Abstract:
A high strength non-magnetic stainless steel comprising by weight ratio, less than 0.20 % C, less than 1.00 % Si, 14-16 % Mn, less than 0.005 % S, 0.2-1.0 % Ni, 15-19 % Cr, 0.30-0.40 % N, and Fe and other impurity elements, of which C+N constitutes 0.40-0.55 % and the Mn equivalent equals 30-33. The stainless steel has a hardness more than Hv 500, a magnetic permeability less than 1.01 after drawing, the steel may be suitably used as the steel for the micro shafts of video tape recorders and electromagnetic valves.

Description:
RELATED APPLICATION 
     This application is a continuation of Ser. No. 222,382, filed July 19, 1988, now abandoned, which is a continuation of Ser. No. 006,241, filed Jan. 20, 1987, now abandoned which is a continuation of Ser. No. 714,044, filed Mar. 18, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a non-magnetic stainless steel which is superior in strength and corrosion resistance and utilized for micro-shafts of video tape recorders (VTR) and electromagnetic valves, and also to a method for manufacturing the stainless steel. 
     2. Description of the Prior Art 
     As VTR equipment has become smaller, the driving mechanisms have changed. Most earlier VTR&#39;s employed a belt, whereas recent VTR&#39;s use a revolving shaft to drive the cartridge directly. 
     The raw material for a micro-shaft must possess a hardness of more than Hv 500, and its magnetic permeability must not exceed 1.01 after drawing. Conventional SUS420J2 is inferior in non-magnetic properties and cannot meet these requirements. 
     18Mn-5Cr steel possesses a hardness of over Hv 500 and a magnetic permeability (μ) of less than 1.01. This steel, however, is very inferior in corrosion resistance, though superior in hardness and non-magnetic properties after drawing. 
     Further, low Ni-high Mn stainless steels which possess high strength and non-magnetic properties are, for example: 
     
         0.1C-0.6Si-12.5Mn-1.6Ni-17.5Cr-0.35N                       (ASTM XM-28) 
    
     
         0.05C-0.6Si-13Mn-3.2Ni-17.5Cr-0.32N                        (ASTM XM-29) 
    
     
         0.1C-0.6Si-16Mn-0.1Ni-18Cr-0.4N                            (ASTM XM-31) 
    
     
         0.18C-0.6Si-15Mn-1.25Ni-17Cr-0.35N                         (205) 
    
     ASTM XM-28 possesses a hardness exceeding Hv 500, but its magnetic permeability is 1.05. 
     ASTM XM-29 is superior in that its magnetic permeability is less than 1.01 but it does not possess a hardness of more than Hv 500. 
     Each of ASTM XM-31 and 205, like XM-29, has a magnetic permeability less than 1.01, but they cannot stably provide a hardness of more than Hv 500. ASTM XM-31 and 205 are inferior also in hot-workability and ductility after drawing because of a large amount of Mn contained therein. Wire drawing of more than 50% was tested and found to be difficult with these steels. 
     All of these steels are low Ni and high Mn steels, and therefore they are inferior to SUS304 in corrosion resistance and ductility after drawing. 
     SUMMARY OF THE INVENTION 
     One of the objects of the present invention is to provide a high-strength non-magnetic stainless steel having a hardness of more than Hv 500 and a magnetic permeability of less than 1.01 after drawing. 
     Another object of the present invention is to provide a high-strength non-magnetic stainless steel which has, in addition to hardness, magnetic permeability and a high corrosion resistance. 
     A further object of the present invention is to provide a method for manufacturing the high-strength non-magnetic stainless steel having a high hardness, low magnetic permeability and high corrosion resistance. 
     Thus, the present invention provides a stainless steel comprising, by weight, not more than 0.20% carbon, not more than 1.00% silicon, 14-16% manganese, not more than 0.005% sulfur, 0.2-1.0% nickel, 15-19% chromium, 0.30-0.40% nitrogen, said carbon and said nitrogen constituting 0.40-0.55% and Mn equivalent being 30-33, the remainder being iron together with impurities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A better understanding of the prior art and of the present invention will be obtained by reference to the detailed description below and to the attached drawings, in which: 
     FIG. 1 is a diagram illustrating the influence of the Mn equivalent and Ni in relation to the hardness after drawing; 
     FIG. 2 is a diagram illustrating the influence of the Mn equivalent in relation to magnetic permeability after drawing; 
     FIG. 3 is a diagram illustrating the influence of S in relation to corrosion resistance; 
     FIG. 4 is a diagram illustrating the influence of S in relation to elongation after drawing; 
     FIG. 5 is a diagram illustrating the relationship between the drawing rates and the hardness after drawing; and 
     FIG. 6 is a diagram illustrating the relationship between low temperature annealing and the tensile strength and hardness of the steels after drawing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention was made as a result of research in the effects of alloying elements on the strength, non-magnetic properties, and corrosion resistance of Cr-Mn-N low-Ni stainless steels. The high-strength non-magnetic stainless steel of this invention needs to satisfy, firstly, a magnetic permeability of less than 1.01 and a high work-hardenability required to provide a hardness exceeding Hv 500. The γ phase of the high-strength non-magnetic stainless steel should be stable, even when high-level drawing is done. Secondly, the high strength non-magnetic stainless steel should contain a large amount of manganese and nitrogen and little nickel to obtain work hardenability, yet the corrosion resistance, hot-workability, and ductility after drawing of the stainless steel do not decrease. The high-strength non-magnetic stainless steel has an optimal composition of such alloying elements as carbon, manganese, chromium, nitrogen, and nickel. 
     The present invention has found that the magnetic permeability is determined by Mn equivalent obtained by the following equation after having researched the effects of such alloying elements as manganese, carbon, chromium, nickel, nitrogen, and silicon on magnetic permeability: 
     
         Mn equivalent=Mn+20C+0.4Cr+Ni+18N+0.35Si. 
    
     According to the present invention the Mn equivalent is required to be equal to or more than 30 in order to obtain magnetic permeability equal to or less than 1.01 at a drawing rate (percentage of reduction of area) of 60% as illustrated in FIG. 2. 
     FIG. 1 shows effects of the Mn equivalent and of the amount of Ni on the hardness after 60% drawing, C+N content being constant at 0.47%. The hardness decreases as the amount of Ni or of Mn equivalent increases. The Mn equivalent should be equal to or less than 33, and the amount of Ni needs to be equal to or less than 1.0% in order to obtain a hardness of more than Hv 500. Further, it is preferable to keep the Mn equivalent at the minimum required to obtain non-magnetic properties because the hot-workability, hardness after drawing and ductility decrease as the Mn equivalent increases. 
     FIGS. 3 and 4 show the relationship of S content and the corrosion resistance and ductility after drawing for a steel containing 0.12% C, 0.61% Si, 14.5% Mn, 17% Cr, 0.8% Ni, and 0.35% N. And, it is apparent from FIGS. 3 and 4 that the decrease of the S content decreases corrosion loss in weight and increases elongation. The corrosion loss of 0.4 g/m 2 .Hr and elongation more than 5% comparable to those of SUS304 can be obtained by keeping the S amount equal to or less than 0.005%. 
     0.40-0.55% (C+N) content and equal to or less than 1.0% Ni are kept to obtain a high work-hardenability, such as a hardness exceeding Hv 500. 
     To stabilize the γ phase, the magnetic permeability is kept less than 1.01 by keeping Mn equivalent between 30 and 33. 
     Further, to compensate the decreases of corrosion resistance due to the decrease of Ni, of the hot-workability and of the ductility after drawing by the increase of N amount, the S amount is kept equal to or less than 0.005%, and the Mn equivalent is kept equal to or less than 33. 
     Thereby, a corrosion resistance, hot-workability and ductility after drawing comparable to those of SUS304 are obtained. 
     The steel of the present invention comprises by weight, not more than 0.2% carbon, not more than 1.00% silicon, 14-16% manganese, not more than 0.005% sulfur, 0.2-1.0% nickel, 15-19% chromium, 0.30-0.40% nitrogen, said carbon and said nitrogen constituting 0.40-0.55% and manganese equivalent being 30-33, the remainder being iron together with impurities. 
     In addition, the steel contains, when necessary not more than 0.1% aluminum, not more than 0.020% phosphorus and not more than 0.0050% oxygen to further improve the corrosion resistance, hot-workability and ductility after drawing. 
     The steel of the present invention is provided with a high work-hardenability. 
     This steel may be required to be work-hardened to stably obtain hardness more than Hv 500. 
     The steel of the present invention was work-hardened by 50-70% drawing as indicated in FIG. 5. In addition, the steel is annealed at low-temperature (250°-550° C.) when additional strength is desired. FIG. 6 shows the relationship between low temperature annealing and the tensile strength and hardness of the steels after drawing. In FIG. 6, the steel at room temperature was not annealed. 
     The reason for limiting the composition of the steel of the present invention is explained hereunder. 
     Carbon is an element which contributes to the work hardenability and also stabilizes the γ phase. The maximum content of C is limited to 0.20%. The corrosion resistance degrades when the C content exceeds 0.20%. 
     The Si content which is required for deoxidizing is limited to 1.00%. When contained more than necessary, silicon causes an imbalance of δ/γ and degradation in hot workability. 
     The minimum content of manganese, one of the main elements of the steel of this invention, is 14%. 
     Manganese contributes to the work-hardenability, the stabilization of the γ phase, thereby making the γ phase with a high work-hardenability, and increasing the N solid solution. 
     The minimum content of manganese is determined to be 14%. The Mn content is required to be not less than 14% to obtain these effects. 
     The maximum content of manganese is limited to 16%. When the Mn content exceeds 16%, Mn over-stabilizes the γ phase and causes the work-hardenability of the γ phase to decrease. Also, hot workability and corrosion resistance are decreased. 
     The maximum S content in the steel of the present invention is 0.005%. S decreases the corrosion resistance, hot-workability of the steel of this invention and the ductility of drawn steel. For this reason, the S content in the steel is required to be minimal. And, preferably the S content should be kept to equal to or less than 0.001%. 
     The minimum Ni content should be 0.2%. Ni stabilizes the γ phase and should constitute at least 0.2% of the steel by weight. If the Ni content exceeds 1.0%, the work-hardenability of the γ phase and the solid solution of N is decreased. Therefore, the maximum Ni content should be 1.0%. 
     The minimum Cr content should be 15%. Cr, another main element of the steel of this invention, increases the steel&#39;s corrosion resistance, the work-hardenability of the phase, the stabilization of the γ phase during drawing, and an increase of solid solution of N. And, the Cr content is required to be more than 15% to obtain these effects. 
     If the Cr content increases, it causes the disruption of the δ/γ balance at high temperature and a degradation of hot-workability. Therefore, the Cr content should be 19% at most. 
     Nitrogen, which facilitates the stabilization of the γ phase, work-hardenability, and corrosion resistance should constitute more than 0.30% of the steel. When the N content exceeds 0.40%, however, nitrogen causes a sharp degradation of the hot-workability, and blow holes develop as the steel ingot solidifies. Therefore, the N content should not exceed 0.40%. 
     The maximum content of phosphorus and oxygen should be 0.020% and 0.0050%, respectively. Phosphorus and oxygen degrade the corrosion resistance, hot workability and ductility after drawing, and must be kept at minimal levels. Preferably, the steel should contain less than 0.015% phosphorus and 0.0040% oxygen. 
     The maximum Al content should be 0.10%. Aluminum improves the corrosion resistance, the hot-workability, and ductility after drawing. When the Al content exceeds 0.10%, aluminum, however, degrades hot-workability. 
     The features of the steel of this invention become more apparent hereunder from the comparison between embodiments of this invention and conventional steels, and comparative steels. 
     Table 1 shows the chemical compositions of the steel employed for the comparison. 
     In Table 1, steels A-F are conventional steels; steel A is SUS420J2, steel B is ASTM XM-28, steel C is ASTM XM-29, steel D is ASTM XM-31, steel E is 205, and steel F is SUS304, steels G-J are comparative steels, and steels K-Q are sample steels of the present invention. 
     
                                           TABLE 1__________________________________________________________________________CHEMICAL COMPOSITION (WEIGHT %)      MnC    Si Mn P  S  Ni Cr N  Al O   C + N                                EQUIVALENT__________________________________________________________________________A 0.310.46   0.75      0.027         0.010            0.08               12.87                  0.01                     0.008                        0.0083                            0.32                                12.5B 0.080.62   12.63      0.025         0.011            1.83               17.55                  0.36                     0.008                        0.0075                            0.44                                29.8C 0.040.59   13.12      0.031         0.011            3.17               18.24                  0.33                     0.010                        0.0078                            0.37                                30.6D 0.100.67   15.98      0.029         0.010            0.08               17.81                  0.40                     0.009                        0.0081                            0.50                                32.6E 0.170.63   15.56      0.030         0.010            1.37               17.48                  0.33                     0.011                        0.0087                            0.50                                33.5F 0.050.54   1.67      0.026         0.012            9.20               18.35                  0.01                     0.011                        0.0073                            0.06                                19.6G 0.120.64   17.34      0.028         0.008            0.58               17.46                  0.34                     0.010                        0.0079                            0.46                                33.6H 0.100.58   14.53      0.025         0.015            0.07               14.77                  0.38                     0.008                        0.0072                            0.48                                29.8J 0.120.46   14.82      0.027         0.009            0.64               17.43                  0.25                     0.010                        0.0079                            0.37                                29.5K 0.100.63   14.36      0.024         0.002            0.69               17.56                  0.33                     0.010                        0.0088                            0.43                                30.2L 0.130.58   14.82      0.027         0.004            0.43               15.83                  0.36                     0.008                        0.0071                            0.49                                30.9M 0.160.67   14.47      0.023         0.003            0.87               16.24                  0.35                     0.008                        0.0068                            0.51                                31.6N 0.120.72   15.76      0.022         0.001            0.63               16.46                  0.31                     0.008                        0.0055                            0.43                                31.2P 0.120.62   14.39      0.015         0.002            0.79               16.68                  0.35                     0.011                        0.0038                            0.47                                30.8Q 0.150.57   14.76      0.012         0.002            0.65               17.29                  0.33                     0.081                        0.0028                            0.48                                31.5__________________________________________________________________________ 
    
     
                                           TABLE 2__________________________________________________________________________HARDNESS (Hv)    AFTER        MAGNETIC  CORROSIONAFTER    LOW TEMPERATURE                 PERMEABILITY                           RESISTANCE                                   ELONGATION                                            HOT-DRAWING  ANNEALING    (μ)    (g/m.sup.2 · Hr)                                   (%)      WORKABILITY__________________________________________________________________________A 520    --           not less than 10                           2.21    --       OB 470    495          1.012     0.55    6        OC 435    450          1.008     0.52    8        OD 525    550          1.003     0.60    3        XE 495    520          1.003     0.56    4        XF 415    445          2.15      0.42    5        OG 495    520          1.003     0.52    4        XH 560    581          1.021     1.15    4        XJ 492    518          1.019     0.67    4        XK 515    540          1.008     0.05    8        OL 532    550          1.005     0.32    6        OM 521    549          1.004     0.17    7        ON 503    521          1.003     0.04    8        OP 510    541          1.006     0.08    9        OQ 515    525          1.003     0.10    8        O__________________________________________________________________________ 
    
     Table 2 shows the hardness, magnetic permeability, corrosion resistance, ductility, and hot-workability of steels A-Q, shown in Table 1. 
     The hardness, magnetic permeability, corrosion resistance and ductility of the steels are measured after drawing (percentage of a reduction of area 60%) and after a low temperature annealing at 400° C. for 20 minutes. 
     The corrosion resistance was measured as the corrosion weight loss of each of the steels which were immersed in an 3.5%NaCl+2%H 2  O 2  aqueous solution at 40° C. for 48 hours. 
     The hot-workability was measured whether or not the occurrence of cracks in the steels when 300 kg ingots of the steel were pressed.  ○ indicates those steels in which cracks did not occur, and × indicates those steels in which cracks occurred. 
     The conventional steel A is superior in hardness (Hv 520) after 60% drawing, but is inferior in magnetic permeability which far exceeds 1.010 and the corrosion resistance which far exceeds 0.50 g/m 2 .Hr. 
     The conventional steel B is superior in ductility and hot-workability, yet the steel B is inferior in hardness which is Hv 470, magnetic permeability (1.012) and corrosion resistance (0.55 g/m 2 .Hr). 
     The conventional steel C is superior in magnetic permeability, yet inferior in hardness, which is Hv 435, and corrosion resistance (0.52 g/m 2 .Hr). 
     The conventional steel D is superior in hardness and magnetic permeability, yet inferior in corrosion resistance, ductility, and hot-workability. 
     The conventional steel E is superior in hardness (Hv 520) after low temperature annealing and magnetic permeability which is 1.003, yet inferior in corrosion resistance, ductility, and hot-workability. 
     The conventional steel F is superior in corrosion resistance, ductility and hot-workability, yet inferior in hardness, which is Hv 415, and magnetic permeability, which is 2.15. 
     None of the conventional steels can provide a high level of hardness, a low magnetic permeability and high corrosion resistance together. 
     As for the comparative steels; 
     The steel G is superior in magnetic permeability and corrosion resistance, yet inferior in hot-workability and hardness, which is Hv 495 after drawing because of a decrease in work-hardenability of the γ phase due to a large Mn content, 17.34%. 
     The comparative steel H is superior in hardness, but inferior in magnetic permeability which is 1.021, and corrosion resistance, which is 1.15 g/m 2 .Hr, because of a low Mn equivalent (29.8) and a low Cr, which is 14.77%. 
     The comparative steel J is inferior in hardness, which is Hv 492, magnetic permeability, which is 1.019, and corrosion resistance which is 0.67 g/m 2 .Hr, because of a low manganese equivalent (29.5) and a low nitrogen 0.25%. 
     The steels K-Q of the present invention contain an optimal amount of Mn, C, Cr, Ni, N, and an Mn equivalent of 30-33, thereby the steels K-Q possess hardnesses exceeding Hv 500 after drawing and more than Hv 520 after low temperature annealing. The magnetic permeabilities of steels K-G are less than 1.010 after 60% drawing, and corrosion losses by weight less than 0.50 g/m 2 .Hr. 
     The steels K-Q are satisfactory in hardness, magnetic permeability, corrosion resistance and superior in ductility and hot-workability. 
     The steels of the present invention possess a magnetic permeability comparable to ASTM XM-29.31 and a corrosion resistance comparable to SUS304, and a superiority in ductility and hot-workability as set forth above. Therefore, the steels of this invention and the method for manufacturing the same can be effectively employed for high-strength non-magnetic stainless steels used for micro-shafts of VTR&#39;s and electromagnetic valves.