Patent Publication Number: US-2007098588-A1

Title: High-nitrogen austenitic stainless steel

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to high-nitrogen austenitic stainless steel and, more specifically, high-nitrogen austenitic stainless steel which is excellent in seawater corrosion resistance and strength, and nonmagnetic.  
      2.Description of the Related Art  
      Austenitic stainless steel is generally excellent in corrosion resistance, but may cause corrosion depending on its using environment. For example, it is known that, when austenitic stainless steel is dipped in a halide aqueous solution, the passive film is broken by a halogen ion such as Cl -  or Br - , causing local corrosion (pitting, crevice corrosion). For prevention of such a local corrosion, the following are known to be effective: 
      (1) Strengthening of passive film by increasing the amount of Cr     (2) Reduction of active dissolution by addition of Mo, increase in amount of Ni, or the like     (3) Addition of N.    

      Therefore, for application in seawater or in marine environment, materials described below, for example, have been used, or uses thereof have been examined: 
      (1) Austenitic stainless steel improved in corrosion resistance by addition of Cr and/or Mo (e.g., SUS316)     (2) Duplex stainless steel rich in Cr and Mo which are effective for corrosion resistance and adjusted in components so that the ratio of austenite phase is about 50% (e.g., SUS329J1, SUS329J4L, etc.)     (3) Austenitic stainless steel largely increased in amounts of Cr and Mo, with Cr(wt %)+3Mo(wt %)+10N (wt %)&gt;38 (so-called super stainless steel, e.g., SUS836L)     (4) Ni-based alloy (e.g., Hastelloy-C276, Inconel 625, etc.)     (5) Ti alloy.    

      The super stainless steel is excellent in seawater corrosion resistance, however, it is hard to say that it has sufficient strength. On the other hand, the application of Ni-based alloy and Ti alloy has also been examined for a portion requiring strength and corrosion resistance, however, the Ni-based alloy and Ti alloy are expensive and of excessive quality. Accordingly, development of inexpensive and reliable stainless steel is requested, and various proposals have been conventionally made.  
      For example, there is disclosed in Patent Reference 1 (Japanese Patent Application Laid-Open No. 10-183303), an austenitic steel alloy, containing 17.5% Cr-4% Mo-11% Mn-0.02% C-0.88% N-0.01% Ni, and the balance Fe; and an austenitic steel alloy, containing 14% Cr-6% Mo-12% Mn-0.9% N, and the balance Fe. It is described inx the same reference that corrosion resistance as high as that of super austenite can be ensured by adopting such a composition.  
      Further, there is disclosed in Patent Reference 2 (Japanese Patent Application Laid-Open No. 2000-309857) a stainless steel, containing C:0.003-0.012 wt %, Cr:15.08-25.02 wt %, Mn:0.01-0.16 wt %, Mo:1.03-9.21 wt %, Ni:2.05-23.41 wt %, N:0.31-1.45 wt %, and the balance Fe, and having a pitting resistance equivalent PRE(=Cr (wt %)+3Mo (wt %)+10N (wt %)) which satisfies a predetermined relation with an area ratio (A %) of nonmetallic inclusion and a diameter (D μm) thereof. It is described in the same reference that, when the area ratio of nonmetallic inclusion and the diameter thereof are certain values or less, then reduction in corrosion resistance which is resulted from that the nonmetallic inclusion becomes coating defect of the passive film can be suppressed.  
      Further, there is disclosed in Patent Reference 3 (Japanese Patent Application Laid-Open No. 2002-235153) a high-strength, high-corrosive resistance nonmagnetic stainless steel, containing C: not more than 0.15%, Si: not more than 1.0%, Mn:3.0-12.0%, P: not more than 0.030%, Ni: not more than 0.50%, Cr:15.0-21.0%, N:0.70-1.50%, Al: not more than 0.020%, O: not more than 0.020%, and the balance Fe. It is described in the same reference that Ni allergy resulted from elution of Ni can be suppressed by controlling the amount of Ni to not more than 0.5%; increased strength and nonmagnetic property can be attained by increasing the amount of N instead of Ni; and excellent corrosion resistance can be ensured by reducing the amount of Mn. 
      [Patent Reference 1] Japanese Patent Application Laid-Open No. 10-183303     [Patent Reference 2] Japanese Patent Application Laid-Open No. 2000-309857     [Patent Reference 3] Japanese Patent Application Laid-Open No. 2002-235153    

      In the austenitic steel alloy disclosed in Patent Reference 1, 11-12% of Mn is added in order to enhance the nitrogen solbility. However, it is hard to say that adaptation of such a composition with high Mn can provide sufficient corrosion resistance (particularly, pitting resistance) in seawater.  
      In the stainless steel disclosed in Patent Reference 2, Mn is hardly added, in order to enhance the corrosion resistance in seawater, while Ni is further added. However, in such a component range for perfectly dissolving Cr nitrides which are harmful to seawater corrosion resistance, solution treatment at an extremely high temperature is needed. Solution treatment at a temperature exceeding 1250° C. is disadvantageous for actual manufacture of a product in respect of grain coarsening and increase in manufacturing cost. Further, non-dissolved Cr nitrides may be left depending on temperature restrictions on existing solution treatment facilities, and sufficient seawater corrosion resistance cannot be obtained.  
      On the other hand, the high-strength, high-corrosion resistant nonmagnetic stainless steel disclosed in Patent Reference 3 is improved in corrosion resistance because a large amount of N is included, and the amount of Mn is suppressed; and has properly high strength. Further, this stainless steel can be used also for accessories, biomedical implants and the like, because it is nonmagnetic and safe for human body. However, further improvement in strength and corrosion resistance is demanded for the use in more severe conditions.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a high-nitrogen austenitic stainless steel having seawater corrosion resistance as high as that of austenitic super stainless steel and strength as high as that of duplex stainless steel.  
      Another object of the present invention is to provide a high-nitrogen austenitic stainless steel capable of reducing Cr nitrides which are harmful to corrosion resistance without increasing the manufacturing cost by performing solution treatment in a practical temperature range of from 1050 to 1250° C.  
      A further object of the present invention is to provide a high-nitrogen austenitic stainless steel having strength equal to or more than that of Ni-based alloy or Ti-based alloy by performing cold working to such a high-nitrogen austenitic stainless steel.  
      Further, an additional object of the present invention is to provide a high-nitrogen austenitic stainless steel which is nonmagnetic.  
      In order to overcome the above-mentioned problems, the high-nitrogen austenitic stainless steel according to the present invention comprises:  
      0.005 mass %≦C≦0.25 mass %;  
      15.0 mass %≦Cr≦35.0 mass %;  
      0.2 mass %&lt;Mn&lt;10.0 mass %;  
      0.05 mass %≦Mo≦8.0 mass %;  
      0.01 mass %≦Cu≦4.0 mass %;  
      0.01 mass %≦Ni&lt;5.0 mass %;  
      0.8 mass %&lt;N≦1.8 mass %;  
      Si≦2.0 mass %;  
      P≦0.03 mass %;  
      S≦0.05 mass %;  
      Al≦0.030 mass %;  
      O≦0.020 mass %; and  
      the balance substantially containing Fe and impurities, and having:  
      PRE represented by the equation (1) and of not less than 5,
 
 PRE= (Cr+3.3Mo+16N)/Mn(mass %)  (1)
 
 and 
 
      CRE represented by the equation (2) and of not less than 27,
 
 CRE= Cr+1.5Mo+2N+Cu(mass %)  (2).
 
      In this case, the high-nitrogen austenitic stainless steel preferably has a composition which can make the diameter of Cr nitrides to not more than 2 μm by the solution treatment of 1050 to 1250° C.  
      The high-nitrogen austenitic stainless steel preferably has an index of stability of austenitic phase: Ni eq −Cr eq  of not less than 0, wherein
 
Ni eq =Ni+Co+0.5Mn+0.3Cu+25N+30C
 
Cr eq =Cr+2Si+1.5Mo+5V+5.5Al+1.75Nb+1.5Ti+0.75W.
 
      When the content of each component element is optimized so that the PRE and the CRE are within predetermined ranges, high strength can be obtained in addition to improvement in seawater corrosion resistance. The optimization of the content of each component element enables dissolution of Cr nitrides with diameter of not less than 2 μm by the solution treatment of 1050 to 1250C. When the content of each component is optimized so that the index of stability of austenite phase: Ni eq −Cr eq  is not less than 0, high-nitrogen austenitic stainless steel which is nonmagnetic even after strong cold working can be obtained. Further, when the high-nitrogen austenitic stainless steel having such a component is subjected to cold working, high strength equal to or more than that of Ni-based alloy or Ti-based alloy can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a view schematically showing a crevice forming tool; and  
       FIG. 1B  is a view schematically showing a test piece with the crevice forming tool fixed thereto. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      One embodiment of the present invention will be then described in detail.  
      The high-nitrogen austenitic stainless steel according to the present invention includes elements described below, and the balance substantially containing Fe and inevitable impurities. The kinds and component ranges of additive elements and the limitation reasons thereof are as follows.  
      In this specification, “mass %” means “mass percent,” which is defined by the equation (A):
 
(mass %)=(amount of each element/total amount of all elements of steel or alloy etc.)×100  (A).
 
      Further, “MPa” is a unit, and 1 MPa equals to 1×10 6  N/m 2 .  
      (1) 0.005 mass %≦C≦0.25 mass %  
      C is an austenite forming element, which is contributable to stabilization of austenite phase and effective for suppression of nitrogen blow. It also contributes to improvement in strength because it is an interstitial element. On the other hand, addition of C exceeding 0.25 mass % reduces the solubility of N, and also reduces dissolved-Cr in a base phase by formation of Cr carbides, causing deterioration of corrosion resistance. Therefore, the addition amount of C is set preferably to from not less than 0.005 mass % to not more than 0.25 mass %, more preferably to from not less than 0.005 mass % to not more than 0.20 mass %, and even more preferably to from not less than 0.01 mass % to not more than 0.15 mass %.  
      (2) 15.0 mass %≦Cr≦35.0 mass %  
      Cr remarkably increases the solubility of N, thus, it is not only effective for suppression of nitrogen blow but also largely contributable to improvement in corrosion resistance and strength. Namely, Cr is an important element. Cr is a ferrite forming element, therefore, excessive addition thereof makes austenite phase unstable, so that nonmagnetic property cannot be kept. Further, the residual amount of non-dissolved Cr nitrides in solution treatment is increased, causing remarkable reduction in corrosion resistance. Further, precipitation of σ-phase which causes deterioration of toughness and ductility is promoted. Therefore, the addition amount of Cr is set preferably to from not less than 15.0 mass % to not more than 35.0 mass %, more preferably to from more than 21.0 mass % to not more than 32.0 mass %, and even more preferably to from more than 24.0 mass % to not more than 30.0 mass %.  
      (3) 0.2 mass %&lt;Mn&lt;10.0 mass %  
      Mn is an austenite forming element, which contributes to stabilization of austenite phase. It remarkably increases the solubility of N, therefore, it is effective for suppression of nitrogen blow. Further, it reduces the temperature at which below-mentioned Cr nitride dissolves. It is effective also as a deoxidizing or desulfurizing element. On the other hand, excessive addition of Mn causes deterioration of pitting resistance. Therefore, the addition amount of Mn is set preferably to from more than 0.2 mass % to less than 10.0 mass %, more preferably to from more than 0.2 mass % to not more than 8.0 mass %, and even more preferably to from not less than 2.0 mass % to not more than 7.0 mass %.  
      (4) 0.05 mass %≦Mo≦8.0 mass %  
      Mo increases the solubility of N and remarkably improves corrosion resistance. It also improves strength as a solid-solution strengthening element. On the other hand, excessive addition of Mo makes austenite phase unstable, inducing N blow, and also makes it difficult to ensure the nonmagnetic property. Further, the excessive addition causes formation of a brittle phase and it causes reduction in toughness and ductility, these are also detrimental to forging. Further, the amount of non-dissolved Cr nitrides in solution treatment is increased, causing remarkable reduction in corrosion resistance. Therefore, the addition amount of Mo is set preferably to from not less than 0.05 mass % to not more than 8.0 mass %, more preferably to from not less than 0.05 mass % to not more than 2.5 mass %, and even more preferably to from not less than 0.10 mass % to less than 2.5 mass %.  
      (5) 0.01 mass %≦Cu≦4.0 mass %  
      Cu is an austenite forming element, which contributes to stabilization of austenite phase. It also contributes to improvement in crevice corrosion resistance. On the other hand, excessive addition of Cu increases the amount of non-dissolved Cr nitrides in solution treatment, causing deterioration of corrosion resistance and reduction in hot workability. Therefore, the addition amount of Cu is set preferably to from not less than 0.01 mass % to not more than 4.0 mass %, more preferably to from not less than 0.02 mass % to not more than 2.0 mass %, and even more preferably to from not less than 0.05 mass % to not more than 1.5 mass %.  
      (6) 0.01 mass %≦Ni&lt;5.0 mass %  
      Ni is an austenite forming element, which contributes to stabilization of austenite phase. On the other hand, excessive addition of Ni increases the amount of non-dissolved Cr nitrides in solution treatment, causing deterioration of corrosion resistance. Therefore, the addition amount of Ni is set preferably to from not less than 0.01 mass % to less than 5.0 mass %, more preferably to from not less than 0.01 mass % to less than 3.0 mass %, and even more preferably to from more than 0.5 mass % to less than 3.0 mass %.  
      (7) 0.8 mass %&lt;N≦1.8 mass %  
      N is one of the most important elements in the present invention. N is an interstitial element, which is very effective for improvement in strength, for stabilization of austenite phase, and for improvement in corrosion resistance. On the other hand, excessive addition of N induces generation of N blow and also makes a large amount of non-dissolved Cr nitrides or large amounts of Ti, Nb, and V nitrides remain in steel in solution treatment, causing remarkable reduction in corrosion resistance. Therefore, the addition amount of N is set preferably to from more than 0.8 mass % to not more than 1.8 mass %, and more preferably to from more than 0.8 mass % to not more than 1.5 mass %.  
      (8) Si≦2.0 mass %  
      Although Al is more effective than Si as a deoxidizing element for general steel, excessive addition of Al in high-nitrogen steel causes formation of AlN which leads to remarkable reduction in corrosion resistance, toughness and ductility. Therefore, as an main deoxidizing element, it is preferable to use Si with an essential element Mn. Since Si is a ferrite forming element, excessive addition thereof makes austenite phase unstable, inducing N blow, and also makes it difficult to ensure the nonmagnetic property. Further, the excessive addition is not only detrimental to forging, but also it remarkably deteriorates the toughness and ductility of steel. The residual amount of non-dissolved Cr nitrides after solution treatment is also increased, causing remarkable reduction in corrosion resistance. Therefore, the addition amount of Si is set preferably to not more than 2.0 mass %, more preferably to from not less than 0.01 mass % to not more than 1.0 mass %, and even more preferably to from not less than 0.01 mass % to not more than 0.50 mass %.  
      (9) P≦0.03 mass %  
      Excessive addition of P causes reduction in hot workability, grain boundary strength, toughness and ductility, while excessive reduction of P causes a rise of manufacturing cost. Therefore, the content of P is preferably set to not more than 0.03 mass %.  
      (10) S≦0.05 mass %  
      Although S is effective for improvement in machinability, excessive addition thereof causes reduction in hot workability and causes deterioration of corrosion resistance by formation of MnS. On the other hand, excessive reduction of S causes a rise of manufacturing cost. Therefore, the content of S is set preferably to not more than 0.05 mass %, and more preferably to not more than 0.01 mass %.  
      (11) Al≦0.030 mass %  
      Al is very effective as a deoxidizing element similarly to Si and Mn. In this steel, however, if the addition amount of Al is more than 0.03 mass %, formation of AlN progresses, causing remarkable reduction in corrosion resistance, toughness and ductility. Therefore, the addition amount of Al is set preferably to not more than 0.030 mass %, more preferably to not more than 0.025 mass %, and even more preferably to not more than 0.020 mass %.  
      (12) O≦0.020 mass %  
      Addition of 0 exceeding 0.020 mass % reduces the cleanliness of steel, causing remarkable deterioration of corrosion resistance. Therefore, the content of 0 is set preferably to not more than 0.020 mass %, more preferably to not more than 0.015 mass %, and even more preferably to not more than 0.010 mass %.  
      The high-nitrogen austenitic stainless steel according to the present invention is characterized in that PRE is not less than 5 and CRE is not less than 27, in addition to that various additive elements are within the above-mentioned ranges.  
      The “PRE” means a pitting resistance equivalent, which is a value represented by the following equation (1). In the equation (1), N, Cr and Mo are used as elements which improve the pitting resistance and Mn is used as element which deteriorates the pitting resistance.
 
 PRE =(Cr+ 3.3Mo+16N)/Mn(mass %)  (1)
 
      When the PRE is not less than 5, a pitting resistance equal to or more than that of super stainless steel (e.g., SUS836L) is obtained. The PRE is more preferably not less than 7.  
      The “CRE” means a crevice corrosion resistance equivalent, which is a value represented by the following equation (2). In the equation (2), Cr, Mo, N and Cu are used as elements which reduce a critical pH.
 
 CRE =Cr+l. 5Mo+2N+Cu (mass %)  (2)
 
      When the CRE is not less than 27, a crevice corrosion resistance equal to or more than that of super stainless steel (e.g., SUS836L) is obtained. The CRE is preferably not less than 30.  
      The high-nitrogen austenitic stainless steel according to the present invention may contain, further, one or more second additive elements as described below, in addition to the various elements described above. The component range of each element and the limitation reason thereof are as follows.  
      (13) 0.01 mass %≦W≦8.0 mass %  
      W contributes to improvement in corrosion resistance similarly to Mo, and also contributes to improvement in strength as a solid-solution strengthening element. On the other hand, excessive addition of W causes reduction in toughness and ductility by formation of a brittle phase similarly to Mo, and is also detrimental to forging. The amount of non-dissolved Cr nitrides in solution treatment is also increased, causing remarkable reduction in corrosion resistance. Therefore, the addition amount of W is set preferably to from not less than 0.01 mass % to not more than 8.0 mass %, and more preferably to from not less than 0.05 mass % to not more than 1.5 mass %.  
      (14) 0.01 mass %≦Co≦5.0 mass %  
      Co contributes to improvement in corrosion resistance and to improvement in strength. On the other hand, excessive addition of Co leads to rise of cost, and it increases the amount of non-dissolved Cr nitrides in solution treatment, causing remarkable reduction in corrosion resistance. Therefore, the addition amount of Co is set preferably to from not less than 0.01 mass % to not more than 5.0 mass %, more preferably to from not less than 0.05 mass % to not more than 4.5 mass %, and even more preferably to from not less than 0.1 mass % to not more than 4.0 mass %.  
      The high-nitrogen austenitic stainless steel according to the present invention may further contain one or more third additive elements as described below, in addition to the above-mentioned various elements or instead of the above-mentioned second additive elements.  
      The component range of each element and the limitation reason thereof are as follows.  
      (15) 0.01 mass %≦Ti≦0.5 mass %  
      Ti bonds to C and/or N to contribute to improvement in strength and to grain refining. On the other hand, excessive addition of Ti makes large amounts of oxides and/or nitrides remain in steel, causing reduction in corrosion resistance. It also reduces the effective amount of dissolved N, causing reduction in strength. Therefore, the addition amount of Ti is set preferably to from not less than 0.01 mass % to not more than 0.5 mass %, more preferably to from not less than 0.02 mass % to not more than 0.4 mass %, and even more preferably to from not less than 0.03 mass % to not more than 0.3 mass %.  
      (16) 0.01 mass %≦Nb≦0.5 mass %  
      Nb bonds to C and/or N similarly to Ti to contribute to improvement in strength and to grain refining. On the other hand, excessive addition of Nb makes large amounts of oxides and/or nitrides remain in steel, causing reduction in corrosion resistance. It also reduces the effective amount of dissolved N, causing reduction in strength. Therefore, the addition amount of Nb is set preferably to from not less than 0.01 mass % to not more than 0.5 mass %, more preferably to from not less than 0.02 mass % to not more than 0.4 mass %, and even more preferably to from not less than 0.03 mass % to not more than 0.3 mass %.  
      (17) 0.01 mass %≦V&lt;1.0 mass %  
      V bonds to C and/or N similarly to Ti and Nb to contribute to improvement in strength and to grain refining. On the other hand, excessive addition of V makes large amounts of oxides and/or nitrides remain in steel, causing reduction in corrosion resistance. It also reduces the effective amount of dissolved N, causing reduction in strength. Therefore, the addition amount of V is set preferably to from not less than 0.01 mass % to less than 1.0 mass %, more preferably to from not less than 0.02 mass % to not more than 0.9 mass %, and even more preferably to from not less than 0.03 mass % to not more than 0.8 mass %.  
      (18) 0.01 mass %≦Ta≦0.5 mass %  
      Ta bonds to C and/or N similarly to Ti, Nb and V to contribute to improvement in strength and to grain refining. On the other hand, excessive addition of Ta makes large amounts of oxides and/or nitrides remain in steel, causing reduction in corrosion resistance. It also reduces the effective amount of dissolved N, causing reduction in strength. Therefore, the addition amount of Ta is set preferably to from not less than 0.01 mass % to not more than 0.5 mass %, more preferably to from not less than 0.02 mass % to not more than 0.4 mass %, and even more preferably to from not less than 0.03 mass % to not more than 0.3 mass %.  
      (19) 0.01 mass %≦Zr≦0.5 mass %  
      Zr contributes to improvement in strength. On the other hand, excessive addition of Zr leads to reduction in toughness and ductility. Therefore, the addition amount of Zr is set preferably to from not less than 0.01 mass % to not more than 0.5 mass %, more preferably to from not less than 0.03 mass % to not more than 0.4 mass %, and even more preferably to from not less than 0.05 mass % to not more than 0.3 mass %.  
      The high-nitrogen austenitic stainless steel according to the present invention may further contain one or more fourth additive elements as described below, in addition to the above-mentioned various elements or instead of the above-mentioned second and/or third additive elements. The component range of each element and the limitation reason thereof are as follows.  
      (20) 0.001 mass %≦B≦0.01 mass %  
      B is effective for improvement in strength and for improvement in hot workability. On the other hand, excessive addition of B rather impairs the hot workability, and deteriorates corrosion resistance. Therefore, the addition amount of B is set preferably to from not less than 0.001 mass % to not more than 0.01 mass %, more preferably to from not less than 0.001 mass % to not more than 0.008 mass %, and even more preferably to from not less than 0.001 mass % to not more than 0.005 mass %.  
      (21) 0.001 mass %≦Ca≦0.01 mass %  
      (22) 0.001 mass %≦Mg≦0.01 mass %  
      Ca and Mg are effective for improving hot workability. Ca is also effective for improving machinability. On the other hand, excessive addition of  
      Ca and Mg rather impairs the hot workability. Therefore, the addition amounts of Ca and Mg are set, respectively, preferably to not less than 0.001 mass % to not more than 0.01 mass %, more preferably to from not less than 0.001 mass % to not more than 0.008 mass %, and even more preferably to from not less than 0.001 mass % to not more than 0.005 mass %.  
      The high-nitrogen austenitic stainless steel according to the present invention may further contain one or more fifth additive elements as described below, in addition to the above-mentioned various elements, or instead of the above-mentioned second, third and/or fourth additive elements. The component range of each element and the limitation reason thereof are as follows.  
      (23) 0.005 mass %≦Te≦0.05 mass %  
      Te contributes to improvement in machinability.  
      On the other hand, excessive addition of Te deteriorates corrosion resistance, toughness, ductility, and hot workability. Therefore, the addition amount of Te is set preferably to from not less than 0.005 mass % to not more than 0.05 mass %, and more preferably to from not less than 0.01 mass % to not more than 0.04 mass %.  
      (24) 0.01 mass %≦Se≦0.20 mass %  
      Se contributes to improvement in machinability. On the other hand, excessive addition of Se deteriorates corrosion resistance, toughness, ductility, and hot workability. Therefore, the addition amount of Se is set preferably to from not less than 0.01 mass % to not more than 0.20 mass %, more preferably to from not less than 0.02 mass % to not more than 0.18 mass %, and even more preferably to from not less than 0.05 mass % to not more than 0.15 mass %.  
      In the high-nitrogen austenitic stainless steel according to the present invention, it is desirable that Ni eq −Cr eq  is not less than 0 in addition that the additive elements are within the above-mentioned ranges.  
      The “Ni eq −Cr eq  ” means an index of stability of austenite phase, which is represented by using contents of main austenite forming elements and contents of main ferrite forming elements. The “Ni eq ” and the “Cr eq ” mean values represented by the following equations, respectively.
 
Ni eq =Ni+Co+0.5Mn+0.3Cu+25N+30C
 
Cr eq =Cr+2Si+1.5Mo+5V+5.5Al+l.75Nb+1.5Ti+0.75W.
 
      When the Ni eq −Cr eq  is not less than 0, the austenite phase can be stably kept even after strong cold working.  
      Further, it is preferable that the high-nitrogen austenitic stainless steel according to the present invention has, particularly, a composition which can make the diameter of Cr nitrides to not more than 2 μm by solution treatment of 1050 to 1250° C. among the above-mentioned compositions.  
      The high-nitrogen austenitic stainless steel according to the present invention is subjected, after forging or rolling, to solution treatment for 0.1 to 2 hours at such a heat treatment temperature as to be suitable for the composition of steel for the purpose of ensuring corrosion resistance. The solution treatment is carried out in order to dissolve Cr nitrides and to uniform microstructure. In this case, extinguishment of Cr nitrides with diameter exceeding 2 μm cannot always be attained in steel having any composition by performing solution treatment at a fixed temperature. Therefore, it is necessary to select an optimum solution treatment temperature according to the composition of the steel.  
      When the solution treatment temperature is excessively low, non-dissolved Cr nitrides with diameter exceeding 2 μm are generally left after solution treatment. Rough Cr nitrides cause reduction in seawater corrosion resistance. On the other hand, an excessively high solution-treatment temperature causes grain coarsening and/or increase in facility cost.  
      In order to suppress the grain coarsening and/or the increase in facility cost while ensuring excellent seawater corrosion resistance, among the above-mentioned compositions, a composition which can make the diameter of non-dissolved Cr nitrides to not more than 2 μm by solution treatment of 1050 to 1250° C. is particularly preferred.  
      The effect of the high-nitrogen austenitic stainless steel according to the present invention will be then described.  
      The high-nitrogen austenitic stainless steel of the present invention is such that a large amount of N is dissolved to make elements such as Cr, Mn, Mo, Ni, Cu and the like to be proper, and the content of each component element is optimized so that the pitting resistance equivalent PRE and the crevice corrosion resistance equivalent CRE are within predetermined ranges. Therefore, seawater corrosion resistance as high as that of austenitic super stainless steel and high strength equivalent to that of duplex stainless steel are obtained.  
      When the addition amount of each component element is optimized, rough Cr nitrides harmful to corrosion resistance can be reduced by solution treatment in a practical temperature range of 1050 to 1250° C. Since the solution treatment can be performed at a relatively low temperature, increase in manufacturing cost can be suppressed.  
      When the content of each component element is optimized so that the index of stability of austenite phase: Ni eq −Cr eq  is not less than 0, the austenite phase can be stably kept even after strong cold working. Therefore, high-nitrogen austenitic stainless steel which is nonmagnetic can be obtained.  
      Further, when the content of each component element is optimized, and Cr nitrides with diameter of 2 μm or more are dissolved by solution treatment of 1050 to 1250° C., the tensile strength after solution treatment becomes 1000 MPa or more by solid-solution strengthening of N.  
      When the content of each component element is optimized, Cr nitrides with diameter of 2 μm or more are dissolved by solution treatment of 1050 to 1250° C., and cold working is performed, then the tensile strength after solution treatment and cold working is increased by solid-solution strengthening and work hardening by N, and high strength equivalent to or more than that of Ni-based alloy or Ti-based alloy can be obtained.  
      Further, the strength after cold working rises in accordance with a reduction ratio. Therefore, if the cold working condition is optimized, the tensile strength after solution treatment and cold working becomes 1800 MPa or more.  
      Further, when the content of each component element is optimized, Cr nitrides with diameter of 2 μm or more are dissolved by solution treatment of 1050 to 1250° C., and the cold working condition is optimized, then the tensile strength after solution treatment and cold working becomes 2000 MPa or more by solid-solution strengthening and work hardening by N. Even in such a high strength state, an elongation of not less than 10% can be ensured.  
     EXAMPLES  
     Examples 1 to 18 and Comparative Examples 1 to 23 
      [1. Preparation of Samples] 
      Each of alloys having chemical components shown in Tables 1 and 2 was melted and cast in a pressurized induction furnace, whereby 50 kg of ingot was obtained. The ingot was homogenized, and made to a round bar with φ24 by hot forging. The resulting round bar was subjected to solution treatment. As the solution treatment condition, the bar was held at 1050 to 1300° C. for 1 hour, and then cooled with water.  
      The same test was carried out with respect to super stainless steel: SUS836L (Comparative Example 14), duplex stainless steel: SUS329J4L (Comparative Example 15), austenitic stainless steel: SUS316 (Comparative Example 16), Ni-based alloy: Inconel 625 (Comparative Example 17), and Ti (Comparative Example 18) as representatives of existing steel products. The heat treatment was performed in a generally adapted temperature condition.  
                                                                           TABLE 1                                   C   Si   Mn   P   S   Cu   Ni   Cr   Mo   Co   W   V   Al   Ti                                                                                                    EXAMPLES   1   0.03   0.13   0.4   0.01   0.01   0.16   0.48   27.3   1.30               0.008               2   0.03   0.21   1.8   0.02   0.01   0.05   0.20   24.9   1.90   0.23   0.12       0.007           3   0.08   0.31   7.9   0.02   0.01   0.23   2.30   31.4   0.92           0.06   0.006           4   0.03   0.20   4.2   0.01   0.01   0.72   0.50   17.9   5.80   0.26   0.13       0.006   0.09           5   0.02   0.11   3.4   0.02   0.01   0.32   1.38   21.2   6.43   0.73           0.005   0.06           6   0.03   0.22   6.5   0.02   0.01   0.16   0.07   27.1   0.21   0.47   0.15       0.005           7   0.05   0.21   7.3   0.02   0.01   0.16   0.09   25.0   2.30   0.32   0.23       0.006   0.02           8   0.03   0.15   7.7   0.02   0.01   0.43   0.03   21.2   2.43               0.007           9   0.06   0.02   9.1   0.02   0.01   0.19   0.09   19.5   3.90           0.07   0.008           10   0.22   0.22   7.1   0.02   0.01   0.16   0.07   24.3   3.80   0.47   0.15       0.005           11   0.05   0.21   5.8   0.02   0.01   1.39   0.09   23.7   1.75               0.006           12   0.06   0.20   5.9   0.02   0.01   0.16   3.80   23.3   3.40       0.05       0.005           13   0.04   0.20   7.2   0.02   0.01   0.16   0.22   24.1   2.40   2.10       0.15   0.005           14   0.04   0.20   5.4   0.02   0.01   0.16   0.18   25.3   1.74   0.28   1.44       0.005   0.15           15   0.01   0.28   6.9   0.01   0.01   0.16   0.22   25.0   2.48               0.011           16   0.03   0.01   7.8   0.02   0.01   0.16   0.10   24.3   2.10       0.15       0.007           17   0.07   0.31   4.5   0.01   0.01   0.13   1.01   23.2   4.20               0.006           18   0.05   0.18   8.8   0.01   0.01   0.13   0.05   24.4   2.33               0.005           19   0.04   0.20   5.4   0.02   0.01   0.63   2.20   24.6   1.80               0.005           20   0.08   0.28   6.9   0.01   0.01   0.16   1.30   25.0   2.45               0.009           21   0.03   0.15   8.4   0.02   0.01   0.20   1.10   24.3   2.30               0.007           22   0.09   0.31   6.8   0.01   0.01   0.13   1.01   22.1   4.20               0.006           23   0.05   0.18   4.5   0.01   0.01   0.51   2.40   24.4   2.33               0.005       COMPARATIVE   1   0.03   0.16   1.5   0.02   0.01   0.52   0.16   24.2   1.94   0.15   0.05       0.010       EXAMPLES   2   0.07   0.21   4.0   0.02   0.01   0.15   0.17   22.2   3.90   0.51       0.31   0.055           3   0.05   0.20   6.0   0.01   0.01   0.15   0.07   19.2   12.90           0.11   0.008   0.05           4   0.05   2.44   8.0   0.02   0.01   0.19   0.20   37.2   0.51           0.009           5   0.04   0.12   0.1   0.02   0.01   0.21   6.20   27.3   0.56   0.05   0.05       0.007           6   0.05   0.14   8.0   0.02   0.01   4.60   0.10   23.2   0.98               0.009           7   0.04   0.19   12.8   0.02   0.01   0.14   0.16   13.0   7.99           0.40   0.009           8   0.06   0.17   9.7   0.01   0.01   0.16   0.06   23.3   1.54           0.15   0.008   0.03           9   0.33   0.24   4.0   0.01   0.01   0.14   0.31   26.8   0.54           0.19   0.006           10   0.08   0.23   1.2   0.01   0.01   0.14   0.24   29.0   1.01               0.009   0.03           11   0.02   0.32   9.2   0.01   0.01   0.13   0.06   18.3   2.21               0.005           12   0.02   0.17   17.6   0.01   0.01   0.23   0.31   18.4   2.12               0.007           13   0.07   0.15   13.0   0.01   0.01   0.20   0.01   17.8   2.02               0.012           14   0.02   0.17   0.8   0.02   0.01   0.92   25.1   19.9   6.22               0.009           15   0.02   0.21   0.9   0.02   0.01       6.4   24.9   3.98               0.010           16   0.04   0.34   1.1   0.03   0.01   0.15   11.8   17.5   2.12               0.025           17   0.05   0.31   0.1   0.03   0.01       Bal.   22.1   9.10               0.011           18   0.01                                                   Bal.                  
 
                                                                             TABLE 2                                                                                   Nieq-           Nb   O   N   B   Mg   Ca   Ta   Zr   Te   Se   PRE   CRE   Creq                                                                                                EXAMPLES   1       0.002   1.31                               131.4   32.0   4.8           2   0.05   0.003   1.47           0.005                   30.4   30.7   10.6           3       0.002   1.33       0.003                       7.1   35.7   8.2           4   0.04   0.002   1.21                               13.4   29.7   6.9           5       0.002   1.22       0.003   0.002   0.09   0.10   0.03   0.10   18.2   33.6   3.8           6       0.001   1.20                               7.2   30.0   6.7           7   0.01   0.003   1.24           0.006                   7.2   31.1   7.5           8       0.002   0.93   0.002   0.003                       5.7   27.1   3.0           9       0.001   0.83                               5.0   27.2   1.5           10       0.001   1.08   0.003                           7.6   32.3   7.2           11       0.003   1.18           0.007                   8.3   30.1   7.6           12       0.003   1.26       0.002   0.003                   9.3   31.1   11.2           13   0.05   0.003   1.08               0.06               6.8   30.0   5.2           14   0.05   0.003   1.06                   0.09           8.9   30.0   1.2           15   0.07   0.002   1.26                       0.03       7.7   31.4   6.1           16       0.005   0.94       0.003   0.006                   5.9   29.5   0.8           17       0.004   1.15                               12.3   31.9   4.0           18       0.003   1.07                               5.6   30.2   4.5           19       0.005   1.14                               9.0   30.2   7.1           20       0.007   1.10                               7.3   31.0   5.4           21       0.005   1.07                               5.8   30.1   4.9           22       0.004   1.10                               7.9   30.7   5.6           23       0.003   1.05                               10.9   30.5   4.3       COMPARATIVE   1       0.029   1.20       0.003           0.11           33.2   30.0   4.6       EXAMPLES   2       0.002   1.03               0.11   0.09           12.9   30.3   0.3           3       0.003   1.19   0.002       0.003   0.10               13.5   41.1   −5.3           4       0.002   1.02                               6.9   40.2   −11.6           5       0.003   1.26               0.11       0.02   0.10   448.3   30.9   10.6           6       0.005   1.21   0.003                           5.7   31.7   12.2           7   0.05   0.002   1.21       0.030   0.004                   4.6   27.5   10.6           8   0.05   0.004   0.69                               4.1   27.2   −2.9           9       0.002   1.11                               11.6   30.0   10.9           10       0.004   1.85           0.002                   51.6   34.4   18.5           11       0.002   0.98   0.0024       0.003                   4.5   23.7   7.5           12       0.03   1.02               0.12               2.4   23.9   13.3           13       0.003   1.01                               3.1   23.1   12.7           14       0.007   0.14                               56.9   30.4   0.1           15       0.004   0.19                               45.6   31.3   −19.1           16       0.005   0.03                               22.7   20.9   −7.2           17       0.005   0.03                               —   —   —           18       0.09   0.01                               —   —   —                    
 [2.Evaluation (1)]
 
      The resulting ingots and round bars were evaluated as follows.  
      (1) Existence of Nitrogen Blow:  
      A test piece was cut out from a bottom portion of each ingot, and visual confirmation of nitrogen blow holes was performed therefor.  
      (2) Selection of the Solution Treatment Temperature:  
      The round bar after hot forging was subjected to solution treatment at various temperatures, after that, optional five visual fields thereof were observed by an optical microscope at 400-fold. Thereby, the lowest temperature at which the circle-converted diameter of Cr-based nitrides becomes 2 μm or less was determined. This lowest temperature was selected as the solution treatment temperature.  
      (3) Tensile Strength, Critical Pitting Temperature (CPT) and Depassivation pH:  
      A test piece was sampled from each round bar heat-treated at the selected solution treatment temperature, and a tensile strength, a critical pitting temperature (CPT) and a depassivation pH thereof were measured.  
      The tensile strength was measured in accordance with JIS Z2241. The critical pitting temperature (CPT) was measured in accordance with JIS G0578.  
      Further, with respect to the depassivation pH, the test piece was dipped in a 4.9% NaCl aqueous solution the pH of which is adjusted with HCl. A spontaneous potential thereof after the lapse of 24 hours was measured, the pH at the time when the potential transits from the active area to the passive area was determined and taken as the depassivation pH. The depassivation pH is correlative with crevice corrosion resistance, and the crevice corrosion resistance is more excellent as the depassivation pH is smaller.  
      The evaluation results of each sample are shown in Table 3.  
      In Comparative Examples 1, 2 and 9, the CPT was reduced, while the depassivation pH was raised because of excessive 0, Al and C. In Comparative Examples 3, 4 and 5, the solution treatment temperature exceeded 1250° C. because of excessive Mo, Cr and Ni. In Comparative Example 6, forging crack was caused because of excessive Cu. In Comparative Examples 7 and 8, the CPT was reduced, while the depassivation pH was raised because of excessive Mn and insufficient N. In Comparative Examples 11, 12 and 13, the CPT was reduced, or the depassivation pH was raised because of low PRE or CRE. In Comparative Example 10, N blow was caused in the ingot because of excessive N.  
      In contrast to this, in Examples 1 to 23, no N blow was caused. The solution treatment temperature was within the range of from 1050 to 1250° C. Further, the tensile strength after solution treatment was 1000 MPa or more in each case, and extremely satisfactory corrosion resistance was shown. The Ni eq −Cr eq  was not less than 0 in each case.  
                                       TABLE 3                                   Existence   Solution treatment   Tensile       Depassivation           of N-blow   (° C.)   Strength (MPa)   CPT (° C.)   pH                                                                EXAMPLES   1   None   1250   1150   &gt;70   &lt;0.5           2   None   1250   1240   &gt;70   &lt;0.5           3   None   1200   1189   &gt;70   &lt;0.5           4   None   1150   1150   &gt;70   &lt;0.5           5   None   1200   1171   &gt;10   &lt;0.5           6   None   1150   1130   &gt;70   &lt;0.5           7   None   1150   1160   &gt;70   &lt;0.5           8   None   1100   1093   &gt;70   0.60           9   None   1050   1072   &gt;70   0.60           10   None   1150   1102   &gt;70   &lt;0.5           11   None   1200   1141   &gt;70   &lt;0.5           12   None   1200   1170   &gt;70   &lt;0.5           13   None   1150   1141   &gt;70   &lt;0.5           14   None   1150   1131   &gt;70   &lt;0.5           15   None   1150   1201   &gt;70   &lt;0.5           16   None   1100   1091   &gt;70   &lt;0.5           17   None   1200   1120   &gt;70   &lt;0.5           18   None   1150   1008   &gt;70   &lt;0.5           19   None   1200   1106   &gt;70   &lt;0.5           20   None   1150   1102   &gt;70   &lt;0.5           21   None   1150   1114   &gt;70   &lt;0.5           22   None   1150   1133   &gt;70   &lt;0.5           23   None   1200   1066   &gt;70   &lt;0.5       COMPARATIVE   1   None   1250   1144   52.5   1.03       EXAMPLES   2   None   1200   1108   47.5   1.13           3   None   1300   1192   &gt;70   &lt;0.5           4   None   1300   1207   &gt;70   &lt;0.5           5   None   1300   1157   &gt;70   &lt;0.5           6   None   Forging crack   Forging crack   Forging crack   Forging crack           7   None   1200   1146   45   0.68           8   None   1100   932   40   0.73           9   None   1250   1125   50   0.98           10   Occured   —   —   —   —           11   None   1150   1098   60   1.23           12   None   1100   1106   22.5   1.23           13   None   1100   1104   27.5   1.28           14   None   1150   692   &gt;70   &lt;0.5           15   None   1100   702   42.5   &lt;0.6           16   None   1100   599   5   1.30           17   None   —   690   &gt;70   &lt;0.5           18   None   —   420   &gt;70   &lt;0.5                  
 
 [3.Evaluation (2)]
 
      With respect to Examples 5, 7, 15 and 16, 50 kg of each ingot was homogenized, and then made into a sheet stock 5 mm in thickness by hot forging and hot rolling. The solution treatment was then performed thereto at a solution treatment temperature selected in Evaluation (1). A test piece was cut out from the resulting sheet stock, and a crevice corrosion dipping test in real seawater for one year was performed. The same test was carried out also with respect to Comparative examples 14, 15, 16 and 18.  
      A crevice forming tool is shown in  FIGS. 1A  and 1B. The crevice forming tool  10  includes a cylindrical projection  12  having an outside diameter of 23 mm and an inside diameter of 20.5 mm at the tip thereof, as shown in  FIG. 1A . The cylindrical projection  12  includes grooves  12   a  having a width of 1 mm and a depth of 0.5 mm so as to form a total of twenty crevice forming parts  12   b.  Such a crevice forming tool  10  was fixed to both sides of a test piece  14  by a Ti-made bolt  16   a  and nut 16 b,  as shown in  FIG. 1B . Namely, a total of forty crevices were formed per one test piece. Three test pieces were suspended in a place 1 m below the sea level and dipped therein for one year. After the test was ended, the number of corroded crevices among a total of one hundred and twenty crevices was measured to determine a crevice corrosion generation rate. The result is shown in Table 4.  
                           TABLE 4                                   Crevice Corrosion               Generation rate (%)   Remarks                                                        Example 5   0               Example 7   0           Example 15   0           Example 16   0           Comparative example 14   0   SUS836L           Comparative example 15   8   SUS329J4L           Comparative example 16   62   SUS316           Comparative example 18   0   Ti                      
 
      In Comparative Examples 15 (corresponding to SUS329J4L) and 16 (corresponding to SUS316), the crevice corrosion generation rates were 8% and 62%, respectively. In Examples 5, 7, 15 and 16, the crevice corrosion generation rate was 0%, and the crevice corrosion resistances were equivalent to those in Comparative Examples 14 (corresponding to SUS836L) and 18 (Ti).  
      [4.Evaluation (3)] 
      For Examples 5, 7 and 15, 50 kg of each ingot was homogenized and made into a wire rod of φ12.5 by hot forging and hot rolling. Then, solution treatment was performed thereto at a solution treatment temperature selected in Evaluation (1). Further, the wire rod was made into a wire rod of φ8.8 by cold working with a reduction ratio of 50%. A test piece was cut out from the resulting wire rod, and a tensile strength and an elongation thereof were measured. The tensile strength and the elongation were measured in accordance with JIS Z2241. The results are shown in Table 5.  
      As is apparent from Table 5, each of the steel stocks has a tensile strength after cold working of not less than 2000 MPa, and an elongation of not less than 10%.  
                           TABLE 5                                   Tensile strength   Elongation           after   after           50%-Cold working   50%-Cold working           (MPa)   (%)                                                        Example 5   2235   12.1           Example 7   2254   11.0           Example 15   2273   10.2                      
 
      The embodiment of the present invention was described above in detail, but the present invention is never limited to the above-mentioned embodiment, and various modifications can be made within the scope not deviating from the gist of the present invention.  
      The high-nitrogen austenitic stainless steel according to the invention can be used in applications requiring seawater resistance, for example, for marine-related equipment, seashore environmental members, structural members for marine structures, seawater-desalination plant members, seawater heat exchanger members, submarine cables, structural members for submarine structures, mooring ropes, aquaculture fish nets, bridge wires for seashore section, seawater pumps, shafts, fastening members such as bolt, nut and screw, and the like.  
      This high-nitrogen austenitic stainless steel can be used also for general high-strength, high-corrosive member such as bolts, nuts, cylinder liners, shafts, hubs, connectors, bearings, races, rails, gears, pins, screws, rolls, turbine blades, molds, dies, drills, valves, valve seats, cutters, nozzles, gaskets, rings, springs, industrial furnace members, chemical plant members, medicine producing members, food producing plant members, food producing device members, oil drilling members, petroleum refining plant members, refuse incinerator members, steam turbine members, gas turbine members, reactor members, aircraft members, biomass plant members, and the like.  
      Further, steel products reduced in content of Ni are particularly applicable to biological materials and accessories, and can be used, for example, for: 
      (1) accessories such as necklaces, pierces or rings, back lids for watches, watch bands, eyeglass frames, interdental brushes and the like, which directly touch human bodies;     (2) dental materials such as artificial roots and correction wires, which are used within living bodies;     (3) implant materials such as plates, bolts, nuts, springs, screws, wires, electrodes, artificial bones, and artificial joints;     (4) medical instruments such as injection needles, knives, surgical knives, scissors, forceps, and drills.    

      Further, steel products sufficiently ensuring stability of austenite phase can be used as nonmagnetic, high-strength, and high-corrosive resistance materials for: 
      (1) springs, shafts, bearings, races, pins, dies, and rails for precision electronic parts;     (2) wires and meshes for printed board manufacturing parts;     (3) biological implant electrodes, MRI parts, and MRI-responding biological implant members; and     (4) medicine producing members, hanger members, linear motor car members, semiconductor manufacturing device parts, pincettes, bearing, scissors, cutters, and the like.