Carburized steel and manufacturing method thereof

Abstract

A carburized steel according to an embodiment of the present invention contains, in wt%, 0.08-0.20% of C, 0.80-1.80% of Si, 0.50% or less (excluding 0) of Mn, 0.90-1.80% of Cr, 0.030% or less of P, 0.0300% or less of S, 0.070% or less (excluding 0) of sol. Al, 0.06% or less of Nb, 0.20% or less of V, 0.005-0.050% of Ti, 0.0005-0.0050% of B, and 0.0100% or less (excluding 0) of N, with the remainder comprising Fe and inevitable impurities, and satisfies Expressions (1) and (2). Expression (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55 Expression (2): [Si] / [C] ≥ 2.9 (where, [C], [Si], [Mn], and [Cr] refer to the content (%) of the respective elements)

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 comprises, in weight%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), the remainder being Fe and unavoidable impurities, and

[0006] The following equations (1) and (2) are satisfied.

[0007] Equation (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55

[0008] Equation (2): [Si] / [C] ≥ 2.9

[0009] (Here, [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element)

[0010] In addition, the carburized steel according to one embodiment of the present invention can satisfy the following formula (3).

[0011] Equation (3): [Cr] / [Mn] ≥ 2.0

[0012] (Here, [Mn] and [Cr] represent the content (%) of the respective elements)

[0013] In addition, the carburized steel according to one embodiment of the present invention can satisfy the following formula (4).

[0014] Equation (4): 0.35<[C]+(0.224×[Si])+(0.056×[Mn])+(0.015×[Cr])+(0.065×[Al])<0.55 (where [C], [Si], [Mn], [Cr], and [Al] represent the content (%) of the respective element)

[0015] In addition, the carburized steel according to one embodiment of the present invention may have a thermal strain of 0.90% or less.

[0016] 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.

[0017] In addition, the carburized steel according to one embodiment of the present invention may have a microstructure of ferrite and martensite.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] A method for manufacturing carburized steel according to another embodiment of the present invention comprises the steps of: preparing a steel material satisfying the following formulas (1) and (2), comprising, in weight%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), and the remainder being Fe and unavoidable impurities; and the steps of carburizing and diffusing the steel material in a temperature range of 880℃ to Ae3+70℃. and the step of cracking the above carburized and diffused steel at a temperature range of 830℃ to Ae3-30℃; is included.

[0022] Equation (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55

[0023] Equation (2): [Si] / [C] ≥ 2.9

[0024] In addition, in the method for manufacturing carburized steel according to one embodiment of the present invention, the steel material may satisfy the following formula (3).

[0025] Equation (3): [Cr] / [Mn] ≥ 2.0

[0026] In addition, a method for manufacturing carburized steel according to one embodiment of the present invention can satisfy the following formula (4).

[0027] Equation (4): 0.35<[C]+(0.224×[Si])+(0.056×[Mn])+(0.015×[Cr])+(0.065×[Al])<0.55

[0028] In addition, a method for manufacturing carburized steel according to one embodiment of the present invention may include the step of quenching the cracked steel in oil at a temperature range of 60°C to 150°C.

[0029] 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.

[0030] According to the present invention, a carburized steel having excellent durability and minimizing thermal deformation occurring during carburization heat treatment can be provided.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0036] The inventors examined various aspects to provide steel with minimized thermal deformation after carburization heat treatment. As a result, they discovered that by optimizing the alloy composition and heat treatment method to secure a ferrite and martensite composite structure in the microstructure of the hardened layer after carburization heat treatment, and by appropriately controlling the fraction of ferrite, it is possible to provide carburized steel capable of minimizing thermal deformation after Q / T heat treatment while ensuring durability equivalent to that of conventional steel, thereby completing the present invention.

[0037] First, a carburized steel according to one aspect of the present invention will be described.

[0038] A carburized steel according to one embodiment of the present invention comprises, in weight%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), the remainder being Fe and unavoidable impurities.

[0039] Hereinafter, the reason for the numerical limitation of the alloy component content in the embodiments of the present invention will be explained.

[0040] C: 0.08~0.20%

[0041] Carbon plays a role in improving the strength of the wire rod. To exhibit this effect, it is desirable to include at least 0.08%. However, if the content is excessive, the deformation resistance of the steel increases rapidly, and this leads to a problem where cold workability deteriorates. Therefore, it is desirable that the upper limit of the carbon content be 0.20%. More preferably, the upper limit of the carbon content may be 0.19%, and most preferably, the upper limit of the carbon content may be 0.18%.

[0042] Si: 0.80~1.80%

[0043] Silicon is a ferrite-stabilizing element, and when added in large quantities, it has the effect of raising the proeutectoid ferrite formation temperature (Ae3). Therefore, in order to secure an ideal temperature range during carburization heat treatment, a content capable of raising the Ae3 temperature to 900°C or higher is required. However, if the content is excessive, the deformation resistance of the steel increases rapidly due to solid solution strengthening, which leads to a problem of deterioration in cold workability. Therefore, it is preferable that the upper limit of the silicon content be 1.80%, and more preferable that it be 1.50%.

[0044] Mn: 0.50% or less (excluding 0)

[0045] If the manganese content is excessive, the strength of the steel itself becomes excessively high, causing a rapid increase in the steel's deformation resistance, which in turn leads to a problem of deterioration in cold workability. Therefore, it is preferable that the upper limit of the manganese content be 0.50%, and more preferable that it be 0.40%.

[0046] Cr: 0.90~1.80%

[0047] Chromium plays a role in promoting ferrite and pearlite transformations during hot rolling. Furthermore, without increasing the inherent strength of the steel beyond what is necessary, it precipitates carbides within the steel to reduce the dissolved carbon content, thereby significantly contributing to the reduction of dynamic strain aging caused by dissolved carbon. To exhibit these effects, it is desirable to include at least 0.90% chromium. On the other hand, if the content is excessive, the inherent strength of the steel becomes excessively high, causing a sharp increase in the steel's deformation resistance, which leads to a problem of deterioration in cold workability. The above chromium content is preferably 1.80% or less, and more preferably 1.60% or less.

[0048] P: 0.030% or less

[0049] 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 to be 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 to 0.03%.

[0050] S: 0.0300% or less

[0051] 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 0.0300%.

[0052] sol.Al: 0.070% or less (excluding 0)

[0053] If the aluminum content exceeds 0.070%, the effect of austenite grain size refinement due to AlN formation becomes greater, thereby reducing cold workability. Therefore, in the present invention, the upper limit of the usable aluminum content is managed at 0.070%. More preferably, the upper limit may be 0.060%, and most preferably, the upper limit may be 0.050%.

[0054] Niobium (Nb): 0.06 wt% or less

[0055] Niobium (Nb) 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 0.06%, there is a problem of forming coarse precipitates. Therefore, the preferred upper limit of the Nb content is 0.06%, more preferably 0.055%, and even more preferably 0.05%.

[0056] Vanadium (V): 0.20 wt% or less

[0057] Vanadium (V), like niobium (Nb), is an element that forms carbides and carbonitrides, thereby limiting grain boundary movement 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 0.20%, there is a problem of forming coarse precipitates. Therefore, it is desirable to limit the content to 0.20% or less. More preferably, it may be 0.19% or less, and most preferably, 0.18% or less.

[0058] Ti: 0.005~0.050%

[0059] Ti combines with nitrogen in steel to form titanium nitride (TiN). This nitride is very stable at high temperatures and forms at 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, resulting in the softening of the steel. Furthermore, the formation of the TiN nitride contributes to a reduction in dissolved nitrogen in the steel, thereby enabling the securing of dissolved boron. If the Ti content is less than 0.005 wt%, these effects cannot be achieved, and if it exceeds 0.050 wt%, coarse titanium nitride precipitates excessively, reducing toughness. Therefore, it is desirable to limit the Ti content to 0.005 to 0.050 wt%. More preferably, it may be 0.005 to 0.045%, and most preferably, 0.005 to 0.040%.

[0060] B: 0.0005~0.0050%

[0061] B is a grain boundary strengthening element for improving hardenability and delayed fracture resistance. When the content of B is less than 0.0005 wt%, the grain boundary strengthening effect or hardenability improvement effect due to grain boundary segregation of boron atoms during heat treatment is insufficient, and when it exceeds 0.0050 wt%, boron carbides precipitate at the grain boundaries, causing a decrease in grain boundary strength. Therefore, it is desirable to limit the content of B to 0.0005 to 0.0050 wt%. More preferably, it can be limited to 0.0005 to 0.0045%, and most preferably, to 0.0005 to 0.0040%.

[0062] N: 0.0100% or less (excluding 0)

[0063] 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 in turn 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 0.01%, more preferable to manage it to 0.0080%, and even more preferable to manage it to 0.0070%.

[0064] The remainder other than the above alloy composition is iron (Fe). In addition, the carburized steel of the present invention may contain other impurities that may be included in the industrial production process of ordinary steel. Since these impurities are known to anyone with ordinary knowledge in the technical field to which the present invention belongs, the present invention does not specifically limit their types and content.

[0065] In addition, the carburized steel according to one embodiment of the present invention satisfies the following formula (1).

[0066] Equation (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55

[0067] (Here, [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element)

[0068] Equation (1) is a relational expression representing the carbon equivalent (Ceq) of carburized steel, which may be 0.35 or higher and 0.55 or lower. If the carbon equivalent (Ceq) is less than 0.35, the strength is low and durability is inferior; if it exceeds 0.55, it may be difficult to secure the target strength. More preferably, it is 0.36 to 0.54. Most preferably, it is 0.37 to 0.53. That is, as the carburized steel according to the present invention satisfies the above Equation (1), the fatigue strength is 80 kgf / mm 2The above is the case, and the fatigue limit ratio may be 50% or more. Here, the fatigue limit ratio means (fatigue strength / tensile strength) × 100 (%). 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] In addition, the carburized steel according to one embodiment of the present invention can satisfy the following formula (2).

[0070] Equation (2): [Si] / [C] ≥ 2.9

[0071] Since Si is a ferrite-stabilizing element and has a significant solid solution strengthening effect, it is an essential element for securing ferrite and martensite structures through phase region heat treatment. Conversely, since C is an austenite-stabilizing element and has a very large solid solution strengthening effect, it is essential for securing strength; however, as its content increases, it becomes difficult to secure the phase region proposed in this invention. Therefore, to secure the strength and phase region temperature range of the carburized steel proposed in this invention, the ratio of [Si] / [C] is preferably 2.9 or higher. More preferably, it is 3.0 or higher. Most preferably, it is 3.1 or higher.

[0072] 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.

[0073] In addition, the carburized steel according to one embodiment of the present invention can satisfy the following formula (3).

[0074] Equation (3): [Cr] / [Mn] ≥ 2.0

[0075] Since Mn is an austenite-stabilizing element and has a very large solid solution strengthening effect, low-temperature structures may occur in segregated regions, and excessive strength increase may occur during cold working. To solve these problems, conversely, if Cr is added instead of Mn, Cr is a ferrite-stabilizing element and has a very small solid solution strengthening effect compared to Mn; therefore, the possibility of low-temperature structures occurring in segregated regions is reduced, and the work hardening rate is lowered during cold working, thereby suppressing the increase in strength. Accordingly, to satisfy the strength and hardenability of the carburized steel proposed in this invention, the ratio of [Cr] / [Mn] is preferably 2.0 or higher. More preferably, it is 2.1 or higher. Most preferably, it is 2.2 or higher.

[0076] Equation (4): 0.35<[C]+(0.224×[Si])+(0.056×[Mn])+(0.015×[Cr])+(0.065×[Al])<0.55

[0077] Equation (4) is a hardness index of steel. When the C content is low, the microstructure of the steel before forging has a significantly higher ferrite fraction than conventional steel (C content of about 0.20%) used for parts that undergo carburization treatment. In this case, since the hardness of the steel is greatly affected not only by the C content (pearite fraction) but also by the hardness of the ferrite, the contribution of each alloying element to the solid solution strengthening amount of ferrite was examined to define Equation (4). If the value is 0.35 or less, the hardness of the steel before forging increases, and the limiting reduction rate decreases. On the other hand, if it is 0.55 or more, the hardness as a carburized part is insufficient. Therefore, the value of Equation (4) is preferably greater than 0.35 and less than 0.55. More preferably, it is greater than 0.355 and less than 0.545. Most preferably, it is greater than 0.360 and less than 0.540. That is, by satisfying equation (4), the surface hardness can be 58 HRC or higher, and the core hardness can be 50 HRC or lower.

[0078] Here, the core refers to the base material, the carburized layer formed on the base material is referred to as the surface, and the hardness of the core and the surface hardness are measured, respectively.

[0079] Hereinafter, a method for manufacturing carburized steel according to another aspect of the present invention will be described.

[0080] A method for manufacturing carburized steel according to another embodiment of the present invention comprises the steps of: preparing a steel material satisfying the following formulas (1) and (2), comprising, in weight%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), and the remainder being Fe and unavoidable impurities; and the steps of carburizing and diffusing the steel material in a temperature range of 880℃ to Ae3+70℃. and the step of cracking the above carburized and diffused steel at a temperature range of 830℃ to Ae3-30℃; is included.

[0081] The above composition and formulas are as described above.

[0082] The temperature during the carburization treatment may be 880°C to Ae3+70°C. Since carbides may precipitate if the temperature is below 880°C and coarse grains may form if the temperature exceeds Ae3+70°C, the carburization treatment temperature is controlled to 880°C to Ae3+70°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.

[0083] Next, crack the carbon-diffusing steel.

[0084] The temperature during the cracking stage (homogenization heat treatment) of the carbon-diffusing steel is controlled to 830°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 830°C and severe thermal deformation may occur if it exceeds Ae3-30°C, the cracking treatment temperature is controlled to 830°C to Ae3-30°C.

[0085] Next, the cracked 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.

[0086] 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.

[0087] 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.

[0088] (Example)

[0089] 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.

[0090] Surface hardness was measured using the Rockwell C scale on the area where the carburized layer was formed (surface) and on the base material where the carburized layer was not formed (core) using the Rockwell C scale. Hardness was evaluated by measuring at 10 random points on the surface and core and calculating the average value.

[0091] 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.

[0092] Classification Alloy Composition (Wet%) CSI Mn PS Al Cr TiBN Mo Nb V Comparative Example 1 0.1 20.9 6 1.0 5 0.0 1 30.0 5 20.0 230.6 7 0.0 220.0 15 0.0 5 6 0.0 11 Comparative Example 2 0.1 80.2 40.8 40.0 1 20.0 46 0.0 1 51.0 5 0.0 48 0.22 Comparative Example 3 0.2 30.4 5 0.7 30.0 1 10.0 5 0 0.0 27 1.6 7 0.0 62 Comparative Example 4 0.2 5 0.7 20.9 20.0 1 10.0 48 0.0370.530.00510.160.05 Invention Example 10.111.170.450.0100.00630.0311.640.0210.00160.00440.0480.12 Invention Example 20.141.350.360.0090.00450.0211.320.0280.00230.00380.05 Invention Example 30.171.460.240.0130.00510.0281.260.0270.00240.00520.029

[0093] Classification Ae1(℃) Ae3(℃) Ae3+70℃ Ae3-30℃ Formula (1) Formula (2) Formula (3) Formula (4) Comparative Example 1 7388709408400.498.000.640.41 Comparative Example 2 7408258957950.461.331.250.30 Comparative Example 3 7568148847840.571.962.29 0.40 Comparative Example 47458319018010.562.880.580.47 Invention Example 17849099798790.4710.643.640.42 Invention Example 27829049748740.479.643.670.48 Invention Example 37819049748740.498.595.250.53 Here, Ceq=[C]+[Si] / 9+[Mn] / 5+[Cr] / 12, where [C], [Si], [Mn], and [Cr] each represent the content (weight%) of the corresponding element.

[0094] Classification J5(mm) J11(mm) Carburization Temperature(°C) Homogenization Temperature(°C) Surface Hardness(HRC) Core Hardness(HRC) Fatigue Strength(kgf / mm²) 2 Fatigue Limit Ratio (%) Thermal Deformation Rate (%) Comparative Example 1 38 359 208 506 0.3388 757 0.98 Comparative Example 2 4 0 329 208 506 0.2449 46 21.06 Comparative Example 3 4 4 389 208 506 1.3479 56 11.12 Comparative Example 4 4 ​​5 359 208 506 1.5489 66 01.25 Inventive Example 1 39 379 508 506 1.8379 59 0.45 Inventive Example 2 4 1 419 508 506 0.4429 66 30.56 Inventive Example 3 4 34 29508 506 1.2469 86 500.65

[0095] The above J5 refers to the hardness measured at a depth of 5 mm from the quenching end. In the steel quenchability test method specified in JIS G0561, if J5 (mm) is less than 30 HRC, the core hardness of the carburized part is insufficient, and the desired strength characteristics cannot be obtained. Therefore, J5 is defined as a predetermined depth position, and the lower limit of hardness at that depth position is specified. J11 is of the same intent, except that the depth is 11 mm from the quenching end. Looking at Table 3 above, Comparative Examples 1 to 4 are cases where one or more of the composition and Equations (1) to (4) do not satisfy the scope of the present invention. In particular, looking at Comparative Example 3, it can be confirmed that Equations (1) and (2) do not satisfy the scope of the present invention, and thus the thermal deformation after carburization heat treatment appears inferior.

[0096] On the other hand, in the case of Inventive Examples 1 to 3, which satisfy the alloy composition and manufacturing conditions of the present invention, not only are all the conditions of Equations (1) to (4) satisfied, but the carburization and homogenization heat treatment temperature conditions also satisfy the range of the present invention, so it can be seen that excellent thermal deformation can be secured while securing normal fatigue strength after carburization. That is, in the case of the Inventive Examples, it can be seen that thermal deformation is reduced by about 50% compared to the Comparative Examples.

[0097] 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 and concept of the claims set forth below.

Claims

1. In wt%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), the remainder being Fe and unavoidable impurities, and Carburized steel satisfying the following formulas (1) and (2). Equation (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55 Equation (2): [Si] / [C] ≥ 2.9 (Here, [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element) 2. In Claim 1, Carburized steel satisfying the following formula (3). Equation (3): [Cr] / [Mn] ≥ 2.0 (Here, [Mn] and [Cr] represent the content (%) of the respective elements) 3. In Claim 1, Carburized steel satisfying the following formula (4). Equation (4): 0.35<[C]+(0.224×[Si])+(0.056×[Mn])+(0.015×[Cr])+(0.065×[Al])<0.55 (Here, [C], [Si], [Mn], [Cr], and [Al] represent the content (%) of the respective element.) 4. In Claim 1, The above carburized steel is a carburized steel having a thermal strain of 0.90% or less.

5. 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.

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

7. In Claim 6, The above composite structure is a carburized steel containing 10% or more of ferrite in area fraction.

8. In Claim 1, The above carburized steel is a carburized steel having a surface hardness of 58 HRC or higher and a core hardness of 50 HRC or lower.

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

10. A step of preparing a steel material comprising, in wt%, C: 0.08~0.20%, Si: 0.80~1.80%, Mn: 0.50% or less (excluding 0), Cr: 0.90~1.80%, P: 0.030% or less, S: 0.0300% or less, sol.Al: 0.070% or less (excluding 0), Nb: 0.06% or less, V: 0.20% or less, Ti: 0.005~0.050%, B: 0.0005~0.0050%, N: 0.0100% or less (excluding 0), the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2); A step of carburizing and diffusing the above steel in a temperature range of 880℃ to Ae3+70℃; and A step comprising cracking the above carburized and diffused steel at a temperature range of 830℃ to Ae3-30℃; Method for manufacturing carburized steel. Equation (1): 0.35 ≤ [C] + [Si] / 9 + [Mn] / 5 + [Cr] / 12 ≤ 0.55 Equation (2): [Si] / [C] ≥ 2.9 (Here, [C], [Si], [Mn], and [Cr] represent the content (%) of the respective element) 11. In Claim 10, The above steel is a method for manufacturing carburized steel that satisfies the following formula (3). Equation (3): [Cr] / [Mn] ≥ 2.0 (Here, [Mn] and [Cr] represent the content (%) of the respective elements) 12. In Claim 10, A method for manufacturing carburized steel satisfying the following formula (4). Equation (4): 0.35<[C]+(0.224×[Si])+(0.056×[Mn])+(0.015×[Cr])+(0.065×[Al])<0.55 (Here, [C], [Si], [Mn], [Cr], and [Al] represent the content (%) of the respective element.) 13. In Claim 10, 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.

14. In Claim 13, A method for manufacturing carburized steel comprising a tempering step at a temperature range of 150℃ to 200℃ after the above-mentioned quenching step.