo): 0.07% to 0.65%, niobium (Nb): 0.015% to 0.045%, nitrogen (N): 100 ppm to 160 ppm, oxygen (O): greater than 0 ppm to less than or equal to 15 ppm, and the remainder being iron (Fe) and other unavoidable impurities, satisfying hardness: 241 HB or less, and having a microstructure including bainite and ferrite, wherein the area fraction of the bainite is 36% to 99% and the area fraction of the ferrite is the remainder. Claim 2 In claim 1, the alloy steel further comprises boron (B): 10 ppm to 35 ppm, an alloy steel for transmission gears. Claim 3 delete Claim 4 A step of hot rolling a steel material comprising, in weight percent, carbon (C): 0.17% ~ 0.25%, silicon (Si): 0.01% ~ 0.70%, manganese (Mn): 0.45% ~ 1.10%, chromium (Cr): 0.95% ~ 2.25%, molybdenum (Mo): 0.07% ~ 0.65%, niobium (Nb): 0.015% ~ 0.045%, nitrogen (N): 100 ppm ~ 160 ppm, oxygen (O): greater than 0 ppm ~ less than or equal to 15 ppm, and the remainder being iron (Fe) and other unavoidable impurities; a step of performing a first heat treatment on the hot-rolled steel material at a temperature greater than the A3 transformation point in order to control the time for the temperature of the hot-rolled steel material to reach a temperature below the A1 transformation point; A method for manufacturing alloy steel for transmission gears, comprising: a step of performing a second heat treatment on the steel material that has undergone the first heat treatment at a temperature below the A1 transformation point; wherein the first heat treatment and the second heat treatment are performed continuously, including a process of changing the temperature in between from a temperature above the A3 transformation point to a temperature below the A1 transformation point, and wherein the first heat treatment step involves heat treating the steel material at a temperature of 850℃ to 900℃, and the second heat treatment step involves heat treating the steel material at a temperature of 700℃ to 750℃. Claim 5 delete Claim 6 delete Claim 7 A method for manufacturing alloy steel for transmission gears according to claim 4, wherein in the first heat treatment step and the second heat treatment step, the heat treatment time is 3 hours or less. Claim 8 A method for manufacturing alloy steel for transmission gears according to claim 4, wherein the steel produced after performing the second heat treatment step satisfies a hardness of 241 HB or less and has a microstructure including bainite and ferrite, wherein the area fraction of bainite is 36% to 99% and the area fraction of ferrite is the remainder. Claim 9 A method for manufacturing alloy steel for transmission gears according to claim 4, wherein the steel further comprises boron (B): 10 ppm to 35 ppm.

Images