Carburized steel and manufacturing method therefor

Abstract

A carburized steel according to the present invention comprises, by wt%, 0.01 to 0.30% of C, 1.00% or less (excluding 0) of Si, 1.50% or less (excluding 0) of Mn, 2.00% or less (excluding 0) of Cr, 0.030% or less of P, 0.0500% or less of S, 0.3% or less of Ni, 0.30% or less of Mo, 0.30% or less of Cu, 0.500-0.01% of sol. Al, 0.100% or less of Nb, 0.30% or less of V, 0.050% or less of Ti, 0.0050% or less of B, 0.0200% or less of N, and the balance of Fe and inevitable impurities, and satisfies the following equations (1) and (2). Equation (1): [Al] / [C] ≥ 2.0 Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8 (wherein [Al], [C], [Si], [Mn], and [Cr] each represent the content (%) of the corresponding element).

Description

Carburized steel and its manufacturing method

[0001] The present invention relates to carburized steel and a method for manufacturing the same.

[0002] Steel used for machine structural parts generally contains a combination of alloys such as Mn, Cr, Mo, and Ni. Machine structural parts include parts that undergo carburization treatment, and the steel used for these carburized parts (hereinafter referred to as carburized parts) has the chemical composition described above and is manufactured by casting, forging, rolling, etc. Carburized parts are manufactured, for example, by the following method. An intermediate product is manufactured from the steel by machining such as forging and cutting. Carburization treatment is performed on the intermediate product to manufacture a carburized part having a carburized layer, which is a hardened layer on the surface, and a core, which is a base material unaffected by the carburization treatment.

[0003] Among the costs of manufacturing carburized parts, the costs associated with machining are very significant. Machining is not only expensive in terms of cutting tools, but it also generates a large amount of cutting chips. Consequently, it is disadvantageous from the perspective of yield. For this reason, attempts are being made to replace machining with forging. Forging methods can be broadly classified into hot forging, warm forging, and cold forging; warm forging is characterized by less scale formation and improved dimensional accuracy compared to hot forging. Additionally, cold forging is characterized by the absence of scale formation and dimensional accuracy close to that of machining. Therefore, methods such as performing rough machining via hot forging followed by finishing via cold forging, performing light machining as a finish after warm forging, or forming the part solely through cold forging have been considered. However, when replacing machining with hot or cold forging, if the deformation resistance of the steel is high, the surface pressure applied to the die increases, and the lifespan of the die decreases. Consequently, the cost advantage of machining diminishes. Furthermore, when forming steel into complex shapes, problems such as cracking occur in areas where large processing is applied. In addition, with the recent increase in the performance of automobile engines, improving the durability of key components has emerged as a critical challenge. Transmission components, which are core power transmission parts, are manufactured using steel with a carbon level of 0.2 wt% and surface treatments such as carburization to achieve high strength, high durability, and sufficient toughness. However, although gear tooth fracture due to insufficient strength rarely occurs due to recent improvements in alloy design technology, the intergranular oxide layer and abnormal grain formation that inevitably occur during carburization heat treatment, as well as gear tooth distortion caused by thermal deformation during carburization quenching, significantly affect the reduction of durability of transmission steel due to tooth surface deterioration and uneven stress; therefore, research on improving these aspects is being reviewed.

[0004] The present invention aims to provide a carburized steel that minimizes thermal deformation occurring during carburization heat treatment and a method for manufacturing the same.

[0005] A carburized steel according to one embodiment of the present invention contains, in weight%, C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, the remainder being Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).

[0006] Equation (1): [Al] / [C] ≥ 2.0

[0007] Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8

[0008] (Here, [Al], [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element)

[0009] In addition, the carburized steel according to one embodiment of the present invention may have a thermal strain of less than 0.90%.

[0010] In addition, the carburized steel according to one embodiment of the present invention has a fatigue strength of 80 kgf / mm 2 This is the case, and the fatigue limit ratio may be 50% or more.

[0011] In addition, the carburized steel according to one embodiment of the present invention may have a microstructure of ferrite and martensite.

[0012] In addition, the carburized steel according to one embodiment of the present invention may include ferrite in an area fraction of 10% or more in the composite structure.

[0013] In addition, the carburized steel according to one embodiment of the present invention may have a surface hardness of 58 HRC or higher and a core hardness of 50 HRC or lower.

[0014] In addition, the carburized steel according to one embodiment of the present invention may have an average austenite grain size of 30 μm or less.

[0015] A method for manufacturing carburized steel according to another embodiment of the present invention comprises the step of preparing a steel material satisfying the following formulas (1) and (2), containing, in weight%, C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, and the remainder being Fe and unavoidable impurities; The method includes the step of carburizing and diffusing the steel at a temperature range of 880℃ to Ae3+50℃; and the step of cracking the carburized and diffusing steel at a temperature range of 820℃ to Ae3-30℃.

[0016] Equation (1): [Al] / [C] ≥ 2.0

[0017] Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8

[0018] (Here, [Al], [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element)

[0019] In addition, a method for manufacturing carburized steel according to one embodiment of the present invention may include a step of quenching the cracked steel in oil at a temperature range of 60°C to 150°C.

[0020] In addition, a method for manufacturing carburized steel according to one embodiment of the present invention may include a step of tempering at a temperature range of 150°C to 200°C after the quenching step.

[0021] According to the present invention, a carburized steel having excellent durability and minimizing thermal deformation occurring during carburization heat treatment can be provided.

[0022] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0023] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0024] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0025] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0026] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0027] First, a carburized steel according to one aspect of the present invention will be described.

[0028] A carburized steel according to one embodiment of the present invention contains, in weight%, C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, and the remainder is Fe and unavoidable impurities.

[0029] Hereinafter, the reason for the numerical limitation of the alloy component content in the embodiments of the present invention will be explained.

[0030] C: 0.01~0.30%

[0031] Carbon plays a role in improving the strength of the wire rod. To exhibit this effect, it is desirable to include at least 0.01%. However, if the content is excessive, the deformation resistance of the steel increases sharply, and there is a problem that the cold workability deteriorates as a result. Therefore, the upper limit of the carbon content may be 0.30%. Preferably, it may be 0.10 to 0.28%, and more preferably, 0.15 to 0.25%.

[0032] Si: 1.00% or less (excluding 0)

[0033] Silicon is a ferrite-stabilizing element, and when added in large quantities, it has the effect of increasing the proeutectoid ferrite formation temperature (Ae3). However, if the content is excessive, the deformation resistance of the steel increases rapidly due to solid solution strengthening, and this causes a problem of deterioration in cold workability. Therefore, the upper limit of the silicon content can be controlled to 1.00%. Preferably, it can be 0.90% or less, and more preferably, 0.80% or less.

[0034] Mn: 1.50% or less (excluding 0)

[0035] If the manganese content is excessive, the strength of the steel itself becomes excessively high, causing a sharp increase in the steel's resistance to deformation, which leads to a problem of deterioration in cold workability. Therefore, the upper limit of the manganese content can be controlled to 1.50%. Preferably, it can be 1.40% or less, and more preferably, 1.30% or less.

[0036] Cr: 2.00% or less (excluding 0)

[0037] Chromium plays a role in promoting ferrite and pearlite transformations during hot rolling. In addition, without increasing the strength of the steel itself beyond what is necessary, it precipitates carbides within the steel to reduce the amount of dissolved carbon, thereby significantly contributing to the reduction of dynamic strain aging caused by dissolved carbon. On the other hand, if the content is excessive, the strength of the steel itself becomes excessively high, causing a sharp increase in the deformation resistance of the steel, which leads to a problem of deterioration in cold workability. The chromium content can be controlled to 2.00% or less. Preferably, it can be 1.80% or less, and more preferably, 1.50% or less.

[0038] P: 0.030% or less

[0039] Phosphorus is an inevitably contained impurity that segregates at grain boundaries and is a major cause of reduced toughness and decreased resistance to delayed fracture in steel; therefore, it is desirable to control its content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0%, but it is inevitably contained due to the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is managed at 0.030%.

[0040] S: 0.0500% or less

[0041] Sulfur is an inevitably contained impurity that segregates at grain boundaries, significantly reducing the ductility of steel, and forms sulfides in steel, which is a major cause of deterioration in resistance to delayed fracture and stress relaxation properties; therefore, it is desirable to control its content to be as low as possible. Theoretically, it is advantageous to control the sulfur content to 0%, but it is inevitably contained due to the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is managed to 0.0500%.

[0042] Ni: 0.3% or less

[0043] Nickel is an element that improves the toughness of steel but increases manufacturing costs and reduces machinability. Considering this, it is desirable to keep the nickel content at 0.3% or less.

[0044] Mo: 0.30% or less

[0045] Molybdenum is an element that improves strength, toughness, and hardenability. However, as it is an expensive element that increases the manufacturing cost of parts, it needs to be minimized in consideration of final strength. With this in mind, the amount of molybdenum added can be managed to be 0.30% or less. Preferably, it can be 0.25% or less, and more preferably, 0.20% or less.

[0046] Cu: 0.30% or less

[0047] Copper is an element that improves strength and corrosion resistance, but excessive addition is a major cause of reduced toughness and surface cracks caused by hot working during the manufacturing process, so it needs to be controlled. Considering this, it is desirable to manage the amount of copper added at 0.30% or less.

[0048] sol.Al: 0.500~1.000%

[0049] Aluminum is a ferrite-stabilizing element, and when added, it has the effect of raising the proeutectoid ferrite formation temperature (Ae3). Therefore, in order to secure an abnormal temperature range during carburization heat treatment, a content capable of raising the Ae3 temperature to 880°C or higher is required. To exhibit this effect, it is desirable to include at least 0.500%. However, if the content exceeds 1.000%, cold workability is reduced due to the formation of a large amount of coarse AlN. Therefore, in the present invention, the upper limit of the usable aluminum content is managed at 1.000%. Preferably, it may be 0.600% to 0.900%, and more preferably, 0.700% to 0.800%.

[0050] Nb: 0.100% or less

[0051] Niobium combines with N and C in steel to form Nb carbonitrides. Nb carbonitrides suppress grain coarsening through a pinning effect. On the other hand, if the Nb content exceeds 0.100%, there is a problem of forming coarse precipitates. Therefore, the preferred upper limit of the Nb content is 0.100%, more preferably 0.090%, and even more preferably 0.080%.

[0052] V: 0.30% or less

[0053] Vanadium, like niobium, is an element that forms carbides and carbonitrides, thereby limiting grain boundary migration in austenite and ferrite. However, since the carbonitrides can act as fracture initiators and reduce impact toughness, it is desirable to add them while adhering to the solubility limit. In the present invention, if the content of V exceeds 0.30%, there is a problem of forming coarse precipitates. Therefore, the content is controlled to be 0.30% or less. Preferably, it may be 0.25% or less, and more preferably, 0.20% or less.

[0054] Ti: 0.050% or less

[0055] Titanium combines with nitrogen in steel to form titanium nitride (TiN). This nitride is very stable at high temperatures and forms at the austenite grain boundaries, inhibiting the growth of austenite grains and refining the microstructure. Due to the refined austenite microstructure, the transformation into ferrite and pearlite, which are soft microstructures, is promoted upon cooling, thereby achieving the effect of softening the steel. However, if it exceeds 0.050%, coarse titanium nitride precipitates excessively, which reduces toughness. Therefore, the titanium content is controlled to be 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0056] B: 0.0050% or less

[0057] Boron is a grain boundary strengthening element for improving hardenability and delayed fracture resistance. However, if it exceeds 0.0050%, boron carbides precipitate at the grain boundaries, causing a decrease in grain boundary strength. Therefore, the upper limit of the boron content can be controlled to 0.0050%. Preferably, it can be 0.0040% or less, and more preferably, 0.0030% or less.

[0058] N: 0.0200% or less

[0059] Nitrogen is an inevitably contained impurity; if its content is excessive, the amount of dissolved nitrogen increases, causing a sharp rise in the deformation resistance of the steel, which leads to a problem of deterioration in cold workability. Theoretically, it is advantageous to control the nitrogen content to 0%, but it is inevitably contained during the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable to manage the upper limit of the nitrogen content to 0.0200%, more preferable to manage it to 0.0180%, and even more preferable to manage it to 0.0150%.

[0060] 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.

[0061] In addition, the carburized steel according to one embodiment of the present invention satisfies the following formula (1).

[0062] Equation (1): [Al] / [C] ≥ 2.0

[0063] Al is a ferrite-stabilizing element and is essential for securing ferrite and martensite structures through phase region heat treatment. Conversely, C is an austenite-stabilizing element and is an element with very high solid solution strengthening, so it is essential for securing strength; however, as its content increases, it becomes difficult to secure the phase region according to the present invention. Therefore, the ratio of [Al] / [C] for securing strength and the phase region temperature range of the carburized steel according to the present invention is controlled to be 2.0 or higher, preferably 2.2 or higher, and more preferably 2.5 or higher.

[0064] Accordingly, the carburized steel may include a composite structure of ferrite and martensite as its microstructure, and may include ferrite in an area fraction of 10% or more. Since the effect of reducing thermal deformation may not be sufficient if the ferrite area fraction is less than 10% or the martensite area fraction exceeds 90%, the abnormal structure is controlled within the above range.

[0065] In addition, the carburized steel according to one embodiment of the present invention can satisfy the following formula (2).

[0066] Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8

[0067] Equation (2) is a hardness index of the steel. When the C content is low, the microstructure of the steel before forging has a significantly higher ferrite fraction than that of conventional steel for parts subjected to carburization treatment (C content of about 0.20%). In this case, the hardness of the steel is greatly affected not only by the C content (pearite fraction) but also by the hardness of the ferrite. Accordingly, the present invention examines the contribution of each alloy element to the solid solution strengthening amount of ferrite and defines Equation (2) to control it. If the value exceeds 0.8, the hardness of the steel before forging increases, causing a problem where the limiting processing rate decreases; on the other hand, if it is less than 0.4, the hardness as a carburized part is insufficient. Therefore, the value of Equation (2) is controlled to be between 0.4 and 0.8.

[0068] That is, the carburized steel according to the present invention satisfies the above equation (1), so that the fatigue strength is 80 kgf / mm 2 The above is the case, and the fatigue limit ratio may be 50% or more. Thermal deformation refers to a change in dimensions (diameter, gap width) before and after quenching of a part, and in the present invention, the thermal deformation rate refers to the rate of change in dimensions (diameter, gap width) after heat treatment relative to the dimensions (diameter, gap width) before heat treatment. The thermal deformation rate during carburization may be 0.90% or less, preferably 0.85% or less, and more preferably 0.80% or less.

[0069] Hereinafter, a method for manufacturing carburized steel according to another aspect of the present invention will be described.

[0070] A method for manufacturing carburized steel according to another embodiment of the present invention comprises the step of preparing a steel material satisfying the following formulas (1) and (2), containing, in weight%, C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, and the remainder being Fe and unavoidable impurities; The method includes the step of carburizing and diffusing the steel at a temperature range of 880℃ to Ae3+50℃; and the step of cracking the carburized and diffusing steel at a temperature range of 820℃ to Ae3-30℃.

[0071] The above composition and formulas are as described above.

[0072] The temperature during the carburization treatment may be 880°C to Ae3+50°C. Since carbides may precipitate if the temperature is below 880°C and coarse grains may form if the temperature exceeds Ae3+50°C, the carburization treatment temperature is controlled to 880°C to Ae3+50°C. Accordingly, the average grain size of the prior austenite according to the present invention can be controlled to 30 μm or less. Here, the prior austenite grains can be obtained by appropriately etching the part after the carburization heat treatment and observing it. For example, the hardened layer formed on the surface of the part was etched in a picric acid solution to expose the prior austenite grain boundaries, the prior austenite grain structure was photographed, and the equivalent diameter was measured using image analyzer software and calculated as an arbitrary 10-point average.

[0073] Next, the carbon-diffusing steel is subjected to crack treatment (homogenization heat treatment).

[0074] The temperature during the cracking treatment step of the carbon-diffusing steel is controlled to 820°C to Ae3-30°C. The temperature can be controlled within the aforementioned range to reduce thermal deformation before cooling. Since carbides may precipitate if the temperature is below 820°C and severe thermal deformation may occur if it exceeds Ae3-30°C, the cracking treatment temperature is controlled to 820°C to Ae3-30°C.

[0075] Next, the crack-treated steel is quenched in oil at 60°C to 150°C. At this time, if the quenching temperature is below 60°C, excessive thermal deformation and cracking may occur. Also, if it exceeds 150°C, it may not be cooled sufficiently, and a stable high-carbon martensite structure may not be properly formed.

[0076] Next, the quenched steel is tempered at 150°C to 200°C. If the tempering temperature is lower than 150°C, the martensite structure may not soften sufficiently, leading to brittleness, and if it exceeds 200°C, the hardness of the carburized area may fall short of the required level, so the tempering temperature is controlled to 150°C to 200°C.

[0077] The following describes the invention in detail through examples. However, the following examples are merely illustrative of the invention, and the scope of the invention is not limited by the following examples.

[0078] (Example)

[0079] In this embodiment, specimens having the elemental compositions and formula values ​​of the carburized steel alloys in Tables 1 and 2 below were manufactured according to the manufacturing method in Table 2 below. Specifically, the alloy design in Table 1 was melted in a vacuum induction melting furnace of approximately 50 kg, reheated at 1150°C, and rolled to 20t and 30t. Spinning bending fatigue specimens and C-ring specimens were machined from the 20t rolled material to evaluate their physical properties after carburization heat treatment, while hardenability evaluation specimens were machined from the 30t rolled material to evaluate them. At this time, the carburization was performed with a carbon potential of 0.8%, and after carburization and homogenization heat treatment at the temperatures corresponding to Table 3 below, the specimens were quenched in 80°C oil. The quenched products were then tempered at 180°C to remove residual stress and ensure toughness, followed by air cooling, and the measured physical properties are shown in Table 3 below.

[0080] Surface hardness was measured using the Rockwell C scale on the area (surface) where the carburized layer was formed. Hardness was evaluated by measuring at 10 points on the surface and calculating the average value.

[0081] Thermal strain was evaluated by fabricating Navy C-Ring specimens as shown in Figure 1. The amount of deformation before and after carburization heat treatment was quantitatively measured using a Zeiss UMM850 measuring instrument, and the inner diameter, outer diameter, and gap width of the C-Ring were measured. Values ​​were derived after measuring at 5 equal intervals for the inner and outer diameters, and the gap width was measured at 3 points: upper, middle, and lower. The dimensional data for each point was measured three times, and the arithmetic mean was calculated. The final thermal strain was calculated by arithmetically averaging the deformation amounts at all points for the inner diameter, outer diameter, and gap width.

[0082] Classification Alloy Composition (Wt%) CSI Mn PS AlCrCuNiMoNbVTiBN Comparative Example 1 0.10 0.5 7 1.05 0.01 20.006 4 0.023 0.21 0.06 0.12 0.07 0.0042 Comparative Example 2 0.19 0.86 0.84 0.01 10.007 5 0.015 0.67 0.18 0.0057 Comparative Example 3 0.27 0.98 0.52 0.01 00.005 20.037 0.84 0.05 0.01 20.015 0.00220 .0069 Invention Example 10.120.421.160.0090.00480.531.050.040.0430.130.0230.00170.0061 Invention Example 20.180.630.620.0130.00320.651.280.030.0320.0240.00240.0053 Invention Example 30.260.720.310.0120.00540.821.470.0260.0210.00260.0042

[0083] Classification Ae1(°C) Ae3(°C) Ae3+50°C Ae3-30°C Formula (1) Formula (2) Comparative Example 1 7238719218410.230.19 Comparative Example 2 7448509008200.080.29 Comparative Example 3 7598408908100.140.37 Inventive Example 1 7579219718914.420.44 Inventive Example 2 7759269768963.610.54 Inventive Example 3 7939229728923.150.69

[0084] Classification Carburization Temperature (°C) Homogenization Temperature (°C) Surface Hardness (HRC) Fatigue Strength (kgf / mm²) 2 Fatigue Limit Ratio (%) Thermal Deformation Rate (%) Comparative Example 19 30 850 60.38 956 0.90 Comparative Example 29 30 850 61.19 26 0.02 Comparative Example 39 30 850 60.89 559 1.19 Inventive Example 19 40 850 61.59 359 0.44 Inventive Example 29 40 850 60.99 56 20.53 Inventive Example 39 40 850 61.39 86 40.62

[0085] As can be seen from Table 3, in the case of Inventive Examples 1 to 3, which satisfy the alloy composition and manufacturing conditions proposed in the present invention, not only do they satisfy all the conditions of Equations 1 and 2, but they also satisfy all the conditions proposed in the present invention regarding the temperature conditions for carburization and homogenization heat treatment. This indicates that excellent thermal deformation can be secured while maintaining normal fatigue strength after carburization. In the case of the Inventive Examples, it can be seen that thermal deformation is reduced by approximately 50% compared to the Comparative Examples. On the other hand, in the case of Comparative Examples 1 to 3, since they do not satisfy at least one of the conditions proposed in the present invention, it can be seen that the thermal deformation after carburization heat treatment is inferior compared to the Inventive Examples. Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope of the concept and scope of the claims described below.

Claims

1. In wt%, containing C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, and the remainder being Fe and unavoidable impurities, Carburized steel satisfying the following formulas (1) and (2). Equation (1): [Al] / [C] ≥ 2.0 Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8 (Here, [Al], [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element) 2. In Claim 1, The above carburized steel is a carburized steel having a thermal strain of less than 0.90%.

3. In Claim 1, The above carburized steel has a fatigue strength of 80 kgf / mm 2 Carburized steel having a fatigue limit ratio of 50% or more.

4. In Claim 1, The microstructure of the above carburized steel comprises a composite structure of ferrite and martensite.

5. In Claim 4, The above composite structure is a carburized steel containing 10% or more of ferrite in area fraction.

6. In Claim 1, Carburized steel with a surface hardness of 58 HRC or higher.

7. In Claim 1, Carburized steel having an average grain size of prior austenite of 30㎛ or less.

8. A step of preparing a steel material satisfying the following formulas (1) and (2), containing, in weight%, C: 0.01~0.30%, Si: 1.00% or less (excluding 0), Mn: 1.50% or less (excluding 0), Cr: 2.00% or less (excluding 0), P: 0.030% or less, S: 0.0500% or less, Ni: 0.3% or less, Mo: 0.30% or less, Cu: 0.30% or less, sol.Al: 0.500~1.000%, Nb: 0.100% or less, V: 0.30% or less, Ti: 0.050% or less, B: 0.0050% or less, N: 0.0200% or less, and the remainder being Fe and unavoidable impurities; A step of carburizing and diffusing the above steel in a temperature range of 880℃ to Ae3+50℃; and A step comprising cracking treatment of the above carburized and diffused steel at a temperature range of 820℃ to Ae3-30℃; Method for manufacturing carburized steel. Equation (1): [Al] / [C] ≥ 2.0 Equation (2): 0.4 ≤ [C]+(0.054×[Si])+(0.042×[Mn])+(0.015×[Cr])+(0.432×[Al]) ≤ 0.8 (Here, [Al], [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element) 9. In Claim 8, A method for manufacturing carburized steel comprising the step of quenching the above-mentioned cracked steel in oil at a temperature range of 60°C to 150°C.

10. In Claim 9, A method for manufacturing carburized steel comprising a tempering step at a temperature range of 150℃ to 200℃ after the above-mentioned quenching step.