Non-magnetic stainless steel

This invention relates to a high strength non-magnetic stainless steel for a spring, which comprises at most 0.2% of carbon, at most 3% of manganese, at most 0.045% of phosphorus, at most 0.03% of sulfur, at most 1% of silicon, 18 to 20% of chromium, 8 to 12% of nickel, 0.08 to 0.25% of nitrogen and the balance of iron and which is subjected to a heat treatment and a cold or warm working.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-magnetic stainless steel and more particularly it is concerned with a low-priced and non-magnetic stainless steel for a spring, having excellent properties.

2. Description of the Prior Art

Up to the present time, SUS 304 stainless steel has widely been used as a high strength stainless steel for a spring. SUS 304 stainless steel is considered to be non-magnetic because of having an austenitic structure after a heat treatment, but SUS 304 obtained as a material for a spring is not non-magnetic because, when subjected to cold working to obtain a high strength as a spring, the austenite is partially transformed into a strain-induced martensite that is ferromagnetic. This strain-induced martensite acts also as a notch or a nucleus of fatigue fracture, resulting in deterioration of the fatigue property. In a case where a non-magnetic property is required, therefore, SUS 316 having a large nickel content has often been used. However, SUS 316 is too expensive because of containing large amounts of nickel and molybdenum and, in addition, SUS 316 is not so suitable for use as a spring steel because of containing large amounts of additional elements so that the strength is only 80 to 90% of that of SUS 304.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a non-magnetic steel having a high strength.

It is another object of the present invention to provide a cheap and non-magnetic stainless steel for a spring, which has various excellent mechanical properties.

It is a further object of the present invention to provide a non-magnetic spring steel which austenitic structure is stabilized by adding nitrogen.

The objects can be attained by a high strength non-magnetic stainless steel for a spring, which consists of 0.20% or less of carbon, 3.00% or less of manganese, 0.045% or less of phosphorus, 0.030% or less of sulfur, 1.00% or less of silicon, 18.00 to 20.00% of chromium, 8.00 to 12.00% of nickel, 0.08 to 0.25% of nitrogen and the balance of iron and which is subjected to a heat treatment and a cold or warm working.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings are to illustrate the principle and merits of the present invention in more detail.

FIG. 1 shows graphically the relation of the wire diameter and tensile strength as to the stainless steels of the present invention and the prior art.

FIG. 2 and FIG. 3 show graphically the results of fatigue tests as to the stainless steels of the present invention and the prior art.

FIG. 4, FIG. 5 and FIG. 6 show graphically the magnetic permeability as to the stainless steels of the present invention and the prior art.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have made efforts to find a stainless steel for a spring, whereby the above described disadvantages of the prior art can be overcome, and consequently, have found that this object can be accomplished by adding nitrogen to the ordinary 18Cr-8Ni type stainless steel and then subjecting to melting, hot rolling, heat treatment, i.e. solution treatment and cold or warm working whereby to give a strength sufficient for a spring. The present invention is based on this finding.

Therefore, in accordance with the present invention, there is provided a high strength non-magnetic stainless steel suitable for a spring, which comprises 0.2% or less, generally 0.01 to 0.20% of carbon, 3.00% or less, generally 0.5 to 3.00% of manganese, 0.045% or less, generally 0.001 to 0.045% of phosphorus, 0.030% or less, generally 0.001 to 0.030% of sulfur, 1.00% or less, generally 0.1 to 1.00% of silicon, 18.00 to 20.00% of chromium, 8.00 to 12.00% nickel, 0.08 to 0.25% of nitrogen and the balance iron, and which is subjected to a heart treatment and a cold or warm working. The percents used in this specification are to be taken as those by weight unless otherwise indicated.

Nitrogen is added for the purpose of stabilizing the austenitic structure in a proportion of generally 0.08 to 0.25%, but the preferable range is 0.15 to 0.22%, since if less than 0.08%, the effect of non-magnetizing is little and if more than 0.25%, melting and hot rolling are difficult. In addition, the steel of the invention can contain 2% or less of molybdenum and 1% or less of copper. Manganese is effective for stabilizing the austenite and thus a martensite does not tend to occur with an increased amount of manganese. However, if more than 3%, there is a problem on the brick life during melting of the steel. In the stainless steel according to the present invention, the austenitic structure can be stabilized by adding nitrogen and the martensitic transformation due to cold or warm working can be suppressed to make the steel non-magnetic. Since this stainless steel has the similar components to SUS 304, rapid hardening takes place by cold working to give a high strength and crystal grains are made fine to raise the toughness. Furthermore, carbonitrides are formed to pin the dislocations and to improve the high temperature strength. The stainless steel of the present invention is also excellent in fatigue property due to its little martensite as well as in resistance to pitting corrosion due to its stabilized austenite by nitrogen resulting in suppression of formation of .delta.-ferrite.

The following examples, which are representative of some of the compositions of the present invention, describe in detail five specific compositions.

EXAMPLES

The specimens of the present invention (Sample A-E) and the prior art having the chemical compositions as shown in Table 1 were drawn and then subjected to measurement of various properties. The drawing was carried out by the ordinary cold drawing method, but in the case of Samples B and E only, the cooling means of a drawing machine was stopped to increase the drawing temperature and to effect the warm drawing thereof. The final wire diameters are shown in Table 2 and the temperatures of the last die at inlet and outlet are shown in Table 3, measured by means of a contact type thermometer.

                                      TABLE 1
    __________________________________________________________________________
    Chemical Composition of Specimens
    C        Si Mn P  S  Cu Cr Ni Mo  N
    __________________________________________________________________________
    SUS 304
          0.069
             0.72
                1.42
                   0.024
                      0.011
                         0.05
                            18.17
                               8.80
                                  0.05
                                     0.021
    Sample A
          0.073
             0.66
                1.44
                   0.032
                      0.012
                         0.21
                            18.28
                               8.50
                                  0.24
                                     0.15
    Sample B
          0.073
             0.66
                1.44
                   0.032
                      0.012
                         0.21
                            18.28
                               8.50
                                  0.24
                                     0.15
    Sample C
          0.065
             0.59
                1.41
                   0.029
                      0.010
                         0.09
                            18.30
                               8.81
                                  0.19
                                     0.10
    Sample D
          0.070
             0.71
                2.18
                   0.028
                      0.011
                         0.15
                            18.31
                               10.01
                                  0.32
                                     0.20
    Sample E
          0.073
             0.62
                1.50
                   0.027
                      0.003
                         0.03
                            18.38
                               8.61
                                  0.03
                                     0.211
    SUS 316
          0.055
             0.62
                1.50
                   0.026
                      0.010
                         0.04
                            16.23
                               12.60
                                  2.05
                                     0.041
    __________________________________________________________________________

			

			

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TABLE 2 ______________________________________ Wire Diameter Used Wire Diameter (mm) Sample 0.3 .phi. 0.6 .phi. 0.7 .phi. 0.9 .phi. 1.0 .phi. 2.0 .phi. ______________________________________ SUS 304 .circle. .circle. .circle. .circle. A .circle. .circle. .circle. .circle. B .circle. .circle. C .circle. D .circle. E .circle. .circle. .circle. SUS 316 .circle. .circle. .circle. .circle. ______________________________________

TABLE 3 ______________________________________ Temperature of Last Die Inlet Outlet ______________________________________ SUS 304 32.degree. C. 120.degree. C. Sample B 155.degree. C. 280.degree. C. SUS 316 30.degree. C. 115.degree. C. ______________________________________

FIG. 1 shows graphically the relation of the wire diameter after drawn and the tensile strength, in which the curves are plotted by for Sample A, .DELTA. for Sample B, .quadrature. for Sample C, .gradient. for Sample D, for Sample E, for SUS 304 (comparison) and x for SUS 316 (comparison). Referring to FIG. 1, the shaded portion is a zone defined by SUS 304 WPB Standard (JIS G 4314, Stainless Steel Spring Wire), in which there are Samples A, B, D and E. This is to say, these samples have an at least 10% higher strength than SUS 316.

In Table 4 the shear modulus important for a spring is shown as to Samples A, B and E according to the present invention and SUS 304 and SUS 316 of the prior art.

                  TABLE 4
    ______________________________________
    Shear Modulus
                 (wire diameter 2.0 mm.phi., torsional
                 pendulum method)
    Sample       Shear Modulus (Kg/mm.sup.2)
    ______________________________________
    SUS 304      7100
    Sample A     7180
    Sample B     7120
    Sample E     7150
    SUS 316      7100
    ______________________________________

                  TABLE 5
    ______________________________________
     Dimension of Spring
    ______________________________________
    Wire Diameter          2.0 mm.phi.
    Coil Mean Diameter    18.5 mm.phi.
    Total Windings         6.5
    Effective Windings     4.5
    Winding Direction     Right
    Free Length           47.0 .+-. 0.3 mm
    ______________________________________

                  TABLE 6
    ______________________________________
    Pitting Potential and Maximum
    Current Density in Active State (in
    5% H.sub.2 SO.sub.4 + 3% NaCl, 35.degree. C. wire diameter
    2mm .phi.)
             Pitting Potential
                          Maximum Current Density
    Sample   (V vs SCE)   in Active State (mA/cm.sup.2)
    ______________________________________
    SUS 304  0.04         2.45
             0.13         3.53
             0.14         3.57
    Sample A 0.42         1.93
             0.31         1.90
             0.33         2.15
    Sample B 0.35         1.98
    Sample E 0.31         3.00
    SUS 316  0.53         3.73
             0.50         1.85
    ______________________________________

                  TABLE 7
    ______________________________________
    Salt Water Spraying Test (5% NaCl
    solution, Test Period 2 months)
    Sample      Surface State after Test
    ______________________________________
    SUS 304   one of three samples: corrosed at end thereof
    Sample A  three samples: no corrosion
    SUS 316   three samples: no corrosion
    ______________________________________

                  TABLE 8
    ______________________________________
     (% by weight)
    Wire Diameter
    Sample 0.3 mm .phi.
                       0.6 mm .phi.
                                   1.0 mm .phi.
                                          2.0 mm .phi.
    ______________________________________
    SUS 304
           50%         45%         35%    15%
    Sample E
           less than 3%
                       less than 3%
                                   0      0
    SUS 316
           less than 3%
                       less than 3%
                                   0      0
    ______________________________________