HIGH STRENGTH CARBURIZING STEEL HAVING IMPROVED DURABILITY

Abstract

A carburizing steel includes, based on a total wt % of the carburizing steel: 0.1 wt % or more and 0.3 wt % or less of C (carbon); 2.0 wt % or more and 2.7 wt % or less of Cr (chrome); 0.4 wt % or more of Si (silicon); 0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum); less than 0.7 wt % of Mn (manganese); and 0.6 wt % or more and 1.5 wt % or less of Ni (nickel).

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to the benefit of Korean Patent Application No. 10-2020-0123021, filed on Sep. 23, 2020 in the Korean intellectual Property Office, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a carburizing steel, and particularly, to a high strength carburizing steel for a vehicle part having improved durability.

BACKGROUND

Generally, gears of a transmission of a vehicle are parts serving to directly transfer engine power to a differential system, and efficiently transfer rotation or power between two or more shafts such that the engine power is attuned to a traveling state of a vehicle, and simultaneously receive bending stress, contact stress, and the like. In the gears, if the durability of a material is insufficient, fatigue breakage (tooth breakage) due to lack of bending fatigue strength and fatigue damage (pitting) due to lack of contact fatigue strength frequently occur. Therefore, in the gears, physical properties such as high hardness, strength, toughness, fatigue strength, and fatigue life are required.

The durability of the carburizing steel for a vehicle part used as an alternative thereof has been improved by 1) increasing a high hardness carbide fraction through an increase in Cr, and 2) increasing matrix strength and softening resistance through an increase in Si.

However, when the contents of Cr/Si are increased, the carbide and the timing point at which pitting causing surface damage by hardness increment effect starts are improved, but due to the matrix toughness decrease effect by Cr/Si, crack propagation and bending strength tend to clearly decrease rather.

The contents described in Description of Related Art are to help the understanding of the background of the present disclosure, and may include what is not previously known to those skilled in the art to which the present disclosure pertains.

SUMMARY OF THE DISCLOSURE

The present disclosure is intended to solve the above problem, and an object of the present disclosure is to provide a carburizing steel capable of simultaneously securing bending strength and pitting resistance strength.

A high strength carburizing steel according to one aspect of the present disclosure comprises. based on a total wt % of the carburizing steel: 0.1 wt % or more and 0.3 wt % or less of C (carbon); 2.0 wt % or more and 2.7 wt % or less of Cr (chrome); 0.4 wt % or more and 0.7 wt % or less of Si (silicon); 0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum); less than 0.7 wt % of Mn (manganese); and 0.6 wt % or more and 1.5 wt % or less of Ni (nickel).

The content of the Si (silicon) may be 0.6 wt % or more and 0.7 wt % or less.

The content of the Ni (nickel) may be 0.6 wt % or more and 1.0 wt % or less.

The content of the Mn (manganese) may be 0.1 wt % or more and 0.5 wt % or less.

The carburizing steel may further contain: one or more of Ti (titanium), V (vanadium), and Nb (niobium).

The sum of the contents of the Ti (titanium), the V (vanadium), and the Nb (niobium) may be 1.0 wt % or less with respect to the total wt %.

The carburizing steel may further contain: 1 to 30 ppm of B (boron).

In the carburizing steel according to the present disclosure, a value of the following formula may be 0.2 or more,

[Ni]+0.3[Mn])/([Cr]+3[Si],

where [Ni], [Mn], [Cr], [Si] mean the wt % of Ni, Mn, Cr, Si, respectively.

A carburizing steel according to another aspect of the present disclosure comprises, based on a total wt % of the carburizing steel: 0.1 wt % or more and 0.3 wt % or less of C (carbon); 2.0 wt % or more and 2.7 wt % or less of Cr (chrome); 0.4 wt % or more and 0.7 wt % or less of Si (silicon); 0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum); less than 0.7 wt % of Mn (manganese); 0.6 wt % or more and 1.5 wt % or less of Ni (nickel); 1.0 wt % or less of the sum of Ti (titanium), V (vanadium), and Nb (niobium); and 1 to 30 ppm of B (boron). A value of the following formula satisfies 0.2 or more, [Ni]+0.3[Mn])/([Cr]+3[Si], where [Ni], [Mn], [Cr], [Si] mean the wt % of Ni, Mn, Cr, Si, respectively.

The content of the Si (silicon) may be 0.6 wt % or more and 0.7 wt % or less.

The content of the Ni (nickel) may be 0.6 wt % or more and 1.0 wt % or less.

The content of the Mn (manganese) may be 0.1 wt % or more and 0.5 wt % or less.

The carburizing hardness is 780 Hv or more, and the bending strength may be 3,000 MPa or more.

The high strength carburizing steel according to the present disclosure may increase the bending strength by 10% or more and the carburizing layer hardness by 20 Hv or more compared to the conventional steel, thereby improving durability.

The conventional high durability carburizing steel generally increases the contents of Cr—Si to improve strength and durability, and in this case, tends to entirely embrittle the material as (Cr, Fe)₇C₃-based carbide having the toughness decrease effect of Si and strong brittleness is produced. The present disclosure optimizes the contents of Ni/Mn even in the relatively low range of Cr/Si based on the ratio of (Ni+0.3Mn)/(Cr+3Si) to secure the toughness of the matrix, thereby suppressing the propagation of the fatigue crack to simultaneously secure strength and toughness.

Further, it is possible to significantly improve the carburizing property by decreasing the contents of Cr and Si, thereby implementing the carburizing heat treatment by only the gas carburizing without applying the pre-heat treatment, such that the cost such as the heat processing cost is saved.

Further, it is possible to control the contents of Ni and Mn with the low content of Cr rather than the high hardness Cr carbide like the conventional steels, thereby securing the toughness of the matrix and improving strength and hardness, and particularly, to suppress the propagation of the fatigue crack, thereby significantly improving durability.

Therefore, it is possible to improve the fatigue life to decrease the surface pitting caused by the fatigue breakage, thereby suppressing the increase in the transmission noise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates hardness, Fe₃C, high-temperature (Cr, Fe)₇C₃ carbide distribution according to the content of Cr.

FIG. 2 illustrates impact toughness, Fe₃C, high-temperature (Cr, Fe)₇C₃ carbide distribution according to the content of Si.

FIG. 3 illustrates a blocky residual austenite distribution according to the content of Ni, and FIGS. 4A and 4B are the tissue pictures illustrating the blocky residual austenite distribution.

FIG. 5 illustrates Fe₃C and high-temperature (Mo, Ni, Fe) composite carbide distribution according to the content of Mo.

FIGS. 6A to 6F illustrate pitting generation amount evaluation result images under severe conditions.

FIGS. 7A to 7C illustrate pitting generation amount evaluation result images under flat conditions.

FIG. 8A illustrates a microstructure according to Comparative Example 8, FIG. 8B illustrates a microstructure according to Example 1, and FIG. 8C illustrates a microstructure according to Example 8.

DESCRIPTION OF SPECIFIC EMBODIMENTS

To fully understand the present disclosure, operational advantages of the present disclosure, and the object achieved by the practice of the present disclosure, reference should be made to the accompanying drawings illustrating exemplary embodiment of the present disclosure and the contents illustrated in the accompanying drawings.

In describing the exemplary embodiment of the present disclosure, the description of the known technology or repetitive description capable of unnecessarily obscuring the subject matter of the present disclosure will be reduced or omitted.

An object of the present disclosure is to provide a carburizing steel capable of simultaneously securing bending strength and pitting resistance strength beyond the conventional limits, as a carburizing steel for a vehicle part manufactured by subjecting a steel to carburizing heat treatment, quenching, tampering, and the like.

To this end, a carburizing steel according to an exemplary embodiment of the present disclosure contains 0.1 wt % or more and 0.3 wt % or less of C (carbon), 2.0 wt % or more and 2.7 wt % or less of Cr (chromium), 0.4 wt % or more of Si (silicon), 0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum), less than 0.7 wt % of Mn (manganese), 0.6 wt % or more and 1.5 wt % or less of Ni (nickel), 1.0 wt % or less of Ti (titanium)+V (vanadium)+Nb (niobium), and 1 to 30 ppm of B (boron), with respect to a total composition 100 wt %, and may contain Fe (iron) and impurities in the remainder; furthermore, reticulated carbide is not produced in a carburizing layer as a value of ([Ni]+0.3[Mn])/([Cr]+3[Si]) satisfies 0.2 or more; and the carburizing steel has the carburizing hardness (Hv) of 780 Hv or more and the bending strength of 3,000 MPa or more.

Here, [Ni], [Mn], [Cr], [Si] mean the wt % of Ni, Mn, Cr, Si, respectively.

That is, this relates to a new Cr—Si—Ni carburizing steel component system which may improve pitting resistance property and bending strength by increasing toughness compared to the conventional high durability high Cr—Si carburizing steel.

In the related art, there exists the carburizing steel component system having improved durability compared to the conventional material by securing the basic toughness with Mn and increasing the contents of Cr—Si advantageous for controlling carbide.

However, since the related art is high likely to excessively decrease toughness by the content of Si of 2% at maximum, the present disclosure utilizes a method for increasing Ni having a better effect than Mn, instead of performing optimization in the range in which the content of Si decreasing the toughness of the material is small, thereby improving the toughness of the matrix, and thus implements the method for utilizing the effect of controlling the formation of the Cr—Si carbide, thereby securing durability.

Hereinafter, each composition and content will be described in more detail with reference to FIGS. 1 to 5.

FIG. 1 illustrates hardness, Fe₃C, high-temperature (Cr, Fe)₇C₃ composite carbide distribution according to the content of Cr. The Cr is generally the alloy element added for securing hardenability of a material, but the present disclosure optimizes the range in order to determine the type and formation temperature of precipitated carbide as well as the hardenability. According to the present disclosure, as the carbide affected by the content of Cr, there are three types of Fe₃C/(Fe, Cr)₃C/(Fe, Cr)₇C₃.

1) Fe₃C: the corresponding carbide is the most basic carbide of the Fe—C component system formed at the Al temperature (−730° C.) or less. In the case of martensite, the heat treatment, particularly, a tampering condition is optimized to implement high strength, thereby intentionally suppressing the precipitation of Fe₃C and supersaturating C, and since the supersaturated C is coupled to Fe by the heat generation during the use in many parts to form Fe₃C, thereby decreasing the strength of the matrix, there are generally many cases of adding Si delaying the precipitation of Fe₃C.

2) (Fe, Cr)₃C: when the content of Cr having C affinity higher than Fe is increased, (Fe, Cr)₃C carbide is formed. The (Fe, Cr)₃C may be precipitated even in the region of 800° C. or more higher than the Al temperature, thereby securing stabilization even during the carburizing heat treatment, and also decreasing the available carbon contained in the matrix to suppress the additional formation of Fe₃C during the use.

3) (Fe, Cr)₇C₃: when Cr is further increased, (Fe, Cr)₇C₃ instead of (Fe, Cr)₃C is formed. The (Fe, Cr)₇C₃ is formed at high temperature but classified as instable transition carbide, and excessively uses C upon precipitation, thereby decreasing strength rather.

The present disclosure does not sufficiently form stable (Fe, Cr)₃C-based carbide at high temperature during carburization if the Cr of less than 2.0% is added.

As a result, since a total amount exceeds 1% while Fe₃C is formed during the use and a rapid softening phenomenon occurs, the lower limit of the Cr in the present disclosure is 2.0%.

On the contrary, if Cr of more than 2.7% is added, the formation of the carbide due to the Cr is very activated to form the (Fe, Cr)₇C₃ having instability and strong brittleness despite high temperature of 900° C. or more to decrease the available carbon amount, thereby degrading the surface hardness strengthening effect. Therefore, the upper limit of the Cr in the present disclosure is 2.7%.

Next, FIG. 2 illustrates impact toughness, Fe₃C, high-temperature (Cr,Fe)₇C₃ carbide distribution according to the content of Si.

Si is known as the element in which the cation solubility with Fe₃C cementite is 0. That is, the Fe₃C may not be formed around the Si element, and thanks to such an effect, most martensite containing Si very effectively suppresses the decrease in hardness due to the formation of the Fe₃C during the use. However, when Si of less than 0.4% is added, the content of the Fe₃C is not effectively suppressed to less than 1%, such that it is disadvantageous for securing hardness and thus the lower limit of the Si in the present disclosure is 0.4%.

Another effect of the Si is to increase the reactivity (activity) of C upon addition, and thus suppresses the formation of the Fe₃C but promotes another form of carbide formation reaction. It is confirmed that the component system according to the present disclosure adds Si in order to suppress the formation of the Fe₃C, but forms instable high-temperature (Cr, Fe)₇C₃ due to the composite effect with Cr when the content of Si exceeds 0.7% to decrease the available carbon amount, thereby being rather disadvantageous for securing hardness and it tends not to sufficiently secure toughness even when Ni is added.

Therefore, the upper limit of the Si in the present disclosure is 0.7%, and more preferably, 0.6 wt % or more and 0.7 wt % or less.

Next, FIG. 3 illustrates a blocky residual austenite distribution according to the content of Ni, and FIGS. 4A and 4B are tissue pictures illustrating the blocky residual austenite distribution.

Ni is a important element in order to implement the intended decrease in the propagation speed of the fatigue crack caused by strengthening the toughness of the material in the present disclosure. Particularly, Ni improves the toughness of the material whereas Cr/Si largely decreases the toughness of the matrix, such that in order to implement the effect of strengthening toughness by Ni, it is necessary to optimize the contents of Ni/Mn in consideration of the synergy effect with the Mn having the small relative effect with the Ni and content with respect to the contents of Cr/Si but performing the same role together.

The toughness decrease effect is required to consider the influence of the Cr and the Si together, and it is confirmed that the present disclosure may secure the effect of sufficiently strengthening toughness only when satisfying 0.2 or more according to the evaluation result based on (Ni+0.3Mn)/(Cr+3Si).

When the content of the Ni exceeds 1.5%, the blocky austenite starts to exist at room temperature due to the strong austenite stabilization effect of the Ni to decrease the fraction of the martensite formed upon heat treatment, thereby improving toughness but causing the adverse effect of decreasing strength. Therefore, the upper limit of the Ni in the present disclosure is 1.5%.

Further, the Ni is the toughness strengthening improvement element and the hardenability improvement element, and when the content of the Ni is less than 0.6%, the toughness is decreased, thereby resultantly increasing the propagation speed of the fatigue crack and thus shortening the fatigue life.

Therefore, the content of the Ni is required to be 0.6 wt % or more and 1.5 wt % or less, and more preferably, 0.6 wt % or more and 1.0 wt % or less.

Next, FIG. 5 illustrates Fe₃C and high-temperature (Mo, Ni, Fe) composite carbide distribution according to the content of Mo. The (Mo, Ni, Fe) composite carbide may be (Mo, Ni, Fe)₂C or (Mo, Ni, Fe)₆C carbide.

The Mo is the main element of suppressing the formation of Fe₃C and causing micronization upon tampering due to the high affinity with C, thereby resultantly strengthening homogeneity. Further, the Mo also serves to delay the movement of the potential to strengthen the strength and toughness of the material. However, when the Mo of less than 0.3% is added, there are almost no effects of suppressing the Fe₃C and causing the micronization and thus the hardness strengthening effect is not implemented, such that the lower limit of the Mo in the present disclosure is 0.3%.

On the other hand, when the Mo exceeds 0.7%, the Mo reacts with Ni to form (Mo, Ni, Fe)-based composite carbide at high temperature to decrease the available carbon amount, such that the phenomenon in which the surface hardness is decreased again occurs.

Therefore, the upper limit of the Mo in the present disclosure is 0.7%.

The effect of the Mn is about ⅓ level, but the Mn is known as the austenite stabilization element capable of strengthening toughness similar to the Ni. Upon addition, the Mn has the effect of strengthening the hardenability and toughness of the material similar to the Ni, but due to the austenite stabilization effect caused by the Ni already contained sufficiently, there occurs the phenomenon in which the hardness is decreased by the decrease in the fraction of the martensite while the blocky austenite is stabilized at room temperature when the content of Mn exceeds 0.6%.

Therefore, the upper limit of the Mn in the present disclosure is 0.6%, and more preferably, 0.1 wt % or more and 0.5 wt % or less.

Nb/Ti/V is the alloy element having the large strength improvement effect by forming MC-based carbide at high temperature to micronize the grain boundary and strengthen precipitation. However, since the Nb/Ti/V is formed at the higher temperature than the Cr carbide, the intended Cr carbide amount necessary for improving durability according to the present disclosure tends to be decreased rather when the Nb/Ti/V is excessively added. Therefore, the sum of the Nb/Ti/V is limited to 1% or less.

B (boron) is known as the element of effectively suppressing the formation of ferrite during cooling even with a small amount, thereby improving hardenability. However, since the B weakens the grain boundary rather when being added excessively due to the characteristic of being preferentially located in the grain boundary, the content of the B in the present disclosure is limited to 1 to 30 ppm.

Hereinafter, test examples and experimental results through Examples and Comparative Examples in a composition range according to the present disclosure will be described.

The test examples are expressed in Table 1 below.

TABLE 1
		PHYSICAL
	COMPOSITION	PROPERTIES
									(Ni + 0.3	Carburizing	Bending
							Ti +	B	Mn)/	hardness	strength
	C	Mn	Ni	Cr	Si	Mo	Nb + V	(ppm)	(Cr + 3 Si)	(Hv 780↑)	(3000 MPa↑)
Example 1	0.2	0.5	0.7	2.0	0.6	0.3	0.200	15	0.22	811	3120
Example 2	0.2	0.5	0.6	2.0	0.6	0.3	0.200	15	0.20	813	3030
Example 3	0.2	0.5	0.7	2.4	0.6	0.3	0.200	15	0.20	821	3060
Example 4	0.2	0.5	0.7	2.0	0.7	0.3	0.200	15	0.20	817	3030
Example 5	0.2	0.5	0.8	2.7	0.7	0.3	0.200	15	0.20	825	3120
Example 6	0.2	0.5	0.7	2.0	0.6	0.6	0.200	15	0.22	809	3210
Example 7	0.2	0.1	0.7	2.0	0.6	0.3	0.200	15	0.22	805	3090
Example 8	0.2	0.1	1.0	2.7	0.7	0.3	0.200	15	0.23	803	3270
Example 9	0.2	0.1	1.5	2.7	0.7	0.3	0.200	15	0.31	796	3240
Example 10	0.2	0.5	1.5	2.7	0.7	0.7	0.200	15	0.34	791	3300
Comparative	0.2	0.5	0.6	2.0	0.6	0.2	0.200	15	0.20	762	3000
Example 1
Comparative	0.2	0.5	0.7	2.0	0.6	0.8	0.200	15	0.22	774	3270
Example 2
Comparative	0.2	0.7	0.7	2.0	0.6	0.3	0.200	15	0.24	754	2970
Example 3
Comparative	0.2	0.5	0.5	2.0	0.6	0.3	0.200	15	0.17	803	2910
Example 4
Comparative	0.2	0.5	1.6	2.0	0.6	0.3	0.200	15	0.46	753	3030
Example 5
Comparative	0.2	0.5	0.7	2.8	0.6	0.3	0.200	15	0.18	817	2940
Example 6
Comparative	0.2	0.5	0.7	2.0	0.8	0.3	0.200	15	0.19	803	2850
Example 7
Comparative	0.2	0.25	0.08	2.4	0.7	0.5	0.25	15	0.03	809	2910
Example 8
Comparative	0.2	0.4	0.6	1.6	0.4	0.3	0.100	10	0.27	771	2820
Example 9
Comparative	0.2	0.5	1.6	2.7	0.7	0.7	0.200	15	0.36	766	3150
Example 10

It may be seen that all of the Examples 1 to 10 satisfy the target values of both the carburizing hardness and the bending strength as the value of (Ni+0.3Mn)/(Cr+3Si) is 0.2 or more, and Examples 1 and 8 may be optimal examples when comprehensively determining the carburizing hardness and the bending strength.

Example 2 is an example of applying the lower limit value of Ni to Example 1, Example 3 is an example in which Cr is increased in Example 1, Example 4 is an example in which Si is increased in Example 1, and Example 5 corresponds to a case where the value of Si/Cr is the maximum in Example 1.

Further, Example 6 is an example in which Mo is increased in Example 1, and Example 7 is an example in which Mn is decreased in Example 1.

Next, Example 9 is an example of applying the upper limit value of Ni based on Example 8, and Example 10 is an example to which the upper limit values of most components are applied.

On the other hand, Comparative Example 1 has Mo less than that of Example 1 and thus show the result that the carburizing hardness is small, and Comparative Example 2 shows the result in which Mo is larger than that of Example 1 and thus the carburizing hardness is small.

It may be seen that Comparative Example 3 has Mn larger than that of Example 1 and thus shows the result that the carburizing hardness and the bending strength are small, and Comparative Example 4 has Ni less than that of Example 1 and the value of (Ni+0.3Mn)/(Cr+3Si) is smaller than 0.2 and thus shows the result that the bending strength is small.

Next, it may be seen that Comparative Example 5 has Ni larger than that of Example 1 and thus shows the result that the carburizing hardness is small, Comparative Example 6 has Cr larger than that of Example 1 and the value of (Ni+0.3Mn)/(Cr+3Si) is smaller than 0.2 and thus shows the result that the bending strength is small, and Comparative Example 7 has Si larger than that of Example 1 and the value of (Ni+0.3Mn)/(Cr+3Si) is smaller than 0.2 and thus shows the result that the bending strength is small.

Further, Comparative Example 8 has Ni less than that of Example 1 and the value of (Ni+0.3Mn)/(Cr+3Si) is smaller than 0.2 and thus shows the result that the bending strength is small, Comparative Example 9 has Cr less than that of Example 1 and thus shows the result that the carburizing hardness and the bending strength are small, and Comparative Example 10 shows the result in which when the Ni larger than that of Example 1 is applied to the upper limit value of most components, the carburizing hardness is small.

FIGS. 6A to 6F illustrate pitting generation amount evaluation result images under severe conditions, in which FIG. 6A illustrates the result of Example 1, FIG. 6B illustrates the result of Example 2, FIG. 6C illustrates the result of Comparative Example 4, FIG. 6D illustrates the result of Comparative Example 5, FIG. 6E illustrates the result of Comparative Example 6, and FIG. 6F illustrates the result of Comparative Example 8.

A test method may use a planetary gear assembly evaluation apparatus for testing a sun gear, a pinion gear, and a carrier, and confirm the pitting generation aspect of the gear surface by driving and then disassembling the planetary gear formed of the carburizing steel according to the present disclosure at a speed of 4,000 RPM based on the sun gear for 24 hours by imposing a torque of 400 Nm applied to the carrier.

As may be seen from the results, it may be visually confirmed that Comparative Examples 4, 5, 6, 8 have the larger pitting generation amounts than those of the Examples 1 and 2.

Further, FIGS. 7A to 7C illustrate pitting generation amount evaluation result images under flat conditions, in which FIG. 7A illustrates the result of Example 1, FIG. 7B illustrates the result of Example 2, and FIG. 7C illustrates the result of Comparative Example 8.

Here, the flat conditions mean the levels lower than the severe conditions in FIGS. 6A to 6F, and the pitting generation aspect of the gear surface may be confirmed by driving and then disassembling the planetary gear formed of the carburizing steel according to the present disclosure at a speed of 3,000 RPM based on the sun gear for 24 hours by imposing a torque of 300 Nm applied to the carrier in the same test method as that of the severe conditions.

As may be seen from the results, it may be visually confirmed that Comparative Example 8 has the larger pitting generation amount that those of Examples 1 and 2.

As described above, the exemplary embodiment of the present disclosure may increase the bending strength by 10% or more, and the carburizing layer hardness by 20 Hv or more compared to the conventional steel, thereby implementing durability.

FIG. 8A illustrates a microstructure according to Comparative Example 8, FIG. 8B illustrates a microstructure according to Example 1, and FIG. 8C illustrates a microstructure according to Example 8.

FIG. 8A illustrates 2.4Cr-0.7Si-0.08Ni steel in Comparative Example 8 and the blocky residual austenite may be confirmed, and FIG. 8B illustrates 2.0Cr-0.6Si-0.7Ni steel in Example 1, and it may be seen that the size and fraction of Fe₃C are decreased by the increase in Ni.

Further, FIG. 8C illustrates 2.7Cr-0.7Si-1.0Ni steel in Example 8, and it may be confirmed that the Fe₃C is suppressed in the grain boundary due to the increase in Cr+Ni, and (Cr, Fe)₇C₃ is formed in some granules.

As described above, the present disclosure has been described with reference to the exemplary drawings, but is not limited to the described exemplary embodiment, and it is apparent to those skilled in the art that the present disclosure may be variously modified and changed without departing from the spirit and scope of the present disclosure. Therefore, these modified examples or changed examples fall within the claims of the present disclosure, and the scope of the present disclosure should be construed based on the appended claims.

Claims

1. A carburizing steel comprising, based on a total wt % of the carburizing steel:

0.1 wt % or more and 0.3 wt % or less of C (carbon):

2.0 wt % or more and 2.7 wt % or less of Cr (chrome);

0.4 wt % or more and 0.7 wt % or less of Si (silicon);

0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum);

less than 0.7 wt % of Mn (manganese); and

0.6 wt % or more and 1.5 wt % or less of Ni (nickel).

2. The carburizing steel of claim 1, further comprising one or more of Ti (titanium), V (vanadium), and Nb (niobium).

3. The carburizing steel of claim 2, wherein the sum of the Ti (titanium), the V (vanadium), and the Nb (niobium) is 1.0 wt % or less with respect to the total wt % of the carburizing steel.

4. The carburizing steel of claim 1, further comprising 1 to 30 ppm of B (boron).

5. The carburizing steel of claim 1, wherein a value of the following formula is 0.2 or more,

[Ni]+0.3[Mn])/([Cr]+3[Si],

where [Ni], [Mn], [Cr], [Si] mean the wt % of Ni, Mn, Cr, Si, respectively.

6. The carburizing steel of claim 1, wherein the content of the Si (silicon) is 0.6 wt % or more and 0.7 wt % or less.

7. The carburizing steel of claim 1, wherein the content of the Ni (nickel) is 0.6 wt % or more and 1.0 wt % or less.

8. The carburizing steel of claim 1, wherein the content of the Mn (manganese) is 0.1 wt % or more and 0.5 wt % or less.

9. The carburizing steel of claim 1, wherein the carburizing steel has a carburizing hardness of 780 Hv or more.

10. The carburizing steel of claim 1, wherein the carburizing steel has a bending strength of 3,000 MPa or more.

11. A carburizing steel comprising, based on a total wt % of the carburizing steel:

0.1 wt % or more and 0.3 wt % or less of C (carbon);

2.0 wt % or more and 2.7 wt % or less of Cr (chrome), 0.4 wt % or more and 0.7 wt % or less of Si (silicon);

0.3 wt % or more and 0.7 wt % or less of Mo (molybdenum);

less than 0.7 wt % of Mn (manganese); 0.6 wt % or more and 1.5 wt % or less of Ni (nickel);

1.0 wt % or less of the sum of Ti (titanium), V (vanadium), and Nb (niobium); and

1 to 30 ppm of B (boron),

wherein a value of the following formula satisfies 0.2 or more, [Ni]+0.3[Mn])/([Cr]+3[Si],

where [Ni], [Mn], [Cr], [Si] mean the wt % of Ni, Mn, Cr, Si, respectively.

12. The carburizing steel of claim 11, wherein the content of the Si (silicon) is 0.6 wt % or more and 0.7 wt % or less.

13. The carburizing steel of claim 11, wherein the content of the Ni (nickel) is 0.6 wt % or more and 1.0 wt % or less.

14. The carburizing steel of claim 11, wherein the content of the Mn (manganese) is 0.1 wt % or more and 0.5 wt % or less.

15. The carburizing steel of claim 11, wherein the carburizing steel has a carburizing hardness of 780 Hv or more.

16. The carburizing steel of claim 11, wherein the carburizing steel has a bending strength of 3,000 MPa or more.