STEEL AND METHOD FOR MANUFACTURING STEEL

Abstract

The present disclosure relates to a steel and a method for manufacturing the steel. Specifically, the present disclosure is characterized by providing a method for providing optimal alloy components that can reduce carburizing thermal deformation by reducing contents of Cr and Mo elements compared to the existing ones, and improving the physical property with fatigue resistance of a carburized steel by adjusting a rolling thermal treatment temperature, a carburizing thermal treatment condition, and the like to increase a fraction of a fine MX precipitate and improve a precipitation strengthening effect.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. § 119(a) the benefit of priority to Korean Patent Application No. 10-2021-0133564 filed on Oct. 8, 2021, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to a steel and a method for manufacturing the steel. Specifically, the present disclosure is characterized by providing a method for providing optimal alloy components that can reduce carburizing thermal deformation by reducing contents of Cr and Mo elements compared to the existing ones, and improving the physical property of fatigue resistance of a carburized steel by adjusting a rolling thermal treatment temperature, a carburizing thermal treatment condition, and the like to increase a fraction of a fine MX precipitate and improve a precipitation strengthening effect.

(b) Background Art

In general, a raw material of a carburized steel is manufactured by processes such as a steelmaking process of manufacturing a steel by removing impurities from molten iron, a casting process of solidifying liquid iron, a rolling process of manufacturing the solidified iron as a reinforcing bar, and thermal treatment. A process of manufacturing a vehicle gear made of the material, and the like is performed by processes such as material→forging→normalizing or annealing→machining (shaving & hobbing)→carburizing thermal treatment.

For a vehicle transmission gear, the physical property should be increased by fatigue resistance for the purpose of reducing the weight of a vehicle body for improving fuel efficiency. At the same time, carburizing thermal deformation caused during a process of manufacturing parts should be reduced to secure an assembly quality and improve vehicle quietness. However, an existing alloy design method for improving the physical property with the fatigue resistance by increasing hardenability improvement elements (Cr and Mo) causes a problem of increasing the amount of carburizing type thermal deformation.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and accordingly it may include information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

An object of the present disclosure is to provide a method for manufacturing a steel, which can reduce carburizing thermal deformation and improve the physical property with fatigue resistance.

The object of the present disclosure is not limited to the aforementioned object.

The object of the present disclosure should be more apparent by the following description and achieved by the means described in the claims and a combination thereof.

A steel according to an embodiment of the present disclosure includes, by weight percent (wt %), C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt % based on the entire composition of 100 wt %, and the residue(s) includes iron and other inevitably mixed impurities. The steel can satisfy Formula 1 below:

6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5  Formula 1

(where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean the amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively). A fraction of a precipitate can satisfy 0.025% to 0.045%.

A contact fatigue L10 life of the steel can satisfy 2,980,000 cycles or more, and at the same time, a bending fatigue life can satisfy 31,000 cycles or more.

A method for manufacturing a steel according to an embodiment of the present disclosure can include preparing a carburized steel including C, Si, Mn, Cr, Ni, Mo, Nb, and V; rolling the carburized steel at a temperature of 1180 to 1460° C.; carburizing and hardening the rolled carburized steel at a temperature of 850 to 920° C. for 150 to 300 minutes; and precipitating an MX precipitate including V and Nb at a fraction of 0.025 to 0.045%.

The carburized steel can include C: 0.15 to 0.25 wt %, Si: 0.5 to 0.9 wt %, Mn: 0.3 to 0.7 wt %, Cr: 1.3 to 1.7 wt %, Ni: 0.2 to 0.5 wt %, Mo: 0.1 to 0.4 wt %, Nb: 0.01 to 0.04 wt %, and V: 0.03 to 0.05 wt % based on the entire composition of 100 wt %. The residue(s) can include iron and other inevitably mixed impurities. The carburized steel can satisfy Formula 1 below,

6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5  Formula 1

(where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean the amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively).

According to the present disclosure, it is possible to provide the method for manufacturing the steel, which can reduce the carburizing thermal deformation and improve the physical property with the fatigue resistance.

The effect of the present disclosure is not limited to the aforementioned effect. The effect of the present disclosure should be understood as including all effects inferable from the following description.

It is understood that the term “automotive” or “vehicular” or other similar term as used herein is inclusive of motor automobiles or vehicles in general such as passenger automobiles including sports utility automotives/vehicles (operation SUV), buses, trucks, various commercial automotives/vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid automotives, electric automotives, plug-in hybrid electric automotives, hydrogen-powered automotives and other alternative fuel automotives or vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid automotive is an automotive that has two or more sources of power, for example both gasoline-powered and electric-powered automotives.

The above and other features of the disclosure are discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present disclosure are described in detail with reference to certain examples thereof illustrated in the accompanying drawings which are given herein below by way of illustration only, and thus are not limitative of the present disclosure, and wherein:

FIG. 1 shows measuring the amount of carburizing thermal deformation according to the present disclosure.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the disclosure. The specific design features of the present disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in section by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent sections of the present disclosure throughout the several figures of the drawing.

DETAILED DESCRIPTION

The above objects, other objects, features, and advantages of the present disclosure should be readily understood through the following embodiments related to the accompanying drawings. However, the present disclosure is not limited to the embodiments described herein and can also be specified in other forms. Rather, the embodiments described herein are provided so that the disclosed contents can be thorough and complete and the spirit of the present disclosure can be sufficiently conveyed to those having ordinary skill in the art.

In the present specification, it should be understood that the term “include” or “have” is intended to specify the presence of features, numbers, steps, operations, components, parts or combinations thereof described in the specification, and does not preclude the possibility of the presence or addition of one or more other features or numbers, steps, operations, components, parts or combinations thereof in advance. In addition, if an element such as a layer, a membrane, a region, a plate, and the like is said to be “on” another portion, this includes not only a case where it is “directly above” another portion, but also a case where it has other parts interposed therebetween. Conversely, if an element such as a layer, a membrane, a region, a plate, and the like is said to be “under” another portion, this includes not only a case where it is “directly under” another portion, but also a case where it has other portions interposed therebetween.

Unless otherwise specified, since all numbers, values, and/or expressions representing components, reaction conditions, polymer compositions, and an amount of mixtures used in the present specification are approximations reflecting various uncertainties of measurements that essentially occur in obtaining these values from the others, it should be understood that all cases are expressed by the term “about”. In addition, if the numerical range is disclosed in the present disclosure, this range is continuous, and includes all values from the minimum value to the maximum value in this range unless indicated otherwise. Furthermore, if this range refers to an integer, all integers including the minimum value to the maximum value are included unless otherwise indicated.

The present disclosure relates to a method for manufacturing a steel and a carburized steel manufactured by the manufacturing method.

Method for Manufacturing the Steel

The present disclosure is characterized by providing a method that can adjust a rolling thermal treatment temperature, a carburizing thermal treatment condition, and the like to increase a fraction of a fine MX precipitate and improve a precipitation strengthening effect, thereby improving the physical property with fatigue resistance of a carburized steel.

Specifically, the method for manufacturing the steel according to the present disclosure includes preparing a carburized steel including C, Si, Mn, Cr, Ni, Mo, Nb, and V, rolling the carburized steel, and carburizing and hardening the rolled carburized steel. The method may also include precipitating an MX precipitate.

Preparing the Carburized Steel

This is a step of preparing the carburized steel including C, Si, Mn, Cr, Ni, Mo, Nb, and V.

The carburized steel is, in one example, characterized by including C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt % based on the entire composition of 100 wt %. The carburized steel is also characterized in that the residues include iron and other inevitably mixed impurities.

Hereinafter, the reason of limiting the composition is described in detail.

Carbon (C) is an element essential to form an MX precipitate after carburizing thermal treatment and to improve hardenability to improve the strength. Therefore, the content of C is added in an amount of 0.17 wt % or more to increase the strength for improving the physical property of fatigue resistance after the carburizing thermal treatment. However, when the content of C is added in an amount of 0.23 wt % or more, a toughness is reduced. Therefore, the content of C is limited to a range of 0.17 wt % to 0.23 wt % to increase the strength of the carburizing thermal treatment material and secure the toughness.

Silicon (Si) is an element for strengthening a base tissue and increasing a tempering softening resistance through solid solution strengthening. Therefore, for the purpose of improving the physical property of the fatigue resistance and improving the softening resistance when a gear is operated, the content of Si is added in an amount of 0.65 wt % or more. However, when the content of Si is excessively added, there occurs a problem of interfering the diffusion of carbon and reducing hardness as an oxide is formed on a carburized surface during the atmospheric carburizing thermal treatment. Therefore, the content of Si is limited to a range of 0.6 to 0.8 wt % to improve the physical property with the fatigue resistance and to prevent the oxide from being formed on the surface during the atmospheric carburizing thermal treatment.

Manganese (Mn) is an element being useful for deoxidation in the process of manufacturing the steel and strengthening the base tissue by strengthening solid solution. The content of Mn needs to be added in an amount of 0.35 wt % or more. However, when Mn is excessively added, there occurs a problem of reducing processability as the hardness of the base increases. Therefore, the content of Mn is limited to a range of 0.35 to 0.65 wt % to strengthen the base tissue and prevent the problem of reducing processability.

Chromium (Cr) is an element for strengthening the base tissue according to the increase in hardenability upon quenching after the carburizing thermal treatment. The content of Cr is added in an amount of 1.35 wt % or more to improve the physical property with the fatigue resistance. However, as hardenability and thus thermal deformation increase when the content of Cr is excessively added, the content of Cr is limited to a range of 1.35 to 1.65 wt % to secure vehicle quietness and assembly stability.

Nickel (Ni) is an element for improving toughness of the base tissue of the steel and the content of Ni is added in an amount of 0.25 wt % or more to improve the physical property with the fatigue resistance. However, since the excessive addition of the Ni element increases a manufacturing cost, the content of Ni is limited to a range of 0.25 to 0.45 wt %.

Molybdenum (Mo) is an element for increasing hardenability similarly to the Cr element and is added in an amount of 0.15 wt % or more to secure the physical property with the fatigue resistance after the carburizing thermal treatment. However, when the Mo element is excessively added in an amount of a certain range or more, the effect of increasing the strength is insignificant. As the thermal deformation due to the increase in hardenability is increased, the content of Mo is limited to a range of 0.15 to 0.25 wt %.

Vanadium (V) is an element for forming a composite MX precipitate like the Nb element during the carburizing thermal treatment. The fine MX precipitate prevents precipitation strengthening and grain boundary coarsening of steel to improve the physical property with the fatigue resistance and make hardenability uniform, thereby preventing the thermal deformation from unevenly occurring. However, when the V element is excessively added in an amount of a certain range or more, the effect of increasing the precipitation strengthening is insignificant and therefore is not effective. Therefore, the content of V is limited to a range of 0.02 to 0.04 wt %.

Niobium (Nb) is an element of forming a composite MX precipitate like the V element and is added in an amount of 0.015 wt % or more to improve the physical property with the fatigue resistance and prevent the uneven thermal deformation. However, the increase in the Nb element increases the rolling solid solution temperature and the non-solid solution Nb element forms a coarse MX precipitate and therefore, the effect of improving the physical property with the fatigue resistance is insignificant. Therefore, the content of Nb is limited to a range of 0.015 to 0.035 wt % to form a fine MX precipitate by dissolving the Nb element as much as possible while rolling and thermally treating the raw material to form the fine MX precipitate.

Rolling Step

This is a step of thermally treating and rolling the carburized steel.

The present disclosure is characterized by dissolving Nb and V included in the carburized steel in the rolling step as much as possible to form a fine MX precipitate in the carburizing and hardening step. In other words, by dissolving the Nb and V elements in the rolling step, it is possible to increase the number of fine MX precipitates in the carburizing and hardening step.

The rolling may be performed at a temperature of 1180 to 1460° C.

Carburizing and Hardening Step

This is a step of carburizing and hardening the rolled carburized steel by carburizing and thermally treating it. More specifically, this is a step of manufacturing the steel by carburizing and thermally treating the thermally treated carburized steel in the rolling step.

The carburizing and thermally treating gives a diffusion temperature and a time in order for the solid solution element to form the fine MX precipitate as much as possible.

The carburizing and thermally treating can be performed at a temperature of 850 to 920° C., and the carburizing and thermally treating can be performed for 150 to 300 minutes.

After the carburizing and thermally treating, by maximizing the fraction of the fine MX precipitate, it is possible to improve the physical property with the fatigue resistance of the steel as the effect of strengthening the precipitation.

Steel

The present disclosure is characterized by providing the steel including the optimal alloy components capable of reducing the carburizing thermal deformation by reducing the contents of the Cr and Mo elements compared to the existing ones.

The steel manufactured by the method for manufacturing the steel according to the present disclosure is, in one example, characterized by including C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt % based on the entire composition of 100 wt %. The steel is also characterized in that the residues include iron and other inevitably mixed impurities.

The composition included in the steel satisfies Formula 1 below,

6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5  Formula 1

(where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively).

When the above value is smaller than 6.7, the life can be reduced by the fatigue resistance as the quenchability is reduced after the carburizing thermal treatment. When the above value is larger than 9.5, there can be a concern about the increase in a part noise and an assembly quality problem as the thermal deformation is increased after the carburizing thermal treatment.

Hereinafter, the present disclosure is described in more detail through specific examples. However, these examples are for illustrating the present disclosure, and the scope of the present disclosure is not limited thereto.

Manufacturing Example

Cylindrical test pieces [10 mm (circle diameter)×100 mm (height)] were manufactured to compare the amounts of the thermal deformation of the carburized hardened material performed under the carburizing thermal treatment condition after applying the rolling thermal treatment temperature conditions. The rolling thermal treatment was performed at 1200° C., and the carburizing thermal treatment performed an oil quenching after maintaining 920° C. for 200 minutes in the atmospheric carburizing thermal treatment furnace.

Measuring Method (Amount of the Thermal Deformation)

As shown in FIG. 1, the amount of the thermal deformation was measured by fixing the test piece to the jig before and after the carburizing thermal treatment to measure the out-of-roundness of the edge end thereof using a micrometer.

Examples 1 to 10

Test pieces of Examples 1 to 10 having compositions in Table 1 below were manufactured in the method of the manufacturing example.

	TABLE 1
	C	Si	Mn	Cr	Ni	Mo	Nb	V
Example 1	0.17	0.60	0.35	1.35	0.25	0.15	0.015	0.020
Example 2	0.23	0.80	0.65	1.65	0.45	0.25	0.035	0.040
Example 3	0.20	0.70	0.50	1.50	0.35	0.20	0.025	0.030
Example 4	0.17	0.80	0.65	1.35	0.25	0.20	0.030	0.025
Example 5	0.19	0.75	0.40	1.40	0.35	0.20	0.035	0.025
Example 6	0.18	0.60	0.65	1.35	0.45	0.15	0.030	0.020
Example 7	0.18	0.80	0.35	1.65	0.25	0.25	0.030	0.030
Example 8	0.19	0.76	0.42	1.49	0.38	0.22	0.030	0.040
Example 9	0.17	0.65	0.45	1.38	0.41	0.24	0.030	0.025
Example 10	0.21	0.78	0.63	1.62	0.40	0.23	0.030	0.025

Comparative Examples 1 to 18

Test pieces of Comparative examples 1 to 18 having composition in Tables 2 and 3 below were manufactured in the method of the manufacturing example.

	TABLE 2
	C	Si	Mn	Cr	Ni	Mo	Nb	V
Comparative	0.17	0.06	0.70	1.30	0.10	0.55	0.025	0
example 1
Comparative	0.20	0.63	0.57	2.4	0.10	0.35	0.025	0
example 2
Comparative	0.20	0.70	0.50	2.00	0.35	0.55	0.025	0
example 3
Comparative	0.20	0.70	0.50	1.50	0.35	0.20	0	0
example 4
Comparative	0.24	0.80	0.65	1.65	0.45	0.25	0.025	0
example 5
Comparative	0.16	0.60	0.35	1.35	0.25	0.15	0.025	0
example 6
Comparative	0.23	0.85	0.65	1.65	0.45	0.25	0.025	0
example 7
Comparative	0.17	0.55	0.35	1.35	0.25	0.15	0.025	0
example 8
Comparative	0.23	0.80	0.70	1.65	0.45	0.25	0.025	0
example 9
Comparative	0.17	0.60	0.30	1.35	0.25	0.15	0.025	0
example 10

	TABLE 3
	C	Si	Mn	Cr	Ni	Mo	Nb	V
Comparative	0.23	0.80	0.65	1.70	0.45	0.25	0.025	0
example 11
Comparative	0.17	0.60	0.35	1.30	0.25	0.15	0.025	0
example 12
Comparative	0.23	0.80	0.65	1.65	0.50	0.25	0.025	0
example 13
Comparative	0.17	0.60	0.35	1.35	0.20	0.15	0.025	0
example 14
Comparative	0.23	0.80	0.65	1.65	0.45	0.30	0.025	0
example 15
Comparative	0.17	0.60	0.35	1.35	0.25	0.10	0.025	0
example 16
Comparative	0.20	0.70	0.50	1.50	0.20	0.20	0.025	0.010
example 17
Comparative	0.20	0.70	0.50	1.50	0.35	0.20	0.010	0.030
example 18

Comparative Example 19

A test piece was manufactured in the same method as that of Example 3 except that the rolling thermal treatment was performed at 1100° C.

Comparative Example 20

A test piece was manufactured in the same method as that of Example 3 except that the rolling thermal treatment was performed at 1150° C.

Comparative Example 21

A test piece was manufactured in the same method as that of Example 3 except that the carburizing thermal treatment was performed at 830° C.

Experimental Example

The test pieces manufactured in Examples and Comparative examples were inspected for the amount of the thermal deformation, the fraction of the precipitate, the contact fatigue L10 life, the bending fatigue, and whether to satisfy Formula 1. The results thereof are expressed in Table 4 below.

	TABLE 4
	Amount of	Fraction	Contact		Whether
	thermal	of	fatigue	Bending	to
	deformation	precipitate	L10 life	fatigue	satisfy
	(um)	(%)	(cycles)	(cycles)	Formula 1
Example 1	13	0.025	2,980,471	31,071	◯
Example 2	107	0.041	3,104,155	40,050	◯
Example 3	60	0.036	3,097,460	35,888	◯
Example 4	59	0.034	3,041,052	35,538	◯
Example 5	49	0.033	3,019,905	34,502	◯
Example 6	42	0.030	3,008,106	33,501	◯
Example 7	71	0.037	3,101,077	37,580	◯
Example 8	61	0.043	3,110,920	36,108	◯
Example 9	47	0.034	3,061,120	34,390	◯
Example 10	94	0.032	3,010,012	39,401	◯
Comparative	49	0.020	2,832,015	34,542	◯
example 1
Comparative	116	0.017	2,802,938	40,278	◯
example 2
Comparative	158	0.021	2,849,199	41,053	X
example 3
Comparative	62	0	2,016,903	31,807	◯
example 4
Comparative	112	0.014	2,401,801	40,091	X
example 5
Comparative	13	0.018	2,819,201	30,992	X
example 6
Comparative	114	0.015	2,539,630	40,208	X
example 7
Comparative	10	0.018	2,814,382	30,651	X
example 8
Comparative	113	0.015	2,653,017	40,130	X
example 9
Comparative	11	0.018	2,810,153	30,902	X
example 10
Comparative	113	0.015	2,710,823	40,149	X
example 11
Comparative	11	0.017	2,807,109	30,722	X
example 12
Comparative	112	0.015	2,558,941	40,108	X
example 13
Comparative	12	0.018	2,812,109	30,945	X
example 14
Comparative	118	0.015	2,691,105	40,458	X
example 15
Comparative	6	0.018	2,810,710	30,260	X
example 16
Comparative	56	0.022	2,854,372	34,988	◯
example 17
Comparative	60	0.022	2,853,191	36,003	◯
example 18
Formula 1*
6.7 ≤ 5[C] + 2[Si] + 2[Mn] + 2[Cr] + 2[Ni] + 5[Mo] ≤ 9.5 (where [C], [Si], [Mn], [Cr], [Ni], and [Mo] mean the amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively.)

Experimental Example (Thermal Treatment Temperature)

The test pieces manufactured in Example 3 and Comparative examples 19 to 21 were inspected for the fraction of the precipitate and the results thereof are expressed in Table 5 below.

TABLE 5
Fraction of precipitate (%)
Example 3              0.036
Comparative example 19 0.018
Comparative example 20 0.026
Comparative example 21 0.012

Claims

1. A steel comprising:

C: 0.17 to 0.23 wt %, Si: 0.6 to 0.8 wt %, Mn: 0.35 to 0.65 wt %, Cr: 1.35 to 1.65 wt %, Ni: 0.25 to 0.45 wt %, Mo: 0.15 to 0.25 wt %, Nb: 0.015 to 0.035 wt %, and V: 0.02 to 0.04 wt %; and

residues comprising iron and other inevitably mixed impurities,

wherein the steel satisfies the following formula: 6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5

wherein [C], [Si], [Mn], [Cr], [Ni], and [Mo] are amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively, and

wherein a fraction of a precipitate satisfies 0.025% to 0.045%.

2. The steel of claim 1,

wherein a contact fatigue L10 life of the steel satisfies 2,980,000 cycles or more and a bending fatigue life satisfies 31,000 cycles or more.

3. A method for manufacturing a steel, the method comprising:

preparing a carburized steel comprising C, Si, Mn, Cr, Ni, Mo, Nb, and V;

rolling the carburized steel at a temperature of 1180 to 1460° C.;

carburizing and hardening the rolled carburized steel at a temperature of 850 to 920° C. for 150 to 300 minutes; and

precipitating an MX precipitate comprising V and Nb at a fraction of 0.025 to 0.045%.

4. The method of claim 3,

wherein the carburized steel comprises: C: 0.15 to 0.25 wt %. Si: 0.5 to 0.9 wt %, Mn: 0.3 to 0.7 wt %, Cr: 1.3 to 1.7 wt %, Ni: 0.2 to 0.5 wt %, Mo: 0.1 to 0.4 wt %, Nb: 0.01 to 0.04 wt %, and V: 0.03 to 0.05 wt %; and residues including iron and other inevitably mixed impurities, and

wherein the carburized steel satisfies the following formula: 6.7≤5[C]+2[Si]+2[Mn]+2[Cr]+2[Ni]+5[Mo]≤9.5

wherein [C], [Si], [Mn], [Cr], [Ni], and [Mo] are amounts of addition (wt %) of C, Si, Mn, Cr, Ni, and Mo, respectively.