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: immerging at least one salt selected from the group consisting of KNO 3 , KNO 2 , Ca(NO 3 ) 2 , NaNO 3  and NaNO 2  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 potassium nitrate (KNO 3 ), potassium nitrite (KNO 2 ), sodium nitrate (NaNO 3 ), sodium nitrite (NaNO 2 ), calcium nitrate (Ca(NO 3 ) 2 ) 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 conventional nitriding methods. As a result, the method can be carried out in various application fields.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for nitriding metal in a salt bath and nitrided metal manufactured using the same; and, more particularly, to a method for nitriding iron or steels by using non-cyanide salt bath, and nitrided iron or steels manufactured using the same. 
         [0003]    2. Background of the Related Art 
         [0004]    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. 
         [0005]    Nitriding is one of the methods for processing the metal surface to impart thereto a corrosion resistance thereof. The nitriding methods include gas nitriding using NH 3  gas, salt bath nitriding using KCN, KCNO etc., gas nitrocarburizing (carbo-nitriding) using a mixture of NH 3  gas and RX gas, i.e., endothermic gas, and ion nitriding involving an insertion of a mixture of N 2  and H 2  gas into plasma. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    Further, the nitriding treatment in a molten salt including cynides is a nitrocarburizing (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. 
       SUMMARY OF THE INVENTION 
       [0010]    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. 
         [0011]    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. 
         [0012]    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 using the same. 
         [0013]    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. 
         [0014]    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 a non-cyanide salt 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. 
         [0015]    In the present invention, it is preferred that the non-cyanide salt includes at least one selected from a group consisting of NaNO 3 , NaNO 2 , KNO 3 , KNO 2  and Ca(NO 3 ) 2 , and the metal is one of iron and steels. 
         [0016]    At this time, the predetermined temperature is within a range of 400° C. to 700° C., and the submerging time is within a range of 1 minute to 24 hours. 
         [0017]    In the present invention, when iron is nitrided in the salt bath including at least one of the group consisting of KNO 3 , KNO 2 , Ca(NO 3 ) 2 , NaNO 3 , and NaNO 2 , the iron can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface. 
         [0018]    In the present invention, when a steel is nitrided in the salt bath including at least one of the group consisting of KNO 3 , KNO 2 , Ca(NO 3 ) 2 , NaNO 3 , and NaNO 2 , the steel can be nitrided into a depth of 0.1 mm to 3.0 mm from its surface. 
         [0019]    The steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel, alloy steel and IF steel. 
         [0020]    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. IF steel has the surface hardness being more than 165 Hv to equal to or less than 400 Hv. The surface hardness of the steels nitrided by the present invention can be improved to a maximum of 420 Hv. The surface hardness of the iron nitrided by the present invention is also improved. 
         [0021]    The ultra-low carbon steel nitrided by the present invention has the tensile strength ranging from more than 35 kgf/mm 2  to equal to or less than 110 kgf/mm 2 . The low carbon steel has the tensile strength ranging from more than 45 kgf/mm 2  to equal to or less than 110 kgf/mm 2 . The medium carbon steel has the tensile strength ranging from more than 45 kgf/mm 2  to equal to or less than 100 kgf/mm 2 . The high carbon steel has the tensile strength ranging from more than 60 kgf/mm 2  to equal to or less than 95 kgf/mm 2 . The alloy steel has the tensile strength ranging from more than 55 kgf/mm 2  to equal to or less than 110 kgf/mm 2 . The tensile strength of IF steel and iron can be improved by the nitriding method of the present invention. 
         [0022]    The salt-bath nitriding method of the present invention can be applied to the iron, the IF steel, the carbon steel including the ultra-low carbon steel having a carbon content of at least 0.0001 wt % to less than 0.13 wt %, the low carbon steel having a carbon content of at least 0.13 wt % to less than 0.2 wt %, the medium carbon steel having a carbon content of at least 0.21 wt % to less than 0.51 wt %, and the high carbon steel having a carbon content of at least 0.51 wt % to less than 2.0 wt %, the steel having a chrome content of 0.1 wt % to 1.5 wt %, the steel having a molybdenum content of 0.05 wt % to 0.5 wt %, the steel having a nickel content of 0.1 wt % to 10 wt %, the steel having a manganese content of 0.1 wt % to 2.0 wt %, the steel having a boron content of 0.001 wt % to 0.1 wt %, the steel having a titanium content of 0.01 wt % to 0.1 wt %, the steel having a vanadium content of 0.05 wt % to 0.15 wt %, the steel having a niobium content of 0.005 wt % to 0.1 wt %, and the steel having an aluminum content of 0.005 wt % to 0.1 wt %. 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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
           [0024]      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; 
           [0025]      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; 
           [0026]      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; 
           [0027]      FIG. 4  is a graph illustrating relationship between the nitriding time and the surface hardness of the steel nitrided in accordance with the fourth embodiment of the present invention; 
           [0028]      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 fourth embodiment of the present invention; 
           [0029]      FIG. 6  is a graph illustrating relationship between the nitriding time and the hardness profile in the steel nitrided in accordance with the fourth embodiment of the present invention; 
           [0030]      FIG. 7  is a graph illustrating the hardness profile in the steel nitrided in accordance with the fifth embodiment of the present invention; 
           [0031]      FIG. 8  is a graph illustrating the hardness profile in the steel nitrided in accordance with the sixth embodiment of the present invention; and 
           [0032]      FIG. 9  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 sixth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    Hereinafter, the present invention will be described in more detail. 
         [0034]    In nitriding of a metal, the present invention incorporates therein the nitrogen dissolution principle involving a non-cyanide molten salt, more particularly, NaNO 3 , NaNO 2 , KNO 3 , KNO 2 , Ca(NO 3 ) 2  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. 
         [0035]    The method for nitriding the metal in accordance with the present invention involves immersing at least one salt from a group consisting of NaNO 3 , NaNO 2  KNO 3 , KNO 2  and Ca(NO 3 ) 2  into a salt bath, melting the salt and maintaining of the molten salt at a predetermined temperature ranging from 400° C. to 700° C. 
         [0036]    Subsequently, the metal to be nitrided is submerged in the bath for 1 minute to 24 hours. 
         [0037]    During this time, nitrogen, oxygen and nitrogen oxides are generated from the non-cyanide molten salts of the present invention, NaNO 3 , NaNO 2  KNO 3 , KNO 2 , Ca(NO 3 ) 2  and mixtures of thereof, by the following reaction formulae 1 to 3. 
         [0038]    The following reaction formula 1 represents nitrogen formation reaction in the molten salt bath of NaNO 3  and NaNO 2 . 
         [0000]      NaNO 3 →NaNO 2 +½O 2    
         [0000]      2 NaNO 2 →Na 2 O+NO 2 +NO 
         [0000]      2NaNO 2 +2NO→2NaNO 3 +N 2   [Reaction formula 1] 
         [0039]    The following reaction formula 2 represents nitrogen formation reaction in the molten salt bath of KNO 3  and KNO 2 . 
         [0000]      KNO 3 →KNO 2 +½O 2    
         [0000]      2KNO 2 →K 2 O+NO 2 +NO 
         [0000]      2KNO 2 +2NO→2KNO 3 +N 2   [Reaction formula 2] 
         [0040]    The following formula 3 shows nitrogen formation reaction in the molten salt bath of Ca(NO 3 ) 2 . 
         [0000]      Ca(NO 3 ) 2 →CaO+2NO 2 +½O 2    
         [0000]      2NO 2 →2O 2 +N 2   [Reaction Formula 3] 
         [0041]    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, IF steel and iron using the salt-bath nitriding method in accordance with the present invention are nitrided to a depth of 0.1 mm to 3.0 mm 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. Reference for the table 1 are as follows: 
         [0042]    K. Funatani, “Low-Temperature Salt Bath Nitriding of Steels”, Metal Science and Heat Temperature, Vol. 46, No. 7, PP. 277-281 (2004). 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Thickness of 
               
               
                   
                 Temperature 
                   
                 diffusion 
               
               
                 Nitriding method 
                 (K) 
                 Type of Steel 
                 layer (μm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Nitride process by 
                 953 
                 Low carbon 
                 3000 
               
               
                 the present 
                   
                 steel 
               
               
                 invention 
                 913 
                 IF steel 
                 1500 
               
               
                 Tufftride TFI 
                 853 
                 1015 
                 800 
               
               
                   
                 853 
                 1045 
                 780 
               
               
                   
                 853 
                 34Cr4 
                 480 
               
               
                   
                 853 
                 X210Cr12 
                 160 
               
               
                 Tufftride NSI 
                 843 
                 1015 
                 780 
               
               
                   
                 843 
                 SCM435 
                 171 
               
               
                 “Soft” Nitriding in 
                 843 
                 SS2250 
                 353 
               
               
                 gas medium 
               
               
                 “Soft” Nitriding in 
                 793 
                 38CrMoAl 
                 78–97 
               
               
                 gas medium 
                 — 
                 40Cr 
                 63–80 
               
               
                 Gas Nitriding 
                 773 
                 SAE9254 
                 49 
               
               
                 Plasma Nitriding 
                 793 
                 722M24 
                 72 
               
               
                   
                 (Pused) 
               
               
                   
                 793 (DC) 
                 722M24 
               
               
                 Plasma Nitriding 
                 833 
                 En40B 
                 100 
               
               
                   
                 813 
                 En19 
                 110 
               
               
                   
                 793 
                 Nitraps 
                 46 
               
               
                   
                 823 
                 36CrMo 
                 100 
               
               
                   
                 793 
                 36CrMo + 0.1Y 
                 200 
               
               
                   
                 823 
                 36CrMo + 0.1Ce 
                 215 
               
               
                 Low-temperature 
                 753 
                 SKD61 
                 150 
               
               
                 salt bath Nitriding 
                 843 
                 SKD61 
                 106 
               
               
                 (palsonite) 
                 753 
                 SCM435 
                 141 
               
               
                   
                 843 
                 SCM435 
                 200 
               
               
                   
               
             
          
         
       
     
         [0043]    Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. 
       First Embodiment 
       [0044]    In accordance with the first embodiment of the present invention, steel is nitrided using the NaNO 3  molten salt. The nitrided steel includes ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel. 
         [0045]    Each of the ultra-low carbon steel, low carbon steel, medium carbon steel, high carbon steel and alloy steel is submerged in the NaNO 3  molten salt bath for 2 hours at a temperature of 500° C. 
         [0046]    Table 2 shows changes in surface hardness and tensile strength of the samples nitrided in the molten salt bath, wherein the hardness was measured using a Vickers hardness tester under a load of 1 kgf. 
         [0047]    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%. 
         [0048]    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%. 
         [0049]    That is, in case of steel, the surface hardness increases by 20% to 120% and the tensile strength increases by 15% to 50%. 
         [0050]    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. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness 
                 Change of Tensile 
               
               
                   
                 (Hv) 
                 Strength (kgf/mm 2 ) 
               
             
          
           
               
                   
                   
                   
                 In- 
                   
                 After 
                 In- 
               
               
                 Type 
                 Before 
                 After 
                 creasing 
                 Before 
                 nitrid- 
                 creasing 
               
               
                 of 
                 nitriding 
                 nitriding 
                 rate 
                 nitriding 
                 ing 
                 rate 
               
               
                 steel 
                 process 
                 process 
                 (%) 
                 process 
                 process 
                 (%) 
               
               
                   
               
             
          
           
               
                 Ultra 
                 128 
                 280 
                 119 
                 34 
                 50 
                 47 
               
               
                 low 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Low 
                 194 
                 286 
                 47 
                 62 
                 74 
                 19 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Medium 
                 183 
                 241 
                 32 
                 56 
                 66 
                 18 
               
               
                 carbon 
               
               
                 steel 
               
               
                 High 
                 230 
                 294 
                 28 
                 73 
                 85 
                 16 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Alloy 
                 226 
                 281 
                 24 
                 71 
                 83 
                 17 
               
               
                 steel 
               
               
                   
               
             
          
         
       
     
         [0051]      FIG. 1  is a graph the showing the hardness distribution in the thickness direction of the ultra-low carbon steel before (As) and after nitriding in the NaNO 3  molten salt bath at 500° C. for 30 minutes, 1 hour, 2 hours and 5 hours, respectively. 
         [0052]    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 0.6 mm from the surface. 
         [0053]      FIG. 2  shows the hardness distribution along the thickness direction of low carbon steel nitrided in the NaNO 3  molten-salt bath at 680° C. for 3, 6, 12 and 24 hours, respectively, wherein the hardness is measured using a Vickers hardness tester under a load of 3 kgf. 
         [0054]    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 mm, which is 6 times deeper than that obtained from the conventional nitriding method. 
         [0055]    Also, the surface hardness after nitriding is 450 Hv, which is more than 4 times higher than that of the non-treated specimen. 
         [0056]    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. 
         [0057]      FIG. 3  shows hardness distributions along the thickness direction of the ultra-low carbon steel before and after nitriding in the NaNO 3  molten-salt bath at 500° C. and 600° C. for 3 hours. The nitrided depth of the steel nitrided at 600° C. is 3 times deeper than that of the steel nitrided at 500° C. The surface hardness of the steel nitrided at 600° C. is 100 Hv higher than that of the steel nitrided at 500° C. That is, the surface hardness and nitrided depth of steel increase with increasing nitriding temperature. 
         [0058]    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 450° C., 500° C., 550° C. and 600° C., respectively, using the salt-bath nitriding method of the first embodiment of the present invention. 
         [0059]    As shown in  FIG. 3 , in case of the nitriding temperature of 450° C., the tensile strength increases by 5%. As the temperature increases, the tensile strength of the steel also increases. Accordingly, when the temperature is 600° C., the tensile strength increases by 134%. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Nitriding 
                 Nitriding 
                 Tensile 
                 Increasing 
               
               
                   
                 temperature 
                 time 
                 strength 
                 rate 
               
               
                 Division 
                 (° C.) 
                 (h) 
                 (kgf/mm 2 ) 
                 (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Before 
                 — 
                 — 
                 34.8 
                 0 
               
               
                 nitriding 
               
               
                 After 
                 450 
                 3 
                 36.6 
                 5 
               
               
                 nitriding 
                 500 
                   
                 50.8 
                 46 
               
               
                   
                 550 
                   
                 64.5 
                 85 
               
               
                   
                 600 
                   
                 81.4 
                 134 
               
               
                   
               
             
          
         
       
     
         [0060]    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. 
       Second Embodiment 
       [0061]    In accordance with the second embodiment of the present invention, steel is nitrided by using the NaNO 2  molten salt. 
         [0062]    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 450° C. for 2 hours. 
         [0063]    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. 
         [0064]    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%. 
         [0065]    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%. 
         [0066]    For alloy steel, the surface hardness increases by 17% and the tensile strength increases by 14%. 
         [0067]    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%. 
         [0068]    Accordingly, the molten salt bath nitriding method in accordance with the second embodiment of the present invention also increases the surface hardness and tensile strength of the steels. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness 
                 Change of Tensile 
               
               
                   
                 (Hv) 
                 Strength (kgf/mm 2 ) 
               
             
          
           
               
                   
                   
                   
                 In- 
                   
                   
                 In- 
               
               
                 Type 
                   
                   
                 creasing 
                   
                 After 
                 creasing 
               
               
                 of 
                 Before 
                 After 
                 rate 
                 Before 
                 nitrid- 
                 rate 
               
               
                 steel 
                 nitriding 
                 nitriding 
                 (%) 
                 nitriding 
                 ing 
                 (%) 
               
               
                   
               
               
                 Ultra- 
                 128 
                 197 
                 54 
                 34 
                 41 
                 21 
               
               
                 low 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Low 
                 194 
                 257 
                 32 
                 62 
                 71 
                 15 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Medium 
                 183 
                 218 
                 19 
                 56 
                 63 
                 13 
               
               
                 carbon 
               
               
                 steel 
               
               
                 High 
                 230 
                 271 
                 18 
                 73 
                 82 
                 12 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Alloy 
                 226 
                 265 
                 17 
                 71 
                 81 
                 14 
               
               
                 steel 
               
               
                   
               
             
          
         
       
     
       Third Embodiment 
       [0069]    In accordance with the third embodiment of the present invention, steels are nitrided using the KNO 2  molten salt. 
         [0070]    The steels including ultra-low carbon steel, low carbon steel, high carbon steel and alloy steel are submerged in the molten salt bath at 480° C. for 2 hours. 
         [0071]    Table 5 shows changes in hardness and tensile strength of the samples submerged in the molten salt bath, wherein the surface hardness is measured using a Vickers hardness tester under a load of 1 kgf. 
         [0072]    For ultra-low carbon steel, the surface hardness increases by 45% and the tensile strength is increases by 15%. For low carbon steel, the surface hardness increases by 25% and the tensile strength increases by 11%. 
         [0073]    For high carbon steel, the surface hardness increases by 17% and the tensile strength increases by 10%. For alloy steel, the surface hardness increases by 12% and the tensile strength increases by 11%. 
         [0074]    That is, when the steels are nitrided using the molten salt bath nitriding method of the third embodiment of the present invention, the surface hardness increases by 10% to 50%, and the tensile strength increases by 10% to 20%. 
         [0075]    Accordingly, the molten salt bath nitriding method in accordance with the third embodiment of the present invention also increases the surface hardness and the tensile strength of the steels. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness 
                 Change of Tensile 
               
               
                   
                 (Hv) 
                 Strength (kgf/mm 2 ) 
               
             
          
           
               
                   
                   
                   
                 In- 
                   
                   
                 In- 
               
               
                 Type 
                   
                   
                 creasing 
                   
                 After 
                 creasing 
               
               
                 of 
                 Before 
                 After 
                 rate 
                 Before 
                 nitrid- 
                 rate 
               
               
                 steel 
                 nitriding 
                 nitriding 
                 (%) 
                 nitriding 
                 ing 
                 (%) 
               
               
                   
               
               
                 Ultra- 
                 128 
                 186 
                 45 
                 34 
                 39 
                 15 
               
               
                 low 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Low 
                 194 
                 243 
                 25 
                 62 
                 69 
                 11 
               
               
                 carbon 
               
               
                 steel 
               
               
                 High 
                 230 
                 268 
                 17 
                 73 
                 80 
                 10 
               
               
                 carbon 
               
               
                 steel 
               
               
                 Alloy 
                 226 
                 252 
                 12 
                 71 
                 97 
                 11 
               
               
                 steel 
               
               
                   
               
             
          
         
       
     
       Fourth Embodiment 
       [0076]    In the fourth embodiment of the present invention, steel is nitrided using the KNO 3  molten salt. 
         [0077]    The steel to be nitrided is Interstitial-Free (IF) steel, which includes carbon (C) of 0.003 wt %, manganese (Mn) of 1.23 wt %, aluminum (Al) of 0.037 wt %, titanium (Ti) of 0.027 wt %, phosphorus (P) of 0.050 wt %, nitrogen (N) of 0.002 wt % and sulfur (S) of 0.008 wt %. 
         [0078]    The IF steel is nitrided in the KNO 3  molten bath at 560° C., 580° C., 600° C., 620° C. and 640° C., respectively. 
         [0079]      FIG. 4  shows the surface hardness of the IF steel nitrided in the KNO 3  molten bath as functions of time and temperature. 
         [0080]    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. 
         [0081]    However, when the nitriding time in the KNO 3  molten salt at 620° C. exceeds 8 hours, or the nitriding time in the KNO 3  molten salt at 640° C. 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. 
         [0082]    In Table 6, the surface hardness values of the IF steel nitrided by the third embodiment of the present invention are given. When the IF steel is nitrided at temperatures of 560° C. to 640° C., the surface hardness increases by 75% to 130%. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness (Hv) 
                   
                 Change of Hardness 
               
               
                   
                 after nitriding for 16 h. 
                   
                 (Hv) after nitriding for 1 h. 
               
             
          
           
               
                   
                   
                   
                 Increasing 
                   
                   
                   
                 Increasing 
               
               
                 Nitriding 
                 Before 
                 After 
                 rate 
                 Nitriding 
                 Before 
                 After 
                 rate 
               
               
                 Temperature 
                 nitriding 
                 nitriding 
                 (%) 
                 Temperature 
                 nitriding 
                 nitriding 
                 (%) 
               
               
                   
               
             
          
           
               
                 560° C. 
                 165 
                 289 
                 75 
                 620° C. 
                 165 
                 336 
                 104 
               
               
                 580° C. 
                 165 
                 329 
                 99 
                 640° C. 
                 165 
                 355 
                 115 
               
               
                 600° C. 
                 165 
                 379 
                 130 
               
               
                   
               
             
          
         
       
     
         [0083]      FIG. 5  shows the hardness distribution along the thickness direction of the IF steel nitrided by the fourth embodiment of the present invention. 
         [0084]    The IF steel is nitrided in the KNO 3  molten salt at 560° C. for 16 hours and at temperatures of 560° C., 580° C., 600° C. and 620° C. for 8 hours. 
         [0085]    Referring to  FIG. 5 , the hardness of the IF steel decreases with increasing depth from the surface because the nitrogen concentration decreases 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 1.38 mm to 1.5 mm, which is 3 to 5 times thicker than the thickness of the nitrided layer formed using the conventional method. 
         [0086]      FIG. 6  is a graph showing hardness distribution along the thickness direction of the IF steel nitrided in the KNO 3  molten salt at 640° C. for 1 hour, 2 hours, 4 hours, 8 hours and 16 hours. 
         [0087]    As shown in the  FIG. 6 , for IF steel, as the nitriding time increases, the difference in hardness between the surface and the interior decreases, resulting in the IF steel having, as well as an increased surface hardness, an increased bulk hardness, as a consequence of nitrogen diffusing into the interior and the difference in concentration thereof between the surface and the interior decreasing. In other word, the nitriding method in accordance with the present invention will lead to an IF steel having an increased surface and bulk hardness, resulting from a nitrogen diffusing into the interior at a higher diffusion rate. 
       Fifth Embodiment 
       [0088]    In the fifth embodiment of the present invention, steel is nitrided using the Ca(NO 3 ) 2  molten salt. 
         [0089]    The steel to be nitrided in the fifth embodiment is low carbon steel. 
         [0090]    Since Ca(NO 3 ) 2  is highly hygroscopic at a room temperature, including combined water, it is preferred to use Ca(NO 3 ) 2  after removing moisture by heating for a predetermined time. 
         [0091]    The fifth embodiment of the present invention includes the process of removing moisture by heating Ca(NO 3 ) 2  for 4 hours at 100° C. to 150° C., heating Ca(NO 3 ) 2  to 580° C. to form the Ca(NO 3 ) 2  molten-salt bath and submerging the low carbon steel in the bath for 3 hours. 
         [0092]      FIG. 7  is a graph showing the surface hardness profile in low carbon steel nitrided by the fifth embodiment of the present invention. 
         [0093]    As shown in  FIG. 7 , the low carbon steel nitrided by the fifth embodiment is nitrided to a depth of 0.5 mm from the surface, and has the surface hardness that is 2 times higher than the surface hardness (As) of the steel before nitriding. 
       Sixth Embodiment 
       [0094]    In the sixth embodiment of the present invention, steel is nitrided using a molten mixture of KNO 3  and NaNO 3 . 
         [0095]    In the sixth embodiment of the present invention, the low carbon steel is nitrided in the molten mixture of KNO 3  and NaNO 3  whose mixture ratios are 1:1, 8:2 and 2:8. 
         [0096]    Table 7 shows the surface hardness values of steels nitrided by the sixth embodiment of the present invention. Various types of steel are submerged in the molten mixture of KNO 3  and NaNO 3  whose ratio is 1:1 for 12 or 24 hours at 650° C. 
         [0097]    At this time, the hardness is measured using a Vickers hardness tester under a load of 3 kg. 
         [0098]    The hardness values of the steels nitrided in the mixture of KNO 3  and NaNO 3  increase by 69% to 251% depending on the steel type. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness (Hv) 
               
             
          
           
               
                   
                 Nitriding 
                   
                   
                 Increasing 
               
               
                   
                 Time 
                 Before 
                 After 
                 rate 
               
               
                 Type of steel 
                 (h) 
                 nitriding 
                 nitriding 
                 (%) 
               
               
                   
               
             
          
           
               
                 Ultra-low 
                 24 
                 128 
                 449 
                 251 
               
               
                 carbon steel 
               
               
                 low carbon 
                 12 
                 194 
                 406 
                 109 
               
               
                 steel 
               
               
                 Medium carbon 
                 12 
                 183 
                 391 
                 114 
               
               
                 steel 
               
               
                 High carbon 
                 24 
                 230 
                 389 
                 69 
               
               
                 steel 
               
               
                 Alloy steel 
                 24 
                 226 
                 387 
                 71 
               
               
                   
               
             
          
         
       
     
         [0099]    Various steels are submerged in the mixture of KNO 3  and NaNO 3  whose ratio is 1:1 at 580° C., and changes in surface hardness and tensile strength of the nitrided steels depending on nitriding time are measured. 
         [0100]    As shown in Table 8, nitriding in accordance with the fifth embodiment of the present invention increases the hardness and the tensile strength of all the steels. The hardness and tensile strength increase with increasing nitriding time. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
             
             
               
                   
                   
               
               
                   
                 Change of Hardness 
                 Change of Tensile 
               
               
                   
                 (Hv) 
                 strength (kgf/mm 2 ) 
               
             
          
           
               
                   
                 Nitriding 
                   
                   
                 Increasing 
                   
                   
                 Increasing 
               
               
                 Type of 
                 Time 
                 Before 
                 After 
                 rate 
                 Before 
                 After 
                 rate 
               
               
                 steel 
                 (h) 
                 nitriding 
                 nitriding 
                 (%) 
                 nitriding 
                 nitriding 
                 (%) 
               
               
                   
               
             
          
           
               
                 Ultra- 
                 3 
                 120 
                 283 
                 136 
                 35 
                 48 
                 37 
               
               
                 low 
                 12 
                 120 
                 421 
                 251 
                 35 
                 92 
                 163 
               
               
                 carbon 
               
               
                 steel 
               
               
                 low 
                 3 
                 200 
                 283 
                 42 
                 45 
                 55 
                 22 
               
               
                 carbon 
                 12 
                 200 
                 403 
                 102 
                 45 
                 79 
                 76 
               
               
                 steel 
               
               
                 Medium 
                 3 
                 130 
                 181 
                 39 
                 45 
                 57 
                 27 
               
               
                 carbon 
                 12 
                 130 
                 398 
                 206 
                 45 
                 88 
                 84 
               
               
                 steel 
               
               
                 High 
                 3 
                 150 
                 201 
                 34 
                 60 
                 76 
                 27 
               
               
                 carbon 
                 12 
                 150 
                 391 
                 161 
                 60 
                 87 
                 45 
               
               
                 steel 
               
               
                 Alloy 
                 3 
                 200 
                 274 
                 37 
                 55 
                 75 
                 36 
               
               
                 steel 
                 12 
                 200 
                 409 
                 105 
                 55 
                 90 
                 64 
               
               
                   
               
             
          
         
       
     
         [0101]      FIG. 8  is a graph showing the hardness profiles of steel nitrided at 680° C. for 200 minutes in the KNO 3  bath, the NaNO 3  bath, the 50% KNO 3 -50% NaNO 3  mixture bath at 680° C. for 200 minutes. 
         [0102]    The hardness was measured using a Vickers hardness tester. 
         [0103]    In  FIG. 8 , the steel nitrided in the mixture bath has a nitrided depth of 1.5 mm and a surface hardness of 160 Hv, 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. 
         [0104]      FIG. 9  is a graph showing the hardness profiles of the low carbon steel nitrided in the 80% KNO 3 -20% NaNO 3  bath and 20% KNO 3 -80% NaNO 3  bath at 650° C. for 4 hours, respectively. 
         [0105]    As shown in  FIG. 9 , the surface hardness of the steel nitrided in the mixture baths is about 2 times higher than that of the steel before nitriding. 
         [0106]    The present invention can solve an environmental pollution problem and can reduce a cost for nitriding steels by using molten non-cyanide salts, such as sodium nitrate (NaNO 3 ), sodium nitrite (NaNO 2 ), calcium nitrate (Ca(NO 3 ) 2 ) and their mixtures. 
         [0107]    Since the present invention can increase the nitrided depth or nitrogen-diffusion depth of steels two to six times higher than that obtained using conventional nitriding methods, thereby nitriding the inner part as well as the surface of the metal, its applications are extended to various fields. 
         [0108]    Since the present invention can be applied to bulk hardening 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. 
         [0109]    The present application contains subject matter related to Korean patent application No. 2006-0049077, filed in the Korean Intellectual Property Office on May 30, 2006, the entire contents of which are incorporated herein by reference. 
         [0110]    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. 
         [0111]    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.