CN111411207A

CN111411207A - Method for heat-treating uppermost surface of nodular cast iron and nodular cast iron heat-treated thereby

Info

Publication number: CN111411207A
Application number: CN201911029190.9A
Authority: CN
Inventors: 权恩秀; 李衡国; 沈容辅
Original assignee: Hyundai Motor Co; Kia Motors Corp
Current assignee: Hyundai Motor Co; Kia Corp
Priority date: 2019-01-07
Filing date: 2019-10-28
Publication date: 2020-07-14
Also published as: US20200216923A1; KR20200085427A

Abstract

The application discloses a method for heat treating the surface of spheroidal graphite cast iron, in particular the uppermost surface of spheroidal graphite cast iron, and spheroidal graphite cast iron heat treated thereby. The method may include a first heat treatment for forming ferrite and a second heat treatment for oxynitridation. The heat-treated ductile iron comprises an oxide layer and a compound layer having a thickness of about 15 to 30 μm, which may be uniform. The heat treatment method can reduce the pearlite fraction of the uppermost surface of the spheroidal graphite cast iron and increase the ferrite fraction by forming ferrite, thereby forming a compound layer having a thickness of about 15 to 30 μm during the oxynitridation heat treatment.

Description

Method for heat-treating uppermost surface of nodular cast iron and nodular cast iron heat-treated thereby

Cross Reference to Related Applications

The present application claims priority from korean patent application No. 10-2019-0001485 filed by 2019 on 7.1.2019 to the korean intellectual property office under 35 u.s.c. § 119, the disclosure of which is incorporated herein by reference in its entirety.

Technical Field

The present invention relates to a method of heat treating a surface of spheroidal graphite cast iron, for example, an uppermost surface of spheroidal graphite cast iron, and spheroidal graphite cast iron heat-treated thereby. Specifically, the method may include oxynitriding heat treatment (second heat treatment) on the uppermost surface of the nodular cast iron.

Background

In order to secure wear resistance and strength, various surface treatments such as high frequency, oxynitridation, and phosphate coating have been performed on steel materials used as various parts of automobiles.

For example, the oxynitridation heat treatment is a heat treatment for improving wear resistance and corrosion resistance by introducing nitrogen [ N ] into a material in a high-temperature atmosphere to generate nitride, and then spraying water vapor to form an oxide layer on the uppermost surface of the compound layer.

As a heat treatment for ensuring durability and wear resistance, oxynitridation heat treatment is often performed on plain carbon steel, but oxynitridation heat treatment has not been well applied to spheroidal graphite cast iron, although oxynitridation heat treatment needs to be applied to ensure wear resistance and the like. This is because the ductile iron contains not only more silicon (Si) than conventional carbon steel but also has a pearlite fraction of 60% or more. Specifically, although the pearlite portion has high toughness and tensile strength, when oxynitridation heat treatment is performed on spheroidal graphite cast iron, a compound layer is disadvantageously formed due to pearlite on the surface of the spheroidal graphite cast iron, so that a surface treatment effect cannot be exhibited since the thickness of the compound layer is formed at a level of 1/2 to 1/3 as compared with conventional carbon steel.

Disclosure of Invention

In a preferred aspect, there is provided a method of forming a compound layer having a predetermined thickness or more by performing oxynitridation heat treatment while reducing the pearlite fraction of the uppermost surface of spheroidal graphite cast iron.

The term "spheroidal graphite cast iron" as used herein refers to cast iron or cast iron alloys that contain graphite (e.g., spheroidal, or spheroidal graphite) as one of the carbon sources. The term "ductile iron" may be used interchangeably with ductile cast iron (ductile cast iron) to have the same grade, mechanical and chemical properties as ductile iron. For example, the ductile iron may suitably comprise carbon in an amount of about 3.2 to 3.60 wt% of the total weight of the composition, as well as other components, such as silicon, manganese, magnesium, phosphorus, sulfur and/or copper, with the balance being iron.

The term "uppermost surface" as used herein refers to a surface region and its lower portion, which may include spaces/regions from the surface to a predetermined depth, preferably from the exposed surface to a depth of up to about 50, 100 or 200 μm, more typically from the exposed surface to a depth of up to about 100 μm.

In one aspect, a method is provided for heat treating a surface of ductile iron, particularly an uppermost surface of ductile iron. The method may include performing a first heat treatment on the ductile iron to form a ferrite, and performing a second heat treatment to oxynitride the ductile iron.

The term "ductile iron" as used herein is an iron also referred to as ductile iron or ductile cast iron, and at high rates (e.g., 100x or rate) the graphite therein may look like balls or nodules.

The term "oxynitriding" as used herein refers to a process for nitriding and oxidizing (oxidizing), which may be combined or a separate sequential process. Preferably, the oxynitridation may comprise sequential processes of respective nitridation and oxidation.

The term "nitriding" or "nitrocarburizing" as used herein refers to a process of diffusing nitrogen on the surface of a metal (e.g., metal alloy, steel, or cast iron) by heat treatment using a gas, salt, or nitrogen medium of a plasma. Preferably, it may be at an elevated temperature, for example at a temperature above about 500 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃ or 1000 ℃, or by applying heatBy using nitrogen-rich gas (e.g. NH)₃) A nitridation process is performed to form a nitride layer.

The term "oxidizing" or "oxidizing" as used herein refers to a process step that induces a reaction in which a metal and oxygen combine to form an oxide or oxide layer or deposit thereof. Exemplary oxidation procedures may include heat treatment or high temperature oxidation at temperatures, for example, greater than about 500 ℃, 600 ℃, 700 ℃, 800 ℃, 900 ℃, or 1000 ℃.

Preferably, the first heat treatment for forming ferrite may include heating ductile iron to a first heating temperature of about 940 to 1,060 ℃ and maintaining the first heating temperature, wherein a total time of the first heat treatment may be about 80 to 100 minutes.

Preferably, the first heat treatment for forming ferrite may further include air-cooling the ductile iron for about 110 to 130 minutes.

Preferably, the second heat treatment for oxynitridation may include nitrocarburizing by introducing a gas, oxidizing the ductile iron, and cooling. The oxynitridation may be performed at the second heat treatment temperature of about 550 ℃ to 570 ℃ for about 720 minutes to 1,200 minutes.

In another aspect, a heat treated ductile iron is provided. Specifically, the uppermost surface of the ductile iron may be treated by the methods described herein. The nodular cast iron can comprise an oxide layer and a compound layer, wherein the thickness of the compound layer is 15-30 mu m. Preferably, the thickness of the compound layer may be uniform.

Preferably, the uppermost surface of the ductile iron may comprise about 60-80% ferrite fraction.

Preferably, the compound layer may include a pore layer and a nitride compound layer. The nitride compound layer may include a γ' phase and a phase. Alternatively, the nitride compound layer may be composed of a γ' phase and a phase. .

Preferably, the compound layer may comprise about 25-35% of the porous layer fraction.

Preferably, the nitride compound layer may contain about 80% or more of the phase.

Preferably, the hardness of the compound layer may be about HV 600 to 1000.

According to various exemplary embodiments, a compound layer having a thickness of 15 to 30 μm may be formed on the uppermost surface of the ductile iron, which may be formed during an oxynitridation heat treatment (e.g., a second heat treatment) by increasing the ferrite fraction.

According to various exemplary embodiments, the wear resistance of ductile iron may be adjusted by controlling the number of porosity layers to be about 25-35%.

Preferably, the compound layer may comprise an oxide layer, a porosity layer, a γ' phase and a phase from the uppermost surface of the ductile iron.

The ductile iron may contain an oxide layer and a compound layer from the uppermost surface of the cast iron.

Additionally provided is a vehicle component, which may comprise the ductile iron described herein.

A vehicle is also provided that may include the vehicle component described herein.

Other aspects of the invention are provided below.

Drawings

Fig. 1 illustrates an exemplary method of heat treating an exemplary cast iron comprising ductile iron at an uppermost surface thereof according to an exemplary embodiment of the present invention.

Fig. 2 illustrates exemplary heat treatment steps for forming ferrites in accordance with an exemplary embodiment of the present invention.

Fig. 3 shows an image of an exemplary microstructure of ductile iron after a heat treatment step for forming ferrite according to an exemplary embodiment of the present invention.

Fig. 4 shows an image of an exemplary microstructure of an exemplary ductile iron heat-treated at a heating temperature lower than that of the heat treatment step for forming ferrite according to the exemplary embodiment of the present invention.

Fig. 5 shows an exemplary microstructure of conventional ductile iron.

Fig. 6 is a detailed setup view of an oxynitridation heat treatment step according to an exemplary embodiment of the invention.

Fig. 7 shows the configuration of the uppermost surface of the spheroidal graphite cast iron after the oxynitriding heat treatment step according to the exemplary embodiment of the present invention.

Fig. 8A is a microstructure of comparative example 2, and fig. 8B is a result of evaluating seizure resistance of comparative example 2.

Fig. 9A is the microstructure of example 2, and fig. 9B is the result of evaluating seizure resistance of example 2.

Fig. 10A is a microstructure of comparative example 3, and fig. 10B is a result of evaluating seizure resistance of comparative example 3.

Fig. 11A is a photograph after evaluating seizure resistance of example 2, and fig. 11B is a photograph after evaluating seizure resistance of comparative example 3.

Detailed Description

Hereinafter, the present invention will be described in detail. However, the present invention is not limited or restricted to the exemplary embodiments, and the purpose and effect of the present invention will be naturally understood or will become apparent from the following description, and the purpose and effect of the present invention are not limited to only the following description. Further, in the description of the present invention, when it is determined that detailed description of well-known technology related to the present invention may unnecessarily make the gist of the present invention unclear, the detailed description thereof will be omitted.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. 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," "comprising," "includes" and/or "including," when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.

It should be understood that the term "vehicle" or "vehicular" or other similar terms as used herein include automobiles in general, such as passenger vehicles including Sport Utility Vehicles (SUVs), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and include 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 having two or more power sources, for example, a vehicle having gasoline power and electric power.

As used herein, unless otherwise indicated herein or otherwise evident from the context, the term "about" is understood to be within the normal tolerance of the art, e.g., within 2 standard deviations of the mean. "about" can be understood as being 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 numbers provided herein are modified by the term "about".

Fig. 1 illustrates an exemplary method of heat treating an uppermost surface of ductile iron according to an exemplary embodiment of the present invention. The method may include a heat treatment step (S101) for forming ferrite and an oxynitridation heat treatment step (S102).

Specifically, by performing the first heat treatment step (S101) for forming ferrite before performing the second heat treatment step (S102) for oxynitridation, the proportion of pearlite that may be formed or present on the uppermost surface of spheroidal graphite cast iron can be reduced, and the proportion of ferrite can be increased. Therefore, during the oxynitridation heat treatment, a compound layer having a predetermined thickness or more may be formed, so that the uppermost surface of the nodular cast iron may be provided with wear resistance characteristics, and a dense and uniform compound layer may be formed. In addition, a microstructure may be formed at the center of pearlite in the deep part of ductile iron, thereby having significantly improved toughness and strength.

As used herein, the uppermost surface of the ductile iron refers to the region from the surface of the cast iron to a depth of about 100 μm. The thickness of the compound layer having a predetermined thickness or more may be about 15 μm or more, or specifically about 15 to 30 μm. By dense and uniform compound layer is meant that a compound layer other than an oxide layer is present and that the compound layer does not crack or break.

Fig. 2 shows an exemplary first thermal treatment step for forming ferrites in accordance with an exemplary embodiment of the present invention. The first heat treatment step for forming ferrite may include heating the ductile iron to a first heat treatment temperature (I) of about 940-1,060 ℃, maintaining the first heat treatment temperature (II), and air-cooling the ductile iron.

Preferably, a uniform austenitizing state can be formed by the heating step and the heating temperature maintaining step. When the uppermost surface of cast iron is austenitized and then air-cooled, those residual stresses may be removed by machining or the like, crystal grains may be miniaturized, and ferrite may be formed on the uppermost surface, so that during the oxynitridation heat treatment, generation of a nitride layer (e.g., a phase and a γ' phase) as one configuration of a compound layer may be promoted. Further, a more dense and uniform-thickness compound layer can be formed over the entire component surface.

The first heat treatment temperature of the ductile iron may be about 940 to 1,060 ℃. When the heating temperature of the spheroidal graphite cast iron is lower than about 940 ℃ and then air-cooled, the ferrite fraction in the uppermost surface of the spheroidal graphite cast iron may be reduced to less than 60%, so that a compound layer having a predetermined thickness or more may not be ensured. Therefore, the heating temperature of the preferred ductile iron may be 940 ℃ or higher. Here, the unit "%" as a unit of the fraction of ferrite means the area occupied by ferrite in the spheroidal graphite cast iron. Meanwhile, when the heating temperature is about 1,060 ℃ or less, since the austenite structure is not transformed into a melt, a uniform austenite structure can be obtained. Therefore, a maximum heating temperature of about 1,060 ℃ is preferred.

Preferably, the total time for performing the heating step (I) and the heating temperature maintaining step (II) may be about 80 to 100 minutes to ensure that the ferrite fraction is 60% or more, and the time for performing the air cooling step (III) may be about 110 to 130 minutes.

The presence of ferrite in the diffusion layer remaining after the formation of the compound layer can improve the elongation characteristics of the component and increase the toughness of the component. Further, during the oxynitridation heat treatment, surface hardness and compressive residual stress may occur due to the intrusive diffusion of carbon and nitrogen in the ferrite layer, and as a result, fatigue strength may be improved.

Since the deep portion is changed from a structure of a medium pearlite state to a fine pearlite structure, the interlamellar spacing of pearlite may be reduced, so that the rigidity of the component may be improved due to the resistance to sintering modification of the component.

Fig. 3 shows an exemplary microstructure of an exemplary ductile iron (hereinafter, referred to as example 1) heat-treated by a first heat treatment step for forming ferrite according to an exemplary embodiment of the present invention, and fig. 4 shows a microstructure of a ductile iron (hereinafter, referred to as comparative example 1) heat-treated at a heating temperature lower than that of the heat treatment step for forming ferrite.

Fig. 5 shows an exemplary microstructure of conventional ductile iron.

Table 1 below summarizes the heating and cooling conditions for the spheroidal graphite cast irons shown in fig. 3 to 5. Table 2 summarizes the ferrite layer thickness, ferrite fraction, and oxynitride layer thickness of the ductile iron shown in fig. 3-5.

TABLE 1

Classification	Example 1	Comparative example 1	General of
				Temperature of heating	970℃	850℃	X
Time of heating	100 minutes	80 minutes	X
				Cooling method	Air cooling	Air cooling	X
Cooling time	120 minutes	120 minutes	X

TABLE 2

As shown in tables 1-2 and FIGS. 3-5, the thickness (T) of the ferrite layer in example 1_F1) 500 μm, ferrite layer thickness (T) in comparative example 1_F2) Is also 500 μm, the same as the ferrite layer thickness in the example 1, but the ferrite part in the example 1 is 60 to 80% which is higher than the ferrite part in the comparative example 1 by 30 to 50%, and thus the thickness (T) of the oxynitride layer in the example 1 is the same as the ferrite layer thickness in the example 1_ON,1) 20 μm, which is the thickness (T) of the oxynitride layer in comparative example 1_ON,2) About 3.3 times of 6 μm.

In the prior art, the conventional spheroidal graphite cast iron, which has not been subjected to heat treatment for forming ferrite, has a ferrite fraction of 30%, and the conventional spheroidal graphite cast iron may not have a separate ferrite layer that can be distinguished from other portions of the spheroidal graphite cast iron. Thickness (T) of the oxynitride layer in conventional cast iron when no ferrite layer is provided_ON,3) It is also possible to form it to be 4 μm or less, which is lower than the thickness (T) of the oxynitride layer in comparative example 1_ON,2) And compared to the thickness (T) of the oxynitride layer in example 1_ON,1) And significantly smaller.

Therefore, in order to secure an oxynitride layer thickness of 15 μm or more, it is necessary to secure 60 to 80% of ferrite fraction by reducing the pearlite fraction on the surface of the spheroidal graphite cast iron. Therefore, it is important to perform a heat treatment step for forming ferrite before the oxynitridation heat treatment, but the spheroidal graphite cast iron may be heated at a temperature of about 940 to 1,060 ℃ for about 80 to 100 minutes, and then may be air-cooled for about 110 to 130 minutes.

Fig. 6 illustrates an exemplary second heat treatment step for an oxynitridation heat treatment, according to an exemplary embodiment of the invention. The oxynitriding heat treatment step may include a gas nitrocarburizing step (IV), an oxidation treatment step (V), and a furnace cooling step (VI).

The number of parts of the porous layer formed during the gaseous nitrocarburizing step can be controlled in order to impart wear resistance characteristics to the compound layer formed by the oxynitriding heat treatment step. The number of pore layers means the thickness of the pore layer/the thickness of the compound layer, and the compound layer means a layer comprising the pore layer and a nitride compound layer composed of a γ' phase and a phase.

When the fraction of the pore layer is less than about 25%, the oil-containing property may be lowered, and thus the effect of reducing the frictional force may be significantly reduced. When the number of the pore layer is more than 35%, the proportion of the soft pore layer may increase, so that the overall hardness of the compound layer may decrease, and as a result, the wear resistance may deteriorate. Therefore, the compound layer may include about 25 to 35% of the number of the pore layer to improve wear resistance.

During nitrocarburizing treatment by introducing a gas, such as gaseous nitrocarburizing, the fraction of the pore layer may be affected by temperature and time. Preferably, the nitrocarburizing treatment may be performed at about 723 ℃ or less, or specifically, at an Ac1 temperature of about 550 ℃ to 570 ℃ for about 720 minutes to 1,200 minutes. When the temperature is below about 550 ℃ and the treatment time is greater than about 1,200 minutes, the fraction of the porous layer may decrease to less than about 25%. Further, when the temperature is greater than about 570 ℃ and the treatment time is less than about 720 minutes, the fraction of the porous layer may be greater than about 35%. Preferably, the oxidation treatment step may be performed at about 560 ℃ for about 30 minutes, and the furnace cooling step may be performed at about 60 ℃ for about 150 minutes.

FIG. 7 shows oxygen and nitrogen in the second heat treatmentAn image of an exemplary uppermost surface of the exemplary ductile iron after the step of heat-treating. The uppermost surface of the spheroidal graphite cast iron subjected to the heat treatment for forming ferrite and the oxynitriding heat treatment may contain Fe₃O₄The oxide layer 10 and the compound layer 20, the compound layer may have a thickness of about 15 to 30 μm, and the thickness may be uniform.

To improve durability, the compound layer may be formed to have a thickness of about 15 μm to 30 μm, which may be uniform. When the compound layer is formed to a thickness of less than about 15 μm, the hardness of the compound layer may be reduced to less than about HV 600, so that the compound layer may be easily peeled off when the component is handled, and as a result, the wear resistance function may be lost. Further, when the thickness of the compound layer is formed to be larger than 30 μm, the compound layer may have sufficient hardness so that brittleness of HV 1200 level occurs, whereby fatigue failure from the compound layer occurs when the component is operated. Therefore, it is preferable that the thickness of the compound layer may be about 15 μm to 30 μm, thereby allowing the compound layer to have a hardness of HV 600 to 1000.

The void layer 21 may be a void layer of an oxynitride layer present under the oxide layer 10 and may be relatively flexible in the compound layer. The porous layer may contain lubricating oil in a transmission, engine, or the like, so that friction may be reduced when operating the component. Therefore, when an optimum number of void layer portions is secured, the influence of occurrence of wear can be minimized by improving the lubrication characteristics with the opposing member. Therefore, the number of the porous layer is preferably about 25 to 35%.

The nitride compound layers 22 and 23 may include a γ' phase 22 and a phase 23. The nitride compound layers 22 and 23 may exist under the pore layer 21, the γ 'phase 22 may be located under the pore layer, and the phase 23 may be located under the γ' phase 22. Also, the fraction of the preferred phase 23 in the nitride compound layers 22 and 23 may be about 80% or more. Meanwhile, the unit% refers to an area occupied by the phase in the nitride compound layer.

Fe is produced when the uppermost surface of the nodular cast iron begins to be solutionized with nitrogen during the gas nitrocarburizing treatment₄N gamma' phase 22. The gamma prime phase 22 may generally have a nitrogen concentration of about 0.1% or greaterGenerated when the nitrogen concentration is about 6% or more, Fe is generated_2,3 Phase 23 of N, and phase 23 may also be transformed into gamma' phase 22.

The gamma' phase 22 may affect the strength of the compound layer and the phase 23 may affect the toughness. Overall, the higher the proportion of phase 23, the better the corrosion and wear resistance properties of the resulting cast iron. In addition, the higher the proportion of phase 23, the more nitrogen can be solutionized. When the phase 23 is about 80% or less, the surface-solubilized nitride may be relatively small, which may be disadvantageous in terms of seizure resistance and wear resistance, and thus the fraction of the phase 23 may be adjusted to about 80% or more. Further, the fraction of the phase 23 can be adjusted to about 80% or more to form a compound layer having a hardness of about HV 600 to 1000.

TABLE 3

Classification	Comparative example 2	Example 2	Comparative example 3
				K_N	1.5	2.0	3.0
Temperature (. degree.C.)	540	570	590
				Time (hr)	22	16	10
Number of pore layers	10％	25％	40％

Table 3 summarizes the conditions and the number of pore layers of the gaseous nitrocarburizing step in comparative example 2, example 2 and comparative example 3. K in Table 3_NIndicating the ratio of the partial pressure of ammonia to the partial pressure of hydrogen in the furnace during the oxynitridation heat treatment

Fig. 8A is a microstructure of comparative example 2, and fig. 8B is a result of evaluating seizure resistance of comparative example 2. Fig. 9A is the microstructure of example 2, and fig. 9B is the result of evaluating seizure resistance of example 2. Fig. 10A is a microstructure of comparative example 3, and fig. 10B is a result of evaluating seizure resistance of comparative example 3. The x-axis and y-axis in fig. 8B, 9B and 10B are time (min) and load (kg), respectively, and the load interval is 500 kg.

Seizure and wear resistance evaluations were based on the FA L EX PIN & VEE B L OCK test, with a load applied to the middle PIN by right and left side V-blocks (V-B L OCK) per ASTM d3233a, and the test was terminated when the BRASS lock PIN (BRASS L locking PIN) broke when seizure occurred between the high speed rotating PIN and block.

As shown in table 3 and fig. 8 to 10, the fixing pin was broken after holding the maximum load of 2,100kg for 1 minute in comparative example 2, while the same maximum load was held for 20 minutes or more in example 2, but the fixing pin was not broken, and the fixing pin was broken after holding the same maximum load for 6 minutes in comparative example 3. Meanwhile, even if Table 3 does not contain this case, when K_N2.7 and the temperature and time of the gaseous nitrocarburizing step were 570 ℃ and 720 minutes, respectivelyThe number of pore layers was 35% in the hour, and the results were at the same level as in example 2 in Table 3.

Fig. 11A is a photograph after evaluating seizure resistance of example 2, and fig. 11B is a photograph after evaluating seizure resistance of comparative example 3. As shown in fig. 11A and 11B, it can be seen that no galling and seizure occurred in example 2, while those occurred in comparative example 3.

As demonstrated in comparative example 2, example 2 and comparative example 3, in order to improve the wear resistance of cast iron, about 25% or more of the number of void layers can be ensured, but less than 40%, or specifically about 35% or less of the number of void layers can be ensured.

According to various exemplary embodiments of the present invention, cast iron may be applied to a control finger (control finger), but is not limited thereto, and a complex oxynitridation heat treatment may be performed compared to a component requiring high strength while requiring durability as compared to conventional cast iron. Since cast iron has been used for various automobile parts such as differential case (diffcase) or control body, a compound layer having a thickness of about 15 μm or more can be formed on the surface layer by subjecting a ductile iron grade FCD450 to 700 to, for example, a composite oxynitriding heat treatment of the present invention.

The present invention has been described in detail through representative embodiments, but it will be understood by those skilled in the art to which the present invention pertains that various modifications may be made to the above embodiments to the extent that they do not depart from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the above-described embodiments, but should be determined not only by the appended claims but also by all changes or modifications derived from the claims and their equivalents.

Claims

1. A method for heat treating the surface of spheroidal graphite cast iron, comprising the steps of:

performing a first heat treatment on the ductile iron to form a ferrite; and

a second heat treatment is performed to oxynitridize the ductile iron.

2. The method of claim 1, wherein the first thermal treatment comprises: heating the nodular cast iron to a first heating temperature of 940-1,060 ℃; and maintaining the first heating temperature, wherein the total time of the first heat treatment is 80 to 100 minutes.

3. The method of claim 2, wherein the first thermal treatment further comprises: and cooling the nodular cast iron in air for 110-130 minutes.

4. The method of claim 1, wherein the second thermal treatment comprises:

nitrocarburizing by introducing gas;

oxidizing the ductile iron; and

cooling;

wherein the nitrocarburizing is performed at a second heat treatment temperature of 550 ℃ to 570 ℃ for 720 minutes to 1,200 minutes.

5. The method of claim 4, wherein the cooling is furnace cooling.

6. A ductile iron comprising:

an oxide layer; and

a layer of a compound, the layer comprising a metal oxide,

wherein the thickness of the compound layer is 15 to 30 μm.

7. The ductile iron of claim 6 wherein the thickness of said compound layer is uniform.

8. The ductile iron according to claim 6, wherein the uppermost surface of the ductile iron comprises 60-80% of ferrite fraction.

9. The ductile iron of claim 8 wherein said compound layer comprises:

a void layer; and

a nitride compound layer comprising a gamma' phase and a phase.

10. The ductile iron according to claim 9, wherein said nitride compound layer consists of a γ' phase and a phase.

11. The ductile iron according to claim 9, wherein said nitride compound layer comprises 80% or more of phase.

12. The ductile iron according to claim 9, wherein said compound layer has a hardness of HV 600-1000.

13. The ductile iron according to claim 9, wherein said compound layer comprises 25-35% parts of a porosity layer.

14. The ductile iron according to claim 10, wherein said compound layer comprises 25-35% parts of a porosity layer.

15. The ductile iron according to claim 9, wherein said compound layer comprises said porosity layer, said γ' phase and said phase from an uppermost surface of said ductile iron.

16. The ductile iron according to claim 6, comprising said oxide layer and said compound layer from an uppermost surface of said ductile iron.

17. A vehicle component comprising the ductile iron according to claim 6.

18. A vehicle incorporating the vehicle component of claim 17.