CARBURIZED STEEL AND METHOD OF MANUFACTURING THE SAME

Abstract

A carburized steel composition of the present invention comprises: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0113991 filed in the Korean Intellectual Property Office on Aug. 12, 2015, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to a carburized steel and a composition thereof, and further relates to a method of manufacturing a carburized steel.

BACKGROUND OF THE INVENTION

[0003] Generally, vehicle gears have been manufactured by sequential processes such as forging steel materials, normalizing or annealing, processing (shaving & hobbing), carburizing heat treatment, and nitriding heat treatment.

[0004] As conventional materials, chromium (Cr) alloy steel, chromium-molybdenum (Cr--Mo) alloy steel, nickel-chromium-molybdenum (Ni--Cr--Mo) alloy steel and the like have been widely used as alloy steels for gears. Since vehicle transmission gears are given a rough shape by forging, and given an accurate shape by processing, those alloy steels should have both excellent forgeability and easy processability.

[0005] For example, the chromium alloy steel and the chromium-molybdenum alloy steel are low-priced, but their fatigue property and impact property are not excellent, and thus, they are mostly used in gears which are not highly loaded.

[0006] Further, the nickel-chromium-molybdenum alloy steel has drawbacks in that due to the content of nickel that is high-priced and difficult to be processed. However, its fatigue property and impact property are excellent, and thus, they are mostly used in gears which are highly loaded.

[0007] In the case of general gears, gears interlock with each other, and are contact-stressed, thereby causing pitting on the surface of a gear part, which may result in durability decrease, and noise generation. Particularly, as a powertrain is down-sized, miniaturized, and high-powered, thereby increasing load to a gear, the occurrence of the problem increases.

[0008] Therefore, there is a demand for the development of gears having improved contact fatigue life (e.g. pitting resistance).

[0009] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain 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 INVENTION

[0010] In one aspect, the present invention provides a carburized steel and composition thereof.

[0011] In another aspect, the present invention provides a method of manufacturing a carburized steel.

[0012] An exemplary embodiment of the present invention provides a carburized steel composition which may comprise: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

[0013] Further, the carburized steel composition may further comprise: carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorus (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the carburized steel composition.

[0014] Further provided are the carburized steel compositions that may consist of, consist essentially of, or essentially consist of the components as described herein. For instance, the carburized steel composition may consist of, consist essentially of, or essentially consist of: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition. In addition, the carburized steel composition may consist of, consist essentially of, or essentially consist of: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt %, nitrogen (N) in an amount of about 0.01 to 0.02 wt %, carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorus (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, aluminum (Al) in an amount of about 0.01 to 0.05 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

[0015] Preferably, for the carburized steel, the following Formula 1 may have a value of about 12.5 to 14 from the contents of Si, Ni, Cr, Mo, V and N:

(5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1=

[0016] In Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to added amounts (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

[0017] Another exemplary embodiment of the present invention provides a method of manufacturing carburized steel that may comprise: carburizing a steel material comprising: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition. In particular, the carburizing may be carried out at a heat treatment temperature of about 930 to 1050.degree. C.

[0018] Preferably, for the steel material, the following Formula 1 may have a value of about 12.5 to 14 from the contents of Si, Ni, Cr, Mo, V and N.

(5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1=

[0019] In the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to added amounts (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

[0020] Preferably, a carbon potential (CP) in the carburizing may be of about 0.85 to 1.1.

[0021] The steel materials may further comprise: carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorus (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the carburized steel composition.

[0022] Preferably, a step of nitriding the steel materials may be further included, after the carburizing. The step of nitriding may be carried out at a temperature in a range of about 820 to 870.degree. C. under an atmosphere containing ammonia gas (NH.sub.3) of about 0.5 to 2 vol %.

[0023] The steel material may be further manufactured into a predetermined gear shape by any one or more of forging, normalizing, annealing and processing.

[0024] Still further provided are vehicle parts that comprise the carburized steel composition as described herein. For instance, the vehicle part may be a gear.

[0025] Additionally, provided are vehicles that comprise a vehicle part such as a gear that comprises the carburized steel composition as described herein.

[0026] According to various exemplary embodiments of the present invention, the carburized steel having improved pitting resistance may be provided. Further, gears having improved pitting resistance may be provided using the carburized steel.

[0027] Other aspects of the present invention are disclosed infra.

DETAILED DESCRIPTION

[0028] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0029] Unless specifically stated or obvious from context, as used herein, the term "about" is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. "About" can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term "about."

[0030] Further, it is understood that the term "vehicle" or "vehicular" or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

[0031] Advantages and features of the present invention and methods to achieve them will be elucidated from exemplary embodiments described below in detail with reference to the accompanying drawings. However, the present invention is not limited to exemplary embodiments disclosed below, but will be implemented in various forms. The exemplary embodiments of the present invention make disclosure of the present invention thorough and are provided so that those skilled in the art can easily understand the scope of the present invention. Therefore, the present invention will be defined by the scope of the appended claims. Throughout the description, like reference numerals denote like elements

[0032] Therefore, in some exemplary embodiments, well-known techniques will not be specifically described, in order to avoid ambiguous interpretation of the present invention. Unless otherwise defined herein, all terms used in the specification (including technical and scientific terms) may have the meaning that is commonly understood by those skilled in the art. The carburized steel composition according to an exemplary embodiment of the present invention may comprise: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

[0033] Further, the carburized steel composition may further comprise: carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorus (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the carburized steel composition.

[0034] The carburized steel composition may satisfy following Formula 1 to have a value of about 12.5 to 14:

(5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1=

[0035] In the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to added amounts (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

[0036] The reasons for limiting the composition are as follows.

[0037] Carbon (C), as used herein, may increase strength and hardness of materials, and precipitate carbides. When carbon is included in an amount of about 0.15 to 0.25 wt %, carbon diffusion when carburizing may be easy. Further, when carbon is included less than about 0.15 wt %, tensile strength may be deteriorated, and when carbon is included greater than about 0.30 wt %, impact toughness may be deteriorated.

[0038] Silicon (Si), as used herein, may be added in an amount of about 0.7 to 1.3 wt % to increase hardness, modulus of elasticity, and the like, and strengthen a ferrite phase. Further, high temperature softening resistance may be improved to reduce the incidence of hardness reduction. When silicon is included less than about 0.7 wt %, intergranular oxidation may occur much, and when silicon is included greater than about 1.3 wt %, elongation and an impact value may be lowered.

[0039] Manganese (Mn), as used herein, may be added in an amount of about 0.3 wt % or greater to increase a quenching property and strength, however, when manganese is included greater than about 1.0 wt %, processability may be deteriorated.

[0040] Phosphorus (P), as used herein, may be added in an amount greater than about 0.03 wt %, more particularly greater than about 0.02 wt %, and thus may form Fe.sub.3P. Fe.sub.3P may be segregated, thereby being difficult to be homogenized even in the case of being annealed, being stretched long by forging rolling to deteriorate impact resistance, and promoting tempering brittleness.

[0041] Sulfur (S), as used herein, may form MnS to improve mechanical processability. However, when sulfur is added greater than about 0.03 wt %, strength may be lowered.

[0042] Chromium (Cr), as used herein, may be added in an amount of about 2.0 wt % or greater, thereby improving tempering softening resistance through precipitation of carbides, and improving a quenching property. However, when chromium is added greater than about 2.8 wt %, problems such as production of network carbides may occur.

[0043] Molybdenum (Mo), as used herein, may be added an amount of about 0.15 wt % or greater to improve hardenability, prevent tempering brittleness, and distribute carbides uniformly. The upper limit of molybdenum is not particularly limited, but molybdenum may be added an amount of about 0.5 wt % or less considering economic feasibility.

[0044] Nickel (Ni), as used herein, may be added an amount of about 0.15 wt % or greater to refine a steel structure. The nickel may be in austenite or ferrite to strengthen matrix, improve hardenability, and increase contact fatigue life. The upper limit of nickel is not particularly limited, but nickel may be added an amount of about 0.5 wt % or less considering economic feasibility.

[0045] Copper (Cu) may, when added greater than about 0.3 wt %, deteriorate thermal processability and produce red shortness.

[0046] Niobium (Nb) may be added an amount of about 0.01 wt % or greater to refine crystallites and carbides. The upper limit of niobium is not particularly limited, but niobium may be added an amount of about 0.5 wt % or less considering economic feasibility.

[0047] Vanadium (V) may be added an amount of about 0.02 wt % or greater to produce fine particulate carbides, thereby refining a steel structure, and improve tempering softening resistance. However, when vanadium is added greater than the predetermined amount, V.sub.2O.sub.5 which is an oxide has high vapor pressure may be formed, thereby being vaporized at high temperature. Thus, vanadium may be added an amount of about 0.1 wt % or less.

[0048] Aluminum (Al) may be added vanadium 0.01 wt % or greater to finely precipitate AlN in steel, thereby refining crystallites of austenite. However, when aluminum is added greater than about 0.05 wt %, embrittlement may occur.

[0049] Nitrogen (N) may be added vanadium 0.01 wt % or greater to increase yield strength, and form nitrides, thereby refining crystallites. However, when nitrogen is added greater than about 0.02 wt %, elongation may be deteriorated.

[0050] In particular, for the carburized steel composition, the following Formula may have a value of about 12.5 to 14.

(5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1=

[0051] In the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to added amounts (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

[0052] Further, when the value of following Formula 1 is less than about 12.5, softening resistance at high temperature may be reduced, thereby increasing an incidence of pitting. In addition, when the value of the Formula 1 is greater than about 14, hardness may be increased, thereby deteriorating processability.

[0053] Further, a smaller particle size of crystallites of the carburized steel may be preferred, since such carburized steel has an advantageous characteristic in that abnormal crystallites occur at a temperature greater than about 1150.degree. C.

[0054] The size of crystallites may be associated with strength, and according to a Hall-Petch equation, the smaller the size of crystallites is, the more improved strength and toughness are.

[0055] The carburized steel as described above may be carburized steel for vehicle gears.

[0056] Hereinafter, a method of manufacturing the carburized steel by an exemplary embodiment of the present invention will be described.

[0057] As described above, the steel material may comprise: silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting the remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

[0058] Preferably, for the steel material, the following Formula 1 may have a value of about 12.5 to 14.

(5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1=

[0059] In the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to added amounts (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

[0060] The steel materials further comprise: carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorus (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the carburized steel composition. The reasons for limiting the composition of the steel materials are the same as the reasons for limiting the composition of the carburized steel as described above.

[0061] In addition, the steel material may be manufactured into a predetermined gear shape by any one or more of forging, normalizing, annealing and processing.

[0062] Particularly, the steel materials may be those manufactured into a predetermined gear shape for a vehicle by hot forging or cold forging materials having the same composition as the steel materials, which are then subjected to normalizing or annealing treatment, and processed (shaving & hobbing).

[0063] The steel material may be carburized. In particular, when carburized, a heat treatment temperature may be of about 930 to 1050.degree. C.

[0064] In a component system of the steel materials by an exemplary embodiment of the present invention, when heat treatment of the steel material is performed at a temperature less than about 930.degree. C., carbides may be precipitated, and when heat treatment is performed at a temperature greater than about 1050.degree. C., crystallites may grow.

[0065] Further, a carbon potential (CP) in the carburizing may be of about 0.85 to 1.1.

[0066] When the CP is less than about 0.85, hardness reduction may occur after carburizing, and when the CP is greater than about 1.1, carbides may be precipitated.

[0067] When the carburizing is completed, a step of nitriding the steel materials may be carried out.

[0068] The step of nitriding may be carried out at a temperature range of about 820-870.degree. C. under a condition of injecting 0.5-2 vol % of ammonia (NH.sub.3) gas (relative to atmosphere in a furnace).

[0069] Herein, when a nitriding temperature is less than about 820.degree. C., ammonia dissociation does not occur, and ammonia gas may not be diffused, and when a nitriding temperature is greater than about 870.degree. C., thermal deformation may occur severely upon etching.

[0070] Further, when NH.sub.3 gas is injected less than about 0.5 vol %, a nitriding effect (increase of residual amount of austenite, and improvement of hardenability) may not be represented, and when NH.sub.3 gas is injected greater than about 2 vol %, durability may be deteriorated by the formation of nitrides.

[0071] Further, since the nitriding treatment time varies with component requirements, it does not have to be particularly specified.

Example

[0072] Hereinafter, the present invention will be described in detail, through examples. However, the following examples only illustrate the present invention, and the disclosure of the present invention is not limited by the following experimental examples.

[0073] In the case of alloy steel having improved pitting resistance, it should have high surface hardness after carburizing thermal treatment, and have excellent tempering softening resistance at about 300.degree. C.

[0074] The elements required for the present invention were determined as being Si, Cr, Ni, Mo and V, and contact fatigue life after carburization was compared, thereby representing the results in following Table 1. In the Table 1, all test data were obtained at CP of 0.9, with P, Cu, S and Al under the same condition, and the test was carried out at a content ratio of P:0.01 wt %, Cu:0.1 wt %, S:0.01 wt % and Al:0.03 wt %.

[0075] However, in the case of Mo, its added amounts were set equally, since the amount is originally large, and a more amount causes burden of cost.

TABLE-US-00001 TABLE 1 Comparison table of main alloy effect Physical property evaluation Material Contact Contact hardness Pitting fatigue life fatigue life Classifi- Main alloy components (wt %) (after resistance (3.51 GPa) (3.51 GPa) cation Purpose C Si Mn Cr Mo N Nb V N annealing) parameter P at 80.degree. C. at 100.degree. C. #1 Conventional 0.2 0.6 0.6 2.0 0.35 0.05 0.035 -- 120 HB 176 11.49 4,530,000 1,320,000 materials ppm #2 Increase of Si 0.2 1.0 0.6 2.4 0.35 0.05 0.035 -- 120 HB 184 12.25 5,120,000 1,920,000 and Cr amounts ppm #3 Addition of Ni 0.2 1.0 0.6 2.4 0.35 0.35 0.035 -- 120 HB 185 12.78 6,110,000 3,970,000 relative to #2 ppm #4 Addition of V 0.2 1.3 0.6 2.4 0.35 0.35 0.035 0.05 120 HB 192 13.45 6,330,000 4,900,000 and Ni relative ppm to #2 #5 Each addition of 0.2 1.3 0.6 2.8 0.35 0.5 0.035 0.11 120 HB 212 14.30 7,750,000 5,530,000 Si, Cr, Ni and V ppm relative to #4 #6 Addition of V 0.2 1.3 0.6 2.8 0.35 0.5 0.035 0.02 120 HB 198 13.78 6,250,000 4,950,000 0.02 to #5 ppm

[0076] 1) Contact fatigue life at 80.degree. C. increases, as pitting resistance parameters (described on the next page) increase from #1 to #5, however, the increment is not large.

[0077] 2) Contact fatigue life at 100.degree. C. increases, as pitting resistance parameters (described on the next page) increase from #1 to #5, and the increment is large.

[0078] The element effect on tempering softening resistance is more remarkable under the condition of a higher temperature.

[0079] 3) The higher a pitting resistance parameter is, the higher the material hardness after annealing is.

[0080] 4) Experimental data around V: 0.02% (#6 experiment), experimental data around V: 0.1% (#5 experiment), experimental data around pitting resistance parameter: 12.5 (#2 and #3 experiments), and experimental data around pitting resistance parameter: 14 (#5 experiment) are shown.

[0081] Further, the invented steel grade and the developed carburizing and nitriding heat treatment process were applied to evaluate physical properties, and as a result, the physical property evaluation results were able to be obtained as shown in following Table 2.

TABLE-US-00002 TABLE 2 Comparison table of physical property evaluation Physical property evaluation Contact fatigue Contact fatigue Network life (3.51 GPa) life (3.51 GPa) at Classification carbides at 80.degree. C. 100.degree. C. Conventional materials + absence 4,530,000 1,320,000 conventional carburizing and nitriding process Invented materials + conventional presence 1,520,000 890,000 carburizing and nitriding process Invented materials + invented absence 6,330,000 4,900,000 carburizing and nitriding process

[0082] 1) In the case of applying the conventional carburizing and nitriding process to the invented materials, the contact fatigue result was not good as compared with the conventional process, due to the network carbides.

[0083] 2) When applying the invented carburizing and nitriding process to the invented material, the best pitting resistance was able to be obtained.

[0084] Although the exemplary embodiments of the present invention has been described with reference to the accompanying drawings, those skilled in the art will appreciate that various modifications and alterations may be made without departing from the spirit or essential feature of the present invention.

[0085] Therefore, it should be understood that the exemplary embodiments described above are not restrictive, but illustrative in all aspects. It should be interpreted that the scope of the present invention is defined by the following claims rather than the above-mentioned detailed description and all modifications or alterations deduced from the meaning, the scope, and equivalences of the claims are included in the scope of the present invention.

[0086] While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A carburized steel composition, comprising: silicon (Si) in an amount of about 0.7 to 1.3 wt %; nickel (Ni) in an amount of about 0.15 to 0.5 wt %; chromium (Cr) in an amount of about 2.0 to 2.8 wt %; molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %; vanadium (V) in an amount of about 0.02 to 0.1 wt %; nitrogen (N) in an amount of about 0.01 to 0.02 wt %; and iron (Fe) constituting remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

2. The carburized steel composition of claim 1, wherein for the carburized steel composition, the following Formula 1 has a value of about 12.5 to 14 from the contents of Si, Ni, Cr, Mo, V and N: (5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1= wherein in the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to the contents (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

3. The carburized steel composition of claim 1, further comprising: carbon (C) in an amount of about 0.15 to 0.25 wt %; manganese (Mn) in an amount of about 0.3 to 1.0 wt %; phosphorous (P) in an amount of about 0.03 wt % or less but greater than 0 wt %; sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %; copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %; niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the carburized steel composition.

4. The carburized steel composition of claim 1, consisting essentially of: silicon (Si) in an amount of about 0.7 to 1.3 wt %; nickel (Ni) in an amount of about 0.15 to 0.5 wt %; chromium (Cr) in an amount of about 2.0 to 2.8 wt %; molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %; vanadium (V) in an amount of about 0.02 to 0.1 wt %; nitrogen (N) in an amount of about 0.01 to 0.02 wt %; and iron (Fe) constituting remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

5. The carburized steel composition of claim 1, consisting essentially of: silicon (Si) in an amount of about 0.7 to 1.3 wt %; nickel (Ni) in an amount of about 0.15 to 0.5 wt %; chromium (Cr) in an amount of about 2.0 to 2.8 wt %; molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %; vanadium (V) in an amount of about 0.02 to 0.1 wt %; nitrogen (N) in an amount of about 0.01 to 0.02 wt %; carbon (C) in an amount of about 0.15 to 0.25 wt %; manganese (Mn) in an amount of about 0.3 to 1.0 wt %; phosphorous (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %; copper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %; niobium (Nb) in an amount of about 0.01 to 0.05 wt %, aluminum (Al) in an amount of about 0.01 to 0.05 wt %, and iron (Fe) constituting remaining balance of the carburized steel composition, all the wt % based on the total weight of the carburized steel composition.

6. A method of manufacturing a carburized steel, comprising: carburizing a steel material comprising silicon (Si) in an amount of about 0.7 to 1.3 wt %, nickel (Ni) in an amount of about 0.15 to 0.5 wt %, chromium (Cr) in an amount of about 2.0 to 2.8 wt %, molybdenum (Mo) in an amount of about 0.15 to 0.5 wt %, vanadium (V) in an amount of about 0.02 to 0.1 wt % and nitrogen (N) in an amount of about 0.01 to 0.02 wt %, and iron (Fe) constituting remaining balance of the carburized steel composition, all the wt % based on the total weight of the steel material, wherein the carburizing is carried out at a heat treatment temperature of about 930 to 1050.degree. C.

7. The method of claim 6, wherein for the steel material, the following Formula 1 has a value of about 12.5 to 14 from the contents of Si, Ni, Cr, Mo, V, N: (5/8*(1+log [Si]))*(log([Ni])+10))+2*(1+log([Cr]))+ ([Mo])*5+ ([V])*3+ ([N])*100 Formula 1= wherein in the Formula 1, [Si], [Ni], [Cr], [Mo], [V] and [N] refer to the contents (wt %) of Si, Ni, Cr, Mo, V and N, respectively.

8. The method of claim 6, wherein a carbon potential (CP) in the carburizing is of about 0.85 to 1.1.

9. The method of claim 6, wherein the steel material further comprises: carbon (C) in an amount of about 0.15 to 0.25 wt %, manganese (Mn) in an amount of about 0.3 to 1.0 wt %, phosphorous (P) in an amount of about 0.03 wt % or less but greater than 0 wt %, sulfur (S) in an amount of about 0.03 wt % or less but greater than 0 wt %, cupper (Cu) in an amount of about 0.3 wt % or less but greater than 0 wt %, niobium (Nb) in an amount of about 0.01 to 0.05 wt %, and aluminum (Al) in an amount of about 0.01 to 0.05 wt %, all the wt % based on the total weight of the steel material.

10. The method of claim 6, further comprising: nitriding the steel materials, after the carburizing.

11. The method of claim 10, wherein the nitriding is carried out at a temperature in a range of about 820 to 870.degree. C. under an atmosphere containing ammonia gas (NH.sub.3) in an amount of about 0.5 to 2 vol %.

12. The method of claim 11, wherein the steel material is manufactured into a predetermined gear shape by any one or more of forging, normalizing, annealing and processing.

13. A vehicle part comprising a carburized steel composition of claim 1.

14. The vehicle part of claim 13 is a gear.

15. A vehicle comprising a vehicle part of claim 13.

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