Carbon steel for rack and method for manufacturing the same

Abstract

The present invention relates to a carbon steel for a rack and a method for manufacturing the same. The carbon steel for the rack contains 0.50 to 0.55wt% carbon (C), 0.15 to 0.35wt% silicon (Si), 0.75 to 0.95wt% manganese (Mn), 0.025wt% or less phosphorus (P), 0.025wt% or less sulfur (S), 0.65 to 0.85wt% chromium (Cr), 0.20wt% or less molybdenum (Mo), 0.001 to 0.02wt% aluminum (Al), 5 to 50ppm boron (B) and the remainder of iron (Fe) and inevitable impurities. The method for manufacturing the rack includes quenching, tempering and drawing the carbon steel, and warm forging the drawn carbon steel.

Description

[0001] Citations of Related Applications

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2020-0160742 filed in the Korean Intellectual Property Office on November 26, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a carbon steel for a rack having excellent mechanical properties such as strength and toughness, and to a method for manufacturing the same. Background Art

[0004] In recent years, environmental problems are emerging on a global scale. As a countermeasure to such environmental problems, methods for reducing the use of fossil fuels throughout the industry are being considered. In order to reduce fossil fuels, methods for improving the efficiency of vehicle engines and methods for reducing the weight of vehicles have been proposed in the automotive field. In this regard, the weight reduction of the vehicle can increase the fuel economy of the vehicle to reduce the use of fossil fuels, but cannot meet the mechanical properties required for the vehicle, such as strength, durability, etc. Therefore, solving this problem has become the biggest goal in the automotive field.

[0005] On the other hand, the steering system of the vehicle rotates the steering wheel in the steering direction desired by the driver. The rotation of the steering wheel is transmitted to the gearbox through the column and finally to the tire. In this process, the rotational force in the column is transmitted to the rack of the gearbox. Since the rack works by meshing with the pinion shaft of the column, the rack is subjected to bending stress, etc., so that strength fixation is useful for the rack. In addition, in the event of a vehicle collision, even if it is deformed under the impact load, the steering components such as the rack should not be damaged to prevent secondary accidents by maintaining the driving direction of the vehicle even after a collision.

[0006] For example, a method for manufacturing a rack with a variable gear ratio is disclosed in Korean Patent Application Publication No. 2012-0081388 (Patent Document 1), which includes: (a) heating a round material at a predetermined temperature for a predetermined time; b) performing plastic working on the heat-treated round material using an upper die and a lower die for warm forging; (c) performing trimming to remove the flash of the gear portion; and (d) shaping the teeth of the gear portion by plastic working using an upper die and a lower die for shaping. However, in the manufacturing method of Patent Document 1, the shape of the rack of the variable gear can be achieved, but it may be difficult to ensure the physical properties of the rack. In the case of a forged part, when quenching and tempering heat treatment is performed after the shape is made, the shape of the gear portion is deformed due to the residual stress during the forging of the gear portion, and the straightness of the rack changes. The gear portion meshes with the pinion and rotates, which requires a very high level of shape accuracy. When the shape accuracy of the gear part is low, noise is generated due to the operating clearance during steering, and excessive wear occurs between components, which may cause safety problems during driving due to the deterioration of the steering function, so the accuracy of the gear is very important.

[0007] Therefore, it is useful to research and develop carbon steel for racks, which ensures excellent mechanical properties of the material such as yield strength, tensile strength, toughness, etc., while ensuring that the shape of the rack gear is achieved with excellent precision, thereby having excellent durability even under high load environments and being able to reduce vehicle weight, and a manufacturing method thereof. Summary of the invention

[0008] The present disclosure is made to solve the above-mentioned problems occurring in the prior art while completely maintaining the advantages achieved by the prior art.

[0009] One aspect of the present disclosure provides a rack having excellent mechanical properties such as yield strength, tensile strength, toughness, etc., thereby having excellent durability even under a high load environment and being able to reduce the weight of a vehicle, and provides a manufacturing method thereof.

[0010] The technical problems conceived to be solved by the present disclosure are not limited to the above-mentioned problems, and any other technical problems not mentioned herein should be clearly understood by those skilled in the art to which the present disclosure belongs based on the following description.

[0011] According to one aspect of the present disclosure, the carbon steel for the rack contains 0.50 to 0.55wt% of carbon (C), 0.15 to 0.35wt% of silicon (Si), 0.75 to 0.95wt% of manganese (Mn), 0.025wt% or less of phosphorus (P), 0.025wt% or less of sulfur (S), 0.65 to 0.85wt% of chromium (Cr), 0.20wt% or less of molybdenum (Mo), 0.001 to 0.02wt% of aluminum (Al), 5 to 50ppm of boron (B), and the balance of iron (Fe) and inevitable impurities.

[0012] According to another aspect of the present disclosure, a rack for a vehicle includes carbon steel.

[0013] According to another aspect of the present disclosure, a vehicle includes a rack for the vehicle.

[0014] According to another aspect of the present disclosure, a method for manufacturing a carbon steel for a rack includes manufacturing a carbon steel comprising 0.50 to 0.55wt% carbon (C), 0.15 to 0.35wt% silicon (Si), 0.75 to 0.95wt% manganese (Mn), 0.025wt% or less phosphorus (P), 0.025wt% or less sulfur (S), 0.65 to 0.85wt% chromium (Cr), 0.20wt% or less molybdenum (Mo), 0.001 to 0.02wt% aluminum (Al), 5 to 50ppm boron (B) and the balance of iron (Fe) and unavoidable impurities. The method also includes quenching, tempering and drawing the carbon steel, and warm forging the drawn carbon steel. DETAILED DESCRIPTION

[0015] Hereinafter, the present disclosure is described in detail.

[0016] Carbon steel for racks

[0017] The carbon steel for a rack according to the present disclosure contains carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), molybdenum (Mo), aluminum (Al), boron (B), iron (Fe), and inevitable impurities.

[0018] Carbon (C)

[0019] Carbon is used to increase the strength and hardness of carbon steel.

[0020] Carbon may be included in the carbon steel in an amount of 0.50 to 0.55 wt % based on the total weight of the carbon steel. When the carbon content is less than the above range, the tensile strength of the carbon steel may be reduced. When the carbon content is higher than the above range, the impact toughness may be reduced.

[0021] Silicon (Si)

[0022] Silicon acts as a deoxidizer for removing oxygen from steel, and is dissolved in steel to enhance the strength of the base material.

[0023] Silicon may be included in the carbon steel in an amount of 0.15 to 0.35 wt % or 0.17 to 0.32 wt % based on the total weight of the carbon steel. When the content of silicon is less than the above range, the tensile strength of the carbon steel may be reduced. When the content of silicon is higher than the above range, machinability may be reduced.

[0024] Manganese (Mn)

[0025] Manganese is used to increase the strength of carbon steel and reacts with sulfur to form manganese sulfide (MnS), which improves machinability.

[0026] Manganese may be included in the carbon steel in an amount of 0.75 to 0.95 wt % or 0.78 to 0.88 wt % based on the total weight of the carbon steel. When the content of manganese is less than the above range, the tensile strength of the carbon steel may be reduced. When the content of manganese is higher than the above range, segregation occurs during heat treatment, so that a decrease in toughness and an increase in deformation resistance may occur.

[0027] Phosphorus (P)

[0028] Phosphorus is an element that acts as an impurity when manufacturing carbon steel, and its content needs to be properly adjusted.

[0029] Phosphorus may be included in the carbon steel in an amount of 0.025 wt% or less, 0.001% to 0.025 wt%, or 0.01% to 0.02 wt%, based on the total weight of the carbon steel. When the phosphorus content is higher than the above range, the hardness of the carbon steel may increase and the toughness may decrease, so that cracks may occur and the cost price may increase.

[0030] Sulfur (S)

[0031] Sulfur reacts with manganese (Mn) to form manganese sulfide (MnS), thereby improving workability.

[0032] Sulfur may be included in the carbon steel in an amount of 0.025 wt% or less, 0.001 to 0.025 wt%, or 0.004 to 0.010 wt%, based on the total weight of the carbon steel. When the sulfur content is less than the above range, workability of the carbon steel may be reduced.

[0033] Chromium (Cr)

[0034] Chromium is used to improve the strength, hardenability and toughness of carbon steel. In addition, chromium plays an important role in improving the resistance to permanent deformation of the final product.

[0035] Chromium may be included in the carbon steel in an amount of 0.65 to 0.85 wt% or 0.70 to 0.80 wt% based on the total weight of the carbon steel. When the chromium content is less than the above range, the problem of insufficient durability of the carbon steel may occur. When the chromium content is higher than the above range, chromium carbide may be precipitated in the grain boundary to reduce the toughness of the carbon steel.

[0036] Molybdenum (Mo)

[0037] Molybdenum is used to improve the strength, hardenability and toughness of carbon steel.

[0038] Based on the total weight of the carbon steel, molybdenum may be included in the carbon steel in an amount of 0.20 wt% or less, 0.001 to 0.20 wt%, or 0.005 to 0.05 wt%. When the content of molybdenum is lower than the above range, the problem of insufficient durability of the carbon steel may occur. When the content of molybdenum is higher than the above range, the strength of the carbon steel may be excessively increased, so that workability is reduced and cost increase may occur.

[0039] Aluminum (Al)

[0040] Aluminum is used to increase strength and toughness by refining the grains in steel.

[0041] Aluminum may be included in the carbon steel in an amount of 0.001 to 0.02 wt% or 0.003 to 0.015 wt% based on the total weight of the carbon steel. When the content of aluminum is less than the above range, grains in the steel may not be refined. When the content of aluminum is higher than the above range, Al 2 O 3 Inclusions reduce the toughness of carbon steel.

[0042] Boron (B)

[0043] Boron strengthens the grain boundaries of carbon steel and improves hardenability.

[0044] Boron may be contained in the carbon steel in an amount of 5 to 50 ppm, or 10 to 30 ppm, based on the total weight of the carbon steel. When the content of boron is less than the above range, the effect obtained by adding boron may be insufficient. When the content of boron is higher than the above range, the improvement of the effect obtained may be insufficient compared to the amount of boron added, so that a reduction in economic feasibility may occur.

[0045] Copper (Cu)

[0046] High-strength carbon steel may further contain copper (Cu). In this regard, copper is an element that acts as an impurity when manufacturing carbon steel, which requires appropriate adjustment of the content.

[0047] Copper may be included in the carbon steel in an amount of 0.10 wt% or less, or 0.01 to 0.10 wt% based on the total weight of the carbon steel. When the copper content is higher than the above range, the hardness of the carbon steel may increase and its toughness may decrease, so that cracks may occur and costs may increase.

[0048] Nickel (Ni)

[0049] The high-strength carbon steel may further contain nickel (Ni). In this regard, nickel plays a role in improving the strength, hardenability and toughness of the carbon steel.

[0050] Nickel may be included in the carbon steel in an amount of 0.30 wt% or less or 0.01 to 0.10 wt% based on the total weight of the carbon steel. When the nickel content is higher than the above range, the strength of the carbon steel may excessively increase and its toughness may decrease, so that cracks may occur and costs may increase.

[0051] Vanadium (V)

[0052] The high-strength carbon steel may further contain vanadium (V). In this regard, vanadium plays a role in improving the strength, hardenability, and toughness of the carbon steel.

[0053] Vanadium may be included in the carbon steel in an amount of 0.06 wt % or less, 0.001 to 0.03 wt %, or 0.001 to 0.01 wt % based on the total weight of the carbon steel. When the content of vanadium is less than the above range, the problem of insufficient durability of the carbon steel may occur. When the content of vanadium is higher than the above range, the strength of the carbon steel may be excessively increased, so that reduced workability and increased cost may occur.

[0054] Iron (Fe) and impurities

[0055] Carbon steel contains iron and other unavoidable impurities.

[0056] Carbon steel may contain a very small amount of inevitable impurities. The inevitable impurities may be contained in such a small amount as not to affect the properties such as strength, workability and durability of the carbon steel.

[0057] Carbon steel may have a tensile strength equal to or higher than 800 MPa, an elongation percentage equal to or higher than 20%, equal to or higher than 23%, or between 23% and 30%, and an electrical conductivity equal to or higher than 100 J / cm 2 , equal to or higher than 150J / cm 2 , equal to or higher than 200J / cm 2 , or between 200 and 300 J / cm 2 Room temperature impact strength between.

[0058] Furthermore, the carbon steel may have a yield strength equal to or higher than 600 MPa, or equal to or higher than 650 MPa.

[0059] As described above, the carbon steel for the rack according to the present disclosure has excellent mechanical properties such as yield strength, tensile strength and toughness, thereby having excellent durability even under high load conditions and being able to reduce the weight of the vehicle. Therefore, the carbon steel for the rack is very suitable for use as a material for vehicle parts, especially for a material for steering parts such as a rack.

[0060] Rack for vehicle and vehicle

[0061] The rack for a vehicle according to the present disclosure includes the carbon steel for a rack as described above or herein. In addition, the vehicle according to the present disclosure includes the rack for a vehicle as described above or herein.

[0062] Method for manufacturing carbon steel for rack

[0063] Further, the manufacturing method of carbon steel for a rack disclosed in the present invention includes: manufacturing carbon steel; quenching, tempering and drawing the carbon steel; and warm forging the drawn carbon steel.

[0064] Manufacturing Carbon Steel

[0065] In this operation, carbon steel containing carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), molybdenum (Mo), aluminum (Al), boron (B), iron (Fe), and inevitable impurities is manufactured. In this regard, the carbon steel may be in the form of a wire rod, but may not be limited thereto.

[0066] Carbon steel contains 0.50 to 0.55wt% of carbon (C), 0.15 to 0.35wt% of silicon (Si), 0.75 to 0.95wt% of manganese (Mn), 0.025wt% or less of phosphorus (P), 0.025wt% or less of sulfur (S), 0.65 to 0.85wt% of chromium (Cr), 0.20wt% or less of molybdenum (Mo), 0.001 to 0.02wt% of aluminum (Al), 5 to 50ppm of boron (B), and the balance of iron (Fe) and inevitable impurities. For example, the carbon steel may contain 0.50 to 0.55 wt % of carbon (C), 0.17 to 0.32 wt % of silicon (Si), 0.78 to 0.88 wt % of manganese (Mn), 0.025 wt % or less of phosphorus (P), 0.025 wt % or less of sulfur (S), 0.10 wt % or less of copper (Cu), 0.30 wt % or less of nickel (Ni), 0.70 to 0.80 wt % of chromium (Cr), 0.20 wt % or less of molybdenum (Mo), 0.003 to 0.015 wt % of aluminum (Al), 10 to 30 ppm of boron (B), and the balance of iron (Fe) and inevitable impurities.

[0067] Furthermore, the carbon steel may contain additional components as described in the carbon steel for the rack.

[0068] Generally, the present operation is not particularly limited as long as it is a method for preparing a material for a rack. For example, the present operation may include a reheating process, a hot rolling process, a coiling process, and a cooling process.

[0069] Specifically, the reheating process may be performed at a slab reheating temperature (SRT) of 900 to 1300° C. or 950 to 1100° C. for 0.5 to 5 hours or 1 to 3 hours. When the slab reheating temperature is lower than the above range, components precipitated during casting cannot be sufficiently redissolved, and the rolling load may become large. On the other hand, when the slab reheating temperature is higher than the above range, decarburization may occur excessively, the austenite grain size may increase, which may make it difficult to ensure strength, and the manufacturing cost of carbon steel may increase due to the excessive heating process.

[0070] In addition, the hot rolling process may be performed at 750 to 900° C. or at 780 to 850° C. When the rolling temperature is higher than the above range, decarburization may occur, pearlite nucleation due to coarse grains may be delayed, and temperature controllability may decrease as the deviation from the coiling temperature increases. On the other hand, when the rolling temperature is too low and lower than the above range, a large rolling load may be applied.

[0071] For example, the winding process may be performed by a laying machine at a temperature of 700 to 850° C. or at a temperature of 750 to 830° C. When the winding temperature is controlled within the above range, it is advantageous to adjust the phase transformation level from austenite to pearlite structure.

[0072] Further, the cooling process can be performed while controlling the average cooling rate equal to or lower than 5°C / s or between 0.3 and 5°C / s after the billet is coiled by the wire laying machine. The microstructure of the carbon steel and the grain size of the microstructure part can be adjusted by adjusting the cooling rate of the cooling process.

[0073] Quenched, tempered and drawn carbon steel

[0074] In this operation, the carbon steel is quenched, tempered and drawn. By this operation, carbon steel in wire form can be manufactured in a straight form.

[0075] For example, quenching can be performed at a temperature of 900 to 1000°C or 930 to 970°C. When the quenching temperature is adjusted to the above range, the strength and toughness of the final product can be further improved. The quenching can be controlled so that the physical properties of carbon steel can be met, for example, the tensile strength is equal to or greater than 880MPa, equal to or greater than 900Mpa, or between 880 and 1200Mpa, the elongation percentage is between 15 and 20% or between 17.0 and 19.5%, and the room temperature impact strength is between 100 and 150J / cm 2 or between 110 and 140 J / cm 2 .

[0076] For example, tempering can be performed at a temperature of 710 to 740° C. or 715 to 735° C. When the tempering temperature is adjusted to the above range, the strength and toughness of the final product can be further improved. Tempering can be controlled so that the physical properties of carbon steel can be met, for example, the tensile strength is equal to or higher than 900 MPa, or between 900 and 1000 MPa, the elongation percentage is between 16 and 21% or between 17.0 and 20%, and the room temperature impact strength is between 100 and 150 J / cm 2 or between 110 and 135 J / cm 2 .

[0077] The drawing can be controlled so that the drawing reduction percentage is between 10% and 20% or between 13% and 15%. In this case, when the final rack product is manufactured, not only the bending direction can be controlled outwardly, but also the bending amount can be further reduced.

[0078] In this regard, in the present operation, quenching, tempering and drawing can be performed while transferring the carbon steel at a speed of 50 to 80 mm / sec or 55 to 75 mm / sec. When the transfer speed is less than the above range, the carbon steel is overheated, so that decarburization may occur. When the transfer speed is higher than the above range, there may be a problem of unevenly heat-treating the carbon steel core.

[0079] Carbon steel after warm forging and drawing

[0080] In this operation, the drawn carbon steel is warm forged. With this operation, a forging die is used to mold the gear teeth of carbon steel in a straight form.

[0081] Warm forging may be performed at 580 to 730° C. or 600 to 700° C. for 15 to 120 minutes or 20 to 60 minutes.

[0082] When the temperature is adjusted within this range during warm forging, the strength and toughness of the final product can be further improved. The warm forging can be controlled so that the physical properties of carbon steel can be met, for example, the tensile strength is equal to or higher than 800 MPa, or between 800 and 950 MPa, the elongation percentage is between 20 and 30%, or between 23 and 29%, and the room temperature impact strength is between 180 and 300 J / cm 2 or between 200 and 280 J / cm 2 .

[0083] The manufacturing method of the present disclosure may further include performing a high frequency hardening heat treatment on the warm forged carbon steel. In this regard, through the high frequency hardening heat treatment, the surface hardness can be increased to prevent the gear teeth of the rack from being worn.

[0084] In this regard, the frequency of the high frequency, the processing time, and the like used for the high frequency hardening heat treatment may be appropriately selected based on the size of the rack, the thickness of the gear teeth, and the like.

[0085] Furthermore, the manufacturing method of the present disclosure may further include straightening the heat-treated carbon steel. In this regard, the overall straightness of the rack may be corrected by straightening.

[0086] The straightening method may not be particularly limited. Further, a method generally used in the field of vehicle steering components may be used. Through this straightening, the outward bending may be corrected, thereby reducing the incidence of defects and further improving productivity.

[0087] The method of manufacturing a carbon steel for a rack according to the present disclosure as described above can manufacture a carbon steel for a rack which minimizes a decrease in tensile strength and significantly improves elongation percentage and room temperature impact strength.

[0088] Hereinafter, the present disclosure is described in more detail by examples. However, the examples are only intended to help understand the present disclosure, and the scope of the present disclosure is not limited to such examples in any sense.

[0089] [Example]

[0090] Example 1 and Comparative Examples 1 to 6. Production of Carbon Steel for Rack

[0091] 1) Carbon steel manufacturing

[0092] A billet including components as shown in Table 1 below was manufactured and then rolled into a billet at 1,250° C. Thereafter, the billet was reheated at 1,000±100° C., then rolled at about 1,000° C., then coiled at a temperature of 850 to 900° C., and then cooled at about 500° C. while being transferred at an average cooling rate of 3 to 12° C. / sec to manufacture carbon steel.

[0093] [Table 1]

[0094]

[0095]

[0096] (2) Quenching

[0097] The carbon steel manufactured in 1) was oil quenched at 950°C while transferring at 65 mm / sec, and then tempered at 725°C at 65 mm / sec, and then the yield strength, tensile strength, elongation percentage, and room temperature impact strength of each carbon steel were measured. Specifically, the yield strength and tensile strength were measured in a test method based on ISO 6892, and the room temperature impact strength was measured in a test method based on ISO 148-1 in a Charpy impact test. The measurement results are shown in Table 2 below.

[0098] [Table 2]

[0099]

[0100] As shown in Table 2, Comparative Example 1 comprising a small amount of chromium and an excess of molybdenum has an insufficient percentage and has poor economic feasibility due to the inclusion of an excess of molybdenum. Further, in Comparative Example 2 of Comparative Example 1 comprising a small amount of chromium and an excess of molybdenum and aluminum, cracks are generated on the surface during quenching due to excessive hardenability. Further, Comparative Example 3 comprising an excess of carbon has insufficient room temperature impact strength, and Comparative Example 4 comprising a small amount of carbon and chromium has insufficient elongation percentage and room temperature impact strength. Further, Comparative Example 5 comprising a small amount of manganese and chromium has insufficient strength. In addition, Comparative Example 6 comprising a small amount of carbon, manganese and chromium and an excess of aluminum and not containing molybdenum has insufficient tensile strength and room temperature impact strength.

[0101] 3) Tempering

[0102] The quenched carbon steel of Example 1 manufactured in 2) was tempered at 650 to 750°C while being transferred at a speed of 65 mm / sec. Then, the yield strength, tensile strength, elongation percentage, and room temperature impact strength of each carbon steel were measured. In this regard, the method for measuring the physical properties was the same as that in Item 2). The measurement results are shown in Table 3.

[0103] [Table 3]

[0104]

[0105] As shown in Table 3, in Example 1-1, the tensile strength of the carbon steel was not sufficient to be equal to or less than 900 MPa, and the yield strength was also insufficient at a tempering temperature of 750°C or higher. In Examples 1-3 to 1-5, the room temperature impact strength of the carbon steel was not sufficient to be equal to or less than 100 J / cm at a temperature of 700°C or lower. 2 Example 1-2 tempered at 725°C has sufficient tensile strength and room temperature impact strength, and also has sufficient yield strength and elongation percentage.

[0106] 4) Drawing and warm forging

[0107] The tempered carbon steel of Example 1-2 manufactured in 3) was drawn so that its drawing reduction rate was equal to or higher than 10%, and then warm forged at a temperature of 600 to 750°C for 30 minutes. Then, the yield strength, tensile strength, elongation percentage, and room temperature impact strength of each carbon steel were measured. In this regard, the method for measuring physical properties is the same as the method in Item 2). The measurement results are shown in Table 4.

[0108] [Table 4]

[0109]

[0110] As shown in Table 4, Example 1-2-4 in which the warm forging temperature was 750° C. had insufficient yield strength and tensile strength.

[0111] Further, when warm forging is performed at less than 600° C., it is difficult to obtain a desired forged shape because the temperature is low and thus moldability is low. Further, wear of the die and damage due to impact may occur.

[0112] Further, the carbon steel of Comparative Example 6 manufactured in 2) was tempered at 725°C while transferring at 65 mm / sec, then drawn so that its drawing reduction percentage became 13%, and then warm forged at a temperature of 600 to 700°C for 30 minutes. Then, its yield strength, tensile strength, elongation percentage, and room temperature impact strength were measured. In this regard, the method for measuring physical properties is the same as the method in Item 2). The measurement results are shown in Table 5.

[0113] [Table 5]

[0114]

[0115] As shown in Table 5, compared with Comparative Example 6, in Example 1-2, the yield strength was improved by 30% or more, the tensile strength was improved by 24% or more, and the room temperature impact strength was improved by 170% or more.

[0116] Test Example: Performance Evaluation

[0117] For the carbon steels for racks of Example 1-2-2 and Comparative Example 6 forged at 650° C. manufactured in Examples and Comparative Examples, the minimum crack load and the maximum load were measured using the following method. The measurement results are shown in Table 6.

[0118] A bending test was performed by vertically pressing a portion with a gear while fixing one side of the rack to measure a load at which an initial crack occurred in the gear portion as an initial crack load and a maximum load when complete fracture occurred.

[0119] [Table 6]

[0120]

[0121] As shown in Table 6, in the carbon steel of Example 1-2-2, the initial crack load was increased by 8.78% and the maximum load was increased by 21.14% compared with Comparative Example 6. Further, in Example 1-2-2, even if an initial crack occurred, the rack did not break, which indicates improved fracture strength. Therefore, it can be seen that Example 1-2-2 has excellent stability against vehicle collision and damage when applied to a vehicle.

[0122] The rack according to the present disclosure has excellent mechanical properties such as yield strength, tensile strength and toughness, etc., and thus has excellent durability even under high load conditions and can reduce vehicle weight. Therefore, the carbon steel used for the rack is very suitable as a material for vehicle parts.

[0123] Although the present disclosure has been described with reference to the embodiments above, the present disclosure is not limited thereto. The disclosed embodiments may be variously modified and altered by a person skilled in the art without departing from the spirit and scope of the present disclosure as claimed in the appended claims.

Claims

1. A method for manufacturing carbon steel for a rack, the method comprising: manufacturing the carbon steel, which is composed of the following components: 0.50 to 0.55 wt% of carbon (C), 0.15 to 0.35 wt% of silicon (Si), 0.75 to 0.95 wt% of manganese (Mn), 0.025 wt% or less of phosphorus (P), 0.025 wt% or less of sulfur (S), 0.65 to 0.85 wt% of chromium (Cr), 0.20 wt% or less of molybdenum (Mo), 0.001 to 0.02 wt% of aluminum (Al), 5 to 50 ppm of boron (B), and the balance of iron (Fe) and inevitable impurities, based on the total weight of the carbon steel; quenching, tempering and drawing the carbon steel; and Warm forging the drawn carbon steel. Wherein, the tempering is carried out at a temperature of 710 to 740°C, and Wherein, the warm forging is carried out at a temperature of 600 to 730°C.

2. The manufacturing method according to claim 1, wherein: The quenching is performed at a temperature of 900 to 1,000°C.

3. The manufacturing method according to claim 1, wherein: The drawing is controlled so that the drawing reduction percentage is between 10% and 20%.

4. The manufacturing method according to claim 1, further comprising: The carbon steel after warm forging is subjected to high frequency hardening heat treatment.

5. The manufacturing method according to claim 4, further comprising: Straightening heat treated carbon steel.

6. A method for manufacturing carbon steel for a rack, the method comprising: manufacturing the carbon steel, which is composed of the following components: 0.50 to 0.55 wt% of carbon (C), 0.17 to 0.32 wt% of silicon (Si), 0.78 to 0.88 wt% of manganese (Mn), 0.025 wt% or less of phosphorus (P), 0.025 wt% or less of sulfur (S), 0.10 wt% or less of copper (Cu), 0.30 wt% or less of nickel (Ni), 0.70 to 0.80 wt% of chromium (Cr), 0.20 wt% or less of molybdenum (Mo), 0.003 to 0.015 wt% of aluminum (Al), 10 to 30 ppm of boron (B), and the balance of iron (Fe) and inevitable impurities, based on the total weight of the carbon steel; quenching, tempering and drawing the carbon steel; and Warm forging the drawn carbon steel. Wherein, the tempering is carried out at a temperature of 710 to 740°C, and Wherein, the warm forging is carried out at a temperature of 600 to 730°C.

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