WO2007142373A1

WO2007142373A1 - Method for nitriding metal in salt bath and metal manufactured by its method

Info

Publication number: WO2007142373A1
Application number: PCT/KR2006/002198
Authority: WO
Inventors: Hee Won Jung; Young June Park; Dong Sam Kim; Dong Nyung Lee; Yinzhong Shen; Kyu Hwan Oh
Original assignee: Iljin Light Metal Co., Ltd.
Priority date: 2006-06-08
Filing date: 2006-06-08
Publication date: 2007-12-13
Also published as: JP4806722B2; JP2009540120A; BRPI0621724A2

Abstract

Provided is a method for nitriding a metal in a salt bath by using a non-cyanide salt and a nitrided metal manufactured using the same. The method includes the steps of: immersing at least one salt selected from a group consisting of Ca(NO), NaNO and NaNO into the salt bath; melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. Nitriding in non-cyanide salts, such as sodium nitrate (NaNO), sodium nitrite (NaNO), calcium nitrate(Ca(NO) ) and their mixtures, is capable of solving an environmental pollution problem and reducing a cost. Also, the method is capable of increasing nitrided depth of the metal two to six times compared to a conventional nitriding methods. As a result, the method can be carried out in various application fields.

Description

METHOD FOR NITRIDING METAL IN SALT BATH AND METAL MANUFACTURED BY ITS METHOD

Technical Field

[1] The present invention relates to a method for nitriding metal in a salt bath and nitrided metal manufactured using the same; and, more specifically, to a method for nitriding iron or steels by using non-cyanide salt bath, and nitrided iron or steels manufactured using the same.

[2]

Background Art

[3] Steels have been widely used for machine parts because of their inherent properties.

To be used for machine parts, steels are usually first heat-treated to impart thereto strength, toughness and durability, all of which are the qualities machine parts require. In addition, for machine parts that are often exposed to corrosive environment, surfaces thereof are further heat-treated to impart thereto corrosion resistance.

[4] Nitriding is one of the methods for processing the metal surface to impart thereto a corrosion resistance. The nitriding methods include gas nitriding using NH gas, salt

3 bath nitriding using of KCN, KCNO etc., gas nitrocarburizing (carbo-nitriding) using a mixture of NH gas and RX gas, i.e., endothermic gas, and ion nitriding involving an insertion of a mixture of N and H gas into plasma.

2 ! ° ^r

[5] Generally, although nitriding is applied to steels to improve their abrasion (wear) resistance and fatigue resistance, it can also be carried out to improve the corrosion resistance thereof.

[6] Of the nitriding methods mentioned hereinabove, the salt bath nitriding is most widely used for a variety of machine parts including automobile components, because properties of chemicals for the salt bath and their melting points can be freely controlled to provide stability through a wide range of process temperatures without eroding the surface of the object being treated. To be more specific, in addition to its excellent thermal conductivity, soaking properties and easily controllable processing conditions, it is cheaper to design and maintain, compared with other nitriding methods. For example, it is easy to operate the salt bath, and the heating rate is 4 times faster in the salt bath than in the atmosphere. The salt bath is especially suitable to heat-treatment of high speed steel which is sensitive to crystal (grain) growth. When a material treated in a salt bath comes into a contact with the atmosphere, a film including the salt bath constituents is formed on the surface thereof, the film preventing oxidation by preventing the material from a direct contact with the atmosphere. Furthermore, the surface of the material thus treated is rather clean, making the salt bath an ideal heat-treatment for both mass production and small- lot-sized production.

[7]

Disclosure of Invention Technical Problem

[8] Cyanide-containing salt is generally used for a salt bath nitriding method, producing cyanide ions inside the bath. Since the cyanide ion is classified as a toxic chemical, it must be carefully and tightly controlled, and this can be an expensive proposition. Also, there is a problem of a cost involved for processing waste water and gas.

[9] Further, the nitriding treatment in a molten salt including cynides is a nitro- carburizing (carbo-nitriding) method involving a simultaneous penetration of carbon and nitrogen. It has a shortcoming in that although the surface hardness of the material thus treated improves significantly, the tensile strength gets only slightly enhanced. The conventional salt bath nitriding method using a cyanide salt also has a problem that its applications are limited to molds or gears since the depth to which the material can be nitrided is limited.

[10] It is, therefore, an object of the present invention to provide a method for nitriding a metal using non-cyanide salts, and a nitrided metal manufactured using the same.

[11] It is another object of the present invention to provide a salt bath nitriding method for nitriding a metal, in which nitrogen penetrates into the metal, and a nitrided metal manufactured using the same.

[12] It is yet another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of increasing hardness and tensile strength of the metal to be treated, and a nitrided metal manufactured by using the same.

[13] It is still another object of the present invention to provide a salt bath nitriding method for nitriding a metal, capable of maximizing a nitriding depth, and a nitrided metal manufactured using the same.

[14]

Technical Solution

[15] In accordance with one aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing at least one salt selected from a group consisting of Ca(NO ) , NaNO and NaNO into

3 2 3 2 the salt bath; b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and c) submerging the metal in the salt bath. [16] In accordance with another aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing a mixed salt including at least one from a group consisting of KNO and KNO , and at least one from a group consisting of Ca(NO ) , NaNO and NaNO into the salt bath; b) melting the salt by heating and maintaining the molten salt at a predetermined temperature; and c) submerging the metal in the salt bath.

[17] At this time, the predetermined temperature is within a range of 400⁰C to 700⁰C, and the submerging time is within a range of 1 minute to 24 hours.

[18] In accordance with another aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing a KNO salt into the salt bath; b) melting the salt by heating and maintaining the molten salt at 400⁰C to 620⁰C; and c) submerging the metal for less than 8 hours in the salt bath.

[19] In accordance with another aspect of the present invention, there is provided a method for nitriding a metal in a salt bath, the method including the steps of: a) immersing a KNO salt into the salt bath; b) melting the salt by heating and maintaining the molten salt at a temperature being more than 62O⁰C to equal to or less than 64O⁰C; and c) submerging the metal for less than 1 hour in the salt bath.

[20] When iron is nitrided in the salt bath of the present invention, the iron can be nitrided into a depth of 0.1 D to 3.0D from its surface.

[21] When the steel is nitrided in the salt bath of the present invention, the steel can be nitrided into a depth of 0.1 D to 3.0D from its surface.

[22] The steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel.

[23] The ultra-low carbon steel nitrided by the present invention has the surface hardness ranging from more than 120 Hv to equal to or less than 450 Hv. The low carbon steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. The medium carbon steel has the surface hardness being more than 130 Hv to equal to or less than 420 Hv. The high carbon steel has the surface hardness being more than 150 Hv to equal to or less than 400 Hv. The alloy steel has the surface hardness being more than 200 Hv to equal to or less than 410 Hv. The surface hardness of the steels nitrided by the present invention can be improved by a maximum of 420 Hv. The surface hardness of the iron nitrided by the present invention is also improved.

[24] The ultra-low carbon steel nitrided by the present invention has the tensile strength ranging from more than 35 kgf/D to equal to or less than 110 kgf/D. The low carbon steel has the tensile strength ranging from more than 45 kgf/D to equal to or less than 110 kgf/D. The medium carbon steel has the tensile strength ranging from more than 45 kgf/ D to equal to or less than 100 kgf/D. The high carbon steel has the tensile strength ranging from more than 60 kgf/D to equal to or less than 95 kgf/D. The alloy steel has the tensile strength ranging from more than 55 kgf/D to equal to or less than 110 kgf/D. The tensile strength of iron can be improved by the nitriding method of the present invention.

[25] The salt-bath nitriding method of the present invention can be applied to the iron, the carbon steel including the ultra-low carbon steel having a carbon content of at least 0.0001wt% to less than 0.13wt%, the low carbon steel having a carbon content of at least 0.13wt% to less than 0.2wt%, the medium carbon steel having a carbon content of at least 0.21wt% to less than 0.51wt%, and the high carbon steel having a carbon content of at least 0.51wt% to less than 2.0wt%, the steel having a chrome content of 0. lwt% to 1.5wt%, the steel having a molybdenum content of 0.05 wt% to 0.5wt%, the steel having a nickel content of 0.1 wt% to 10wt%, the steel having a manganese content of 0. lwt% to 2.0wt%, the steel having a boron content of 0.001wt% to 0. lwt%, the steel having a titanium content of 0.00 lwt% to 0. lwt%, the steel having a vanadium content of 0.05 wt% to 0.15wt%, the steel having a niobium content of 0.005 wt% to 0.1wt%, and the steel having an aluminum content of 0.005 wt% to 0. lwt%. Also, the salt bath nitriding method of the present invention can be applied to the alloy steel including at least two kinds of the steels suggested above.

[26]

Advantageous Effects

[27] The present invention is directed to nitriding steels in non-cyanide salts, such as sodium nitrate (NaNO ), sodium nitrite (NaNO ), calcium nitrate(Ca(NO ) ) and their

3 2 3 2 mixtures. Nitriding steels in non-cyanide salts is capable of solving an environmental pollution problem and reducing a cost. [28] The present invention is capable of increasing nitrided depth of the metal two to six times compared to conventional nitriding methods, so as to be carried out in various application fields. [29] In addition, since the present invention can be applied to bulk hardnening as well as surface hardening of steels by increasing hardness and tensile strength of the metal, it is possible to apply the present invention to many fields including light and highly strong automobile components and diverse structural members which require improved wear resistance, corrosion resistance and fatigue life. [30]

Brief Description of the Drawings [31] Fig. 1 is a graph illustrating relationship between a nitriding time and a hardness profile in a steel nitrided in accordance with a first embodiment of the present invention; [32] Fig. 2 is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention; [33] Fig. 3 is a graph illustrating relationship between a nitriding temperature and the hardness profile in the steel nitrided in accordance with the first embodiment of the present invention; [34] Fig. 4 is a graph illustrating relationship between the nitriding time and the surface hardness of the steel nitrided in accordance with the third embodiment of the present invention; [35] Fig. 5 is a graph illustrating relationship between the nitriding temperature and time and the hardness profile in the steel nitrided in accordance with the third embodiment of the present invention; [36] Fig. 6 is a graph illustrating hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention; [37] Fig. 7 is a graph illustrating the hardness profile in the steel nitrided in accordance with the fifth embodiment of the present invention; and [38] Fig. 8 is a graph illustrating relationship between a mixture ratio of a mixed salt and the hardness profile in the steel nitrided in accordance with the fifth embodiment of the present invention. [39]

Mode for the Invention

[40] Hereinafter, the present invention will be described in more detail.

[41] In nitriding of a metal, the present invention incorporates therein the nitrogen dissolution principle involving a non-cyanide molten salt, more particullarly, NaNO ,

NaNO , Ca(NO ) and mixtures thereof as a molten salt, as opposed to a conventional nitriding method such as a nitrocarburizing (carbo-nitriding) method involving the use of cyanides, e.g., KCN and NaCN, as the molten salt wherein carbon and nitrogen are simultaneously diffused into the metal. [42] The method for nitriding the metal in accordance with the present invention involves immersing at least one salt from a group consisting of NaNO , NaNO and

Ca(NO ) into a salt bath, melting the salt and maintaining the molten salt at a pre-

3 2 determined temperature ranging from 400⁰C to 700⁰C. Subsequently, the metal to be nitrided is submerged in the bath for 1 minute to 24 hours.

[43] During this time, nitrogen, oxygen and nitrogen oxides are generated from the non- cyanide molten salts of the present invention, NaNO , NaNO and Ca(NO ) and mixtures thereof, by the following reaction formula 1 and reaction formula 2.

[44]

[45] The following reaction formula 1 represents nitrogen formation reaction in the molten salt bath of NaNO and NaNO .

3 2

[46] [Reaction formula 1] [47] NaNO → NaNO + 1/20

3 2 2

[48] 2NaNO → Na 0 + NO + NO

2 2 2

[49] 2NaNO + 2NO → 2NaNO + N

2 3 2

[50]

[51] The following formula 2 shows nitrogen formation reaction in the molten salt bath of Ca(NO ) .

[52] [Reaction Formula 2]

[53] Ca(NO₃)₂ → CaO + 2NO₂ + 1/2O₂

[54] 2NO → 20 + N

2 2 2

[55] As shown in Table 1, those metals nitrided, including carbon steel(including ultra- low carbon steel, low carbon steel, medium carbon steel and high carbon steel), alloy steel and iron using the salt-bath nitriding method in accordance with the present invention are nitrided to a depth of O. ID to 3.OD from the surface. The range of nitrided depth/diffusion layer thickness obtained through the present invention is 2 to 6 times larger than that obtained using the conventional nitriding methods, meaning that a nitrided/diffusion layer formed using the nitriding method of the present invention extends from the surface to the metal inner part, and consequently the surface hardness and tensile strength of the metal also improve compared to those of the metal nitrided using the conventional nitriding method. References for the table 1 are as follows:

[56] [1] B. Finnem, Bad und Gasnitrieren. Vol.18, Betriebsbuecher Carl-Hausner-Verlag

, Muenchen(1965)

[57] [2] Tufftride Information 15. DEGUSSA Durfeerit Abteilung

[58] [3] H.Eiraku, K.Shinkawa, Y.Yoneyama, and M.Higashi, "Characteristics of

Palsonite (Low temperature salt bath nitriding)," JSHT Conf., No. l, 49-50(1998)

[59] [4] E.A.Mattision, K.Frisk, and A. Melander, "Microstructure evolution during the combination hardening process of nitriding and induction hardening," in: 5 ASM- HTSE Europe(2002), pp. 209-219

[60] [5] W.Junyi, P.Lin, and Z.Hul,"Effect of rare earth on ionic nitriding process," in:

1st Conf. Heat Treatment of Materials, May(1998), pp. 57-61

[61] [6] S.Kondo, Y.Izawa, O. Nakano, S.Uchida, and M.Onoda, "Influence of white layer produced by gas nitriding on fatigue strength of compressive spring. " J. JSHT, 36(1), pp. 34-40(1996)

[62] [7] J. Georges, "TC plasma nitriding," in: 12th IFHTSE Melbourne, Australia

(2000), p229; Heat treatment Met., No.2, pp. 33-37(2001)

[63] [8] T.Bell, Y.Sun, K.Mao, and P.Buchhagen, "Modeling plasma nitriding,"

Advanced Mater. Pro., No.8, 40Y-40BB(1996)

[64] [9] T.Bell, Y.Sun, Z.Lin, and M.Yan, "Rare earth surface engineering," Heat

Treatment Mat., 27(1), pp. 12-13(2000) [65] Table 1

[66] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. [67]

[68] [First Embodiment]

[69] In accordance with the first embodiment of the present invention, steel is nitrided using the NaNO molten salt. [70] The nitrided steel includes ultra- low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel. [71] Each of the ultra- low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel is submerged in the NaNO molten salt bath for 2 hours at a temperature of 500⁰C . [72] Table 2 shows changes in surface hardness and tensile strength of the samples being nitrided in the molten salt bath, wherein the hardness was measured using a Vickers hardness tester under a load of 1 kgf. [73] In case of ultra- low carbon steel, the surface hardness increases by 119% and the tensile strength increases by 47%. In case of low carbon steel, the surface hardness increases by 47% and the tensile strength increases by 19%. [74] In case of medium carbon steel, the surface hardness increases by 32% and the tensile strength increases by 18%. In case of high carbon steel, the surface hardness increases by 28% and the tensile strength increases by 16%. In case of alloy steel, the surface hardness increases by 24% and the tensile strength increases by 17%. [75] That is, in case of steel, the surface hardness increases by 20% to 120% and the tensile strength increases by 15% to 50%. [76] The differences in the amount of increases shown in the surface hardness depending on the steel type can be attributed to the differences in the nitrogen diffusion rate associated with each type of steels determined by the carbon content therein. [77] Table 2

[78] Fig. 1 is a graph showing the hardness distribution in the thickness direction of the ultra-low carbon steel before(As) and after nitriding in the NaNO molten salt bath at 500⁰C for 30 minutes, 1 hour, 2 hours and 5 hours, respectively.

[79] The nitrided depth or the diffusion depth increases with increasing nitriding time, and the hardness decreases with increasing distance from the surface because the nitrogen concentration decreases with increasing distance from the surface. When the steel is nitrided for 5 hours, it can be seen that the steel is nitrided to a depth of about 0.6mm from the surface.

[80] Fig. 2 shows the hardness distribution along the thickness direction of low carbon steel nitrided in the NaNO molten-salt bath at 68O⁰C for 3, 6, 12 and 24 hours, respectively, wherein the hardness is measured using a Vickers hardness tester under a load of 3 kgf.

[81] As shown in Fig. 2, the nitrided depth or the diffusion depth of the steel increases with increasing nitriding time. The nitrided depth of the steel after nitriding for 24 hours is about 3.0mm, which is 6 times deeper than that obtained from the conventional nitriding method.

[82] Also, the surface hardness after nitriding is 450 Hv, which is more than 4 times higher than that of the non-treated specimen.

[83] Accordingly, the nitriding method of the present invention can increase the nitrided depth of the steel by 2 to 6 times compared to the conventional cyanide-based salt bath nitriding method.

[84] Fig. 3 shows hardness distributions along the thickness direction of the ultra-low carbon steel before and after nitriding in the NaNO molten-salt bath at 500⁰C and 600⁰C for 3 hours. The nitrided depth of the steel nitrided at 600⁰C is 3 times deeper than that of the steel nitrided at 500⁰C. The surface hardness of the steel nitrided at 600⁰C is 100 Hv higher than that of the steel nitrided at 500⁰C. That is, the surface hardness and nitrided depth of steel increase with increasing nitriding temperature.

[85] Table 3 shows changes in tensile strength of ultra-low carbon steel depending on the nitriding temperature wherein the samples are nitrided for 3 hours at 450⁰C, 500⁰C, 55O⁰C and 600⁰C, respectively, using the salt-bath nitriding method of the first embodiment of the present invention.

[86] As shown in Fig. 3, in case of the nitriding temperature of 450⁰C, the tensile strength increases by 5%. As the temperature increases, the tensile strength of the steel also increases. Accordingly, when the temperature is 600⁰C, the tensile strength increases by 134%.

[87] Table 3

[88] That is, since it is possible to simultaneously improve the hardness and the tensile strength by nitriding the steel according to the first embodiment, the present invention can be applied to diverse fields including diverse components and structural members.

[89] [90] [Second Embodiment] [91] In accordance with the second embodiment of the present invention, steel is nitrided by using the NaNO molten salt.

[92] Steels including ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel are submerged in the salt bath at 450⁰C for 2 hours. [93] Table 4 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf.

[94] For ultra- low carbon steel, the surface hardness increases by 54% and the tensile strength increases by 21%. For low carbon steel, the surface hardness increases by 32% and the tensile strength increases by 15%. [95] For medium carbon steel, the surface hardness increases by 19% and the tensile strength increases by 13%. For high carbon steel, the surface hardness increases by 18% and the tensile strength increases by 12%.

[96] For alloy steel, the surface hardness increases by 17% and the tensile strength increases by 14%. [97] That is, in case that steels are nitrided by the molten salt bath nitriding method of the second embodiment of the present invention, the surface hardness increases by 15% to 60%, and the tensile strength increases by 10% to 25%.

[98] Accordingly, the molten salt bath nitriding method in accordance with the second embodiment of the present invention also increases the surface hardness and the tensile strength of the steels.

[99] Table 4

[100] [101] [Third Embodiment] [102] In the third embodiment of the present invention, steel is nitrided using the KNO molten salt.

[103] The steel to be nitrided is Interstitial-Free (IF) steel, which includes carbon (C) of 0.003wt%, manganese (Mn) of 1.23wt%, aluminum (Al) of 0.037wt%, titanium (Ti) of 0.027wt%, phosphorus (P) of 0.050wt%, nitrogen (N) of 0.002wt% and sulfur (S) of 0.008wt%.

[104] The IF steel is nitrided in the KNO molten bath at 56O⁰C, 580⁰C, 600°C, 62O⁰C and 64O⁰C, respectively. [105] Fig. 4 shows the surface hardness of the IF steel nitrided in the KNO molten bath

3 as functions of time and temperature. [106] As shown in Fig. 4, as the nitriding time and temperature increase, the surface hardness increases under most temperature conditions. Although the increase of the hardness can be explained as solution strengthening, the present invention is not limited to this theory.

[107] However, when the nitriding time in the KNO molten salt at 620⁰C exceeds 8 hours, or the nitriding time in the KNO molten salt at 640⁰C exceeds one hour, the surface hardness decreases. It is understood that this decrease in the surface hardness is caused by the formation of the nitrided layer in the grain boundaries of the IF steel.

[108] Fig. 5 shows the hardness distributions along the thickness direction of the IF steel nitrided by the third embodiment of the present invention.

[109] The IF steel is nitrided for 16 hours in a KNO molten salt at 56O⁰C and for 8 hours in a KNO molten salt at 56O⁰C, 58O⁰C 600⁰C and 62O⁰C, respectively.

3

[110] Referring to Fig. 5, the hardness of the IF steel decreases with increasing depth from the surface because the nitrogen concentration deceases with increasing distance from the steel surface. When the nitrided depth is defined as the distance between the surface and the position where the hardness value is equaled to 110% of that of the center of the IF steel before nitriding, the nitrided depth formed in each condition ranges from about 1.38mm to 1.5mm, which is 3 to 5 times thicker than the thickness of the nitrided layer formed using the conventional method.

[I l l]

[112] [Fourth Embodiment]

[113] In the fourth embodiment of the present invention, steel is nitrided using the Ca(NO

) molten salt.

[114] The steel to be nitrided in the fourth embodiment is low carbon steel.

[115] Since Ca(NO ) is highly hygroscopic at a room temperature, including combined water, it is preferred to use Ca(NO ) after removing moisture by heating for a pre-

3 2 determined time.

[116] The fourth embodiment of the present invention includes the processes of removing moisture by heating Ca(NO ) for 4 hours at 100⁰C to 15O⁰C , heating Ca(NO ) to

3 2 3 2

58O⁰C to form the Ca(NO ) molten-salt bath and submerging the low carbon steel in the bath for 3 hours. [117] Fig. 6 is a graph showing the surface hardness profile in low carbon steel nitrided by the fourth embodiment of the present invention. [118] As shown in Fig. 6, the low carbon steel nitrided by the fourth embodiment is nitrided to a depth of 0.5D from the surface, and has the surface hardness that is 2 times higher than the surface hardness (As) of the steel before nitriding. [119]

[ 120] [Fifth embodiment]

[121] In the fifth embodiment of the present invention, steel is nitrided using a molten mixture of KNO and NaNO .

3 3

[122] In the fifth embodiment of the present invention, the low carbon steel is nitrided in the molten mixture of KNO and NaNO whose mixture ratios are 1:1, 8:2 and 2:8. [123] Table 5 shows the surface hardness values of steels nitrided by the fifth embodiment of the present invention. Various types of steel are submerged in the molten mixture of KNO and NaNO whose ratio is 1:1 for 12 or 24 hours continuously at 65O⁰C.

[124] At this time, the hardness is measured using a Vickers hardness tester under a load of 3 Df. [125] The hardness values of the steels nitrided in the mixture of KNO and NaNO

3 3 increase by 69% to 251% depending on the steel type. [126] Table 5

[127] Various steels are submerged in the mixture of KNO and NaNO whose ratio is 1 : 1 at 58O⁰C, and changes in surface hardness and tensile strength of the nitrided steels depending on nitriding time are measured.

[128] As shown in Table 6, nitriding in accordance with the fifth embodiment of the present invention increases the hardness and the tensile strength of all the steels.

[129] The hardness and tensile strength increase with increasing nitriding time. [130] Table 6

[131] Fig. 7 is a graph showing the hardness profiles of steel nitrided at 68O⁰C for 200 minutes in the KNO bath, the NaNO bath and the 50%KNO -50%NaNO mixture

3 3 3 3 bath.

[132] The hardness was measured using a Vickers hardness tester.

[133] In Fig. 7, the steel nitrided in the mixture bath has a nitrided depth of 1.5D and a surface hardness of 160Hv, which is higher than that of the steel nitrided in the single salt baths and 3 times higher than that of the steel before nitriding. [134] Fig. 8 is a graph showing the hardness profiles of the low carbon steel nitrided in the 80%KNO -20%NaNO bath and 20%KNO -80%NaNO bath at 65O⁰C for 4 hours,

3 3 3 3 respectively.

[135] As shown in Fig. 8, the surface hardness of the steel nitrided in the mixture baths is about 2 times higher than that of the steel before nitriding.

[136] The terms and words used in the present specification and claims should not be construed to be limited to the common or dictionary meaning, because an inventor defines the concept of the terms appropriately to describe his/her invention as best he/ she can. Therefore, they should be construed as a meaning and concept fit to the technological concept and scope of the present invention.

[137] Therefore, the embodiments and structure described in the present specification are nothing but one preferred embodiment of the present invention, and do not represent all of the technological concept and scope of the present invention. Therefore, it should be understood that many equivalents and modified embodiments that can substitute those described in this specification exist.

Claims

[1] A method for nitriding a metal in a salt bath, comprising the steps of: immersing at least one salt selected from a group consisting of Ca(NO ) , NaNO and NaNO into the salt bath:

2 melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. [2] A method for nitriding a metal in a salt bath, comprising the steps of: immersing a mixed salt including at least one selected from a group consisting of KNO and KNO , and at least one selected from a group consisting of Ca(NO ) , NaNO and NaNO into the salt bath;

3 2 melting the salt by heating and maintaining the molten salt at a predetermined temperature; and submerging the metal in the salt bath. [3] The method as recited in claim 1 or 2, wherein the predetermined temperature is within a range of 400⁰C to 700⁰C.

[4] The method as recited in claim 1 or 2, wherein, in the step of submerging the metal in the salt bath, a submerging time is within a range of 1 minute to 24 hours.

[5] A method for nitriding a metal in a salt bath, comprising the steps of: immersing a KNO salt into the salt bath; melting the salt by heating and maintaining the molten salt at a temperature ranging from 400⁰C to 62O⁰C; and submerging the metal for less than 8 hour in the salt bath.

[6] A method for nitriding a metal in a salt bath, comprising the steps of: immersing a KNO salt into the salt bath;

3 melting the salt by heating and maintaining the molten salt at a temperature ranging more than 620⁰C to equal to or less than 640⁰C; and submerging the metal for less than 1 hour in the salt bath.

[7] The method as recited in any one of claims 1, 2, 5 or 6, wherein the metal is one of iron and steels.

[8] A metal nitrided in a salt bath including at least one selected from a group consisting of Ca(NO ) , NaNO and NaNO , wherein the metal is iron and the

3 2 3 2 iron is nitrided to a depth of 0.1D to 3.0D from the surface.

[9] A metal nitrided in a mixture salt bath including at least one selected from a group consisting of KNO and KNO , and at least one selected from a group

3 2 consisting of Ca(NO ) , NaNO and NaNO , the metal comprising: iron, wherein the iron is nitrided into a depth of 0.1 D to 3.0D from the surface.

[10] A metal nitrided in a salt bath including at least one selected from a group consisting of Ca(NO ) , NaNO and NaNO , the metal comprising: steel, wherein the steel is nitrided into a depth of 0.1 □ to 3.0D from the surface.

[11] A metal nitrided in a mixture salt bath including at least one selected from a group consisting of KNO and KNO , and at least one selected from a group consisting of Ca(NO ) , NaNO and NaNO , the metal comprising:

3 2 3 2 steel, wherein the steel is nitrided into a depth of 0.1 □ to 3.0D from the surface. [12] The metal as recited in claim 10 or 11, wherein the steel is at least one selected from a group consisting ultra- low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel. [13] The metal as recited in claim 12, wherein the ultra- low carbon steel has a surface hardness being more than 120Hv to equal to or less than 450Hv. [14] The metal as recited in claim 12, wherein the low carbon steel has a surface hardness being more than 200Hv to equal to or less than 410Hv. [15] The metal as recited in claim 12, wherein the medium carbon steel has a surface hardness being more than 130Hv to equal to or less than 420Hv. [16] The metal as recited in claim 12, wherein the high carbon steel has a surface hardness being more than 150Hv to equal to or less than 400Hv. [17] The metal as recited in claim 12, wherein the alloy steel has a surface hardness being more than 200Hv to equal to or less than 410Hv. [18] The metal as recited in claim 13, wherein the ultra- low carbon steel has a tensile strength being more than 35 kgf/D to equal to or less than 110 kgf/D. [19] The metal as recited in claim 14, wherein the low carbon steel has a tensile strength being more than 45 kgf/D to equal to or less than 110 kgf/D. [20] The metal as recited in claim 15, wherein the medium carbon steel has a tensile strength being more than 45 kgf/D to equal to or less than 100 kgf/D. [21] The metal as recited in claim 16, wherein the high carbon steel has a tensile strength being more than 60 kgf/D to equal to or less than 95 kgf/D. [22] The metal as recited in claim 17, wherein the alloy steel has a tensile strength being more than 55 kgf/D to equal to or less than 110 kgf/D. [23] The metal as recited in claim 22, wherein a chrome content of the steel ranges from 0.1 wt% to 1.5wt%. [24] The metal as recited in claim 22, wherein a molybdenum content of the steel ranges from 0.05 wt% to 0.5wt%. [25] The metal as recited in claim 22, wherein a nickel content of the steel ranges from 0.1wt% to 10wt%. [26] The metal as recited in claim 22, wherein a manganese content of the steel ranges from 0.1 wt% to 2.0wt%. [27] The metal as recited in claim 22, wherein a boron content of the steel ranges from 0.001wt% to 0.1wt%. [28] The metal as recited in claim 22, wherein a titanium content of the steel ranges from 0.001wt% to 0.1 wt%. [29] The metal as recited in claim 22, wherein a vanadium content of the steel ranges from 0.05wt% to 0.15wt%. [30] The metal as recited in claim 22, wherein a niobium content of the steel ranges from 0.005 wt% to 0.1 wt%. [31] The metal as recited in claim 22, wherein an aluminum content of the steel ranges from 0.005wt% to 0.1wt%. [32] The metal as recited in claim 18, wherein the ultra- low carbon steel has a carbon content ranging from at least 0.0001wt% to less than 0.13wt%. [33] The metal as recited in claim 19, wherein the low carbon steel has a carbon content ranging from at least 0.13wt% to less than 0.2wt%. [34] The metal as recited in claim 20, wherein the medium carbon steel has a carbon content ranging from at least 0.21wt% to less than 0.51wt%. [35] The metal as recited in claim 21, wherein the high carbon steel has a carbon content ranging from at least 0.51wt% to less than 2.0wt%.