Abstract:
Ferritic stainless steel, compirsing the following composition by weight:  
     0%&lt;C≦0.030%  
     1%≦Si≦3%  
     0%&lt;Mn≦0.5%  
     10%≦Cr≦13%  
     0%&lt;Ni≦0.5%  
     0%&lt;Mo≦3%  
     N≦0.030%  
     Cu≦0.5%  
     Ti≦0.5%  
     Nb≦1%  
     Ca≧1 10 −4 %  
     O≧10 10 −4 %  
     S≦0.030%  
     P≦0.030%  
     the remainder being iron and the impurities which are inevitable from the production of the steel.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention concerns a ferritic stainless steel which can be used for ferromagnetic parts.  
           [0002]    Ferritic stainless steels are characterised by a given composition, the ferritic structure being notably provided, after hot rolling and cooling of the composition, by a thermal annealing treatment conferring the said structure on them.  
           [0003]    Amongst the major classes of ferritic stainless steels, defined notably according to their chromium and carbon content, there are:  
           [0004]    the ferritic stainless steels which can contain up to 0.17% carbon. These steels, after the cooling which follows their production, have a two-phase austeno-ferritic structure. They may however be converted into ferritic stainless steels after annealing in spite of a relatively high carbon content;  
           [0005]    the ferritic stainless steels whose chromium content is around 11 or 12%. They are fairly close to martensitic steels containing 12% chromium, but different through their carbon content, which is relatively low.  
           [0006]    During the hot rolling of stainless steels, the structure of the steel can be two phase, ferritic and austenitic. If the cooling is, for example, energetic, the final structure is ferritic and martensitic. If it is slower, the austenite decomposes partially into ferrite and carbides, but with a higher carbide content than the surrounding matrix, the austenite having solubilised hot more carbon than ferrite. In both cases, a tempering or annealing must be performed on the hot-rolled and cooled steels in order to generate a completely ferritic structure. The tempering can be carried out at a temperature of approximately 820° C. lower than the Ac1 alpha→gamma transition temperature, which gives rise to a precipitation of carbides.  
           [0007]    In the field of ferritic steels intended for an application using magnetic properties, the ferritic structure is obtained by limiting the quantity of carbides, and it is for this reason that the ferritic stainless steels developed in this field have a carbon content below 0.02%.  
         DESCRIPTION OF THE PRIOR ART  
         [0008]    Steels are known which can be used for their magnetic properties, such as for example in the document U.S. Pat. No. 5,769,974, which describes a method of manufacturing a corrosion-resistant ferritic steel able to reduce the value of the coercive field of the said steel. The steel used in the method is a steel of the resulfurated type. The sulfur reduces the cold deformation properties. The steel obtained by the method is therefore difficult to use for the production of cold-forged parts.  
           [0009]    The patent U.S. Pat. No. 5,091,024 is also known, in which there are presented corrosion-resistant magnetic articles formed by an alloy consisting essentially of a composition with a low carbon content and a low silicon content, that is to say respectively below 0.03% and 0.5%. However, in the magnetic domain, it is important for the steel to contain a high silicon content in order to increase the resistivity of the material and to reduce eddy currents.  
           [0010]    The purpose of the present invention is to present a stainless steel with a ferritic structure which can be used for magnetic parts with strong magnetic properties and presenting good properties of use in terms of cold forging and good machinability properties.  
         SUMMARY OF THE INVENTION  
         [0011]    The object of the invention is a ferritic stainless steel which can be used for ferromagnetic parts which comprises, in its composition by weight:  
           [0012]    0%&lt;C≦0.030%  
           [0013]    1%≦Si≦3%  
           [0014]    0%&lt;Mn≦0.5%  
           [0015]    10%≦Cr≦13%  
           [0016]    0%&lt;Ni≦0.5%  
           [0017]    0%&lt;Mo≦3%  
           [0018]    N≦0.030%  
           [0019]    Cu≦0.5%  
           [0020]    Ti≦0.5%  
           [0021]    Nb≦1%  
           [0022]    Ca≧1 10 −4 %  
           [0023]    O≧10 10 −4 %  
           [0024]    S≦0.030%  
           [0025]    P≦0.030%  
           [0026]    the remainder being iron and the impurities inevitable from the production of the steel.  
           [0027]    The other characteristics of the invention are:  
           [0028]    the composition by weight also includes calcium and oxygen so that:  
           [0029]    Ca&gt;30 10 −4 %  
           [0030]    O&gt;70 10 −4 %  
           [0031]    the ratio between the calcium and oxygen content Ca/O being  
           [0032]    0.2≦Ca/O≦0.6  
           [0033]    the steel contains inclusions of lime silico-aluminate of the anorthite and/or pseudo-wollastonite and/or gehlenite type;  
           [0034]    preferably the steel comprises, in its composition by weight:  
           [0035]    0%&lt;C 0.015%  
           [0036]    1%≦Si≦3%  
           [0037]    0≦Mn≦0.4%  
           [0038]    10%≦Cr≦13%  
           [0039]    0%&lt;Ni≦0.2%  
           [0040]    0.2%≦Mo ≦2%  
           [0041]    N≦0.015%  
           [0042]    Cu≦0.2%  
           [0043]    Ti≦0.2%  
           [0044]    Nb≦1%  
           [0045]    Ca≧30 10 −4 %  
           [0046]    O≧70 10 −4 %  
           [0047]    S≦0.003%  
           [0048]    P≦0.030%  
           [0049]    the remainder being iron and the impurities inevitable from the production of the steel;  
           [0050]    preferably the steel comprises, in its composition by weight:  
           [0051]    0%&lt;C≦0.015%  
           [0052]    1%≦Si≦3%  
           [0053]    0≦Mn≦0.4%  
           [0054]    10%≦Cr≦13%  
           [0055]    0%&lt;Ni≦0.2%  
           [0056]    0.2%≦Mo ≦2%  
           [0057]    N≦0.015%  
           [0058]    Cu≦0.2%  
           [0059]    Ti≦0.2%  
           [0060]    Nb≦1%  
           [0061]    Ca≧30 10 −4 %  
           [0062]    O≧70 10 −4 %  
           [0063]    0.015≦S≦0.03%  
           [0064]    P≦0.030%  
           [0065]    the remainder being iron and the impurities inevitable from the production of the steel.  
           [0066]    The invention also concerns a method of producing a ferritic steel wherein the composition by weight is subjected, after hot rolling and cooling, to a thermal annealing treatment and then a modification of cross-section of the drawing or stretch forming type.  
           [0067]    The drawn or stretch-formed steel can subsequently be subjected to an additional recrystallisation annealing in order to perfect the magnetic properties of the part.  
         DESCRIPTION OF THE PREFERRED EMBODIMENTS  
         [0068]    The following description and the single FIGURE, the whole given by way of non-limitative example, will give a clear understanding of the invention. 
       
    
    
       [0069]    The single FIGURE presents a ternary diagram giving the general composition of the inclusions of aluminosilicates of lime. 
     
    
       [0070]    The invention concerns a steel with the following general composition:  
         [0071]    0%&lt;C≦0.030%  
         [0072]    1%≦Si≦3%  
         [0073]    0%&lt;Mn≦0.5%  
         [0074]    10%≦Cr≦13%  
         [0075]    0%&lt;Ni≦0.5%  
         [0076]    0%&lt;Mo≦3%  
         [0077]    N≦0.030%  
         [0078]    Cu≦0.5%  
         [0079]    Ti≦0.5%  
         [0080]    Nb≦1%  
         [0081]    Ca≧1 10 −4 %  
         [0082]    O≧10 10 −4 %  
         [0083]    S≦0.030%  
         [0084]    P≦0.030%  
         [0085]    the remainder being iron and the impurities inevitable from the production of the steel.  
         [0086]    From the metallurgical point of view, certain elements contained in the composition of a steel promote the appearance of the ferritic phase with body-centred cubic structure. These elements are known as alphagenes. Amongst these appear notably chromium and molybdenum. Other elements known as gammagenes promote the appearance of the gamma-austenitic phase with a face-centred structure. Amongst these elements are nickel as well as carbon and nitrogen. It is therefore necessary to reduce the proportion of these elements and it is for these reasons that the steel according to the invention has in its composition less than 0.030% carbon, less than 0.5% nickel and less than 0.030% nitrogen.  
         [0087]    Carbon is harmful with respect to forging, corrosion and machinability. In general terms, in the field of magnetic properties, the precipitates must be reduced since they constitute obstacles to the movements of Bloch walls.  
         [0088]    Concerning the other elements in the composition, the nickel, manganese and copper in the composition, due to the industrial production of steel, are merely residual elements which it is sought to reduce and even to eliminate.  
         [0089]    Titanium and/or niobium form compounds including titanium and/or niobium carbide, which prevents the formation of chromium carbides and nitrides. They thereby promote corrosion resistance and notably the corrosion resistance of welds.  
         [0090]    Sulfur is limited so as to optimise the behaviour of the steel in the field of cold forging and to optimise the magnetic properties.  
         [0091]    Silicon is necessary for increasing the resistivity of the steel in order to reduce eddy currents, and is favourable to corrosion resistance.  
         [0092]    Steels according to the invention can also contain 0.2% to 3% molybdenum, an element improving corrosion resistance and promoting the formation of ferrite.  
         [0093]    In the field of their use, ferritic stainless steels pose problems of machinability.  
         [0094]    This is because a major drawback of ferritic steels is the poor conformation of the swarf. They produce long tangled swarf, which is very difficult to fragment. This drawback may become very detrimental in machining methods where the swarf is confined, such as for example in deep drilling or sawing.  
         [0095]    One solution afforded in order to mitigate the problems of machining ferritic steels is to introduce sulfur into their composition or elements of the lead, tellurium or selenium type which impair either the mechanical properties of cold deformation or corrosion resistance, or the magnetic properties. The said ferritic steels normally contain hard inclusions of the chromite type (Cr Mn, Al Ti)O, alumina (AlMg)O, silicate (SiMn)O, abrasives for cutting tools.  
         [0096]    According to the invention, the ferritic stainless steel can also contain in its composition by weight more than 30 10 −4 % calcium and more than 70 10 −4 % oxygen.  
         [0097]    The introduction of calcium and oxygen in a controlled and intentional fashion satisfying the relationship 0.2≦Ca/O≦0.6 promotes, in the ferritic steel, the formation of malleable oxides of the silicoaluminate of lime type as presented in FIG. 1, which is an Al 2 O 3 ; SiO 2 ; CaO ternary diagram, the malleable oxides being chosen in the area of the anorthite, gehlenite and pseudo-wollastonite triple point.  
         [0098]    The presence of calcium and oxygen consequently reduces the formation of hard and abrasive inclusions of the chromite, alumina and silicate type. On the other hand, the formation of inclusions of silicoaluminates of lime promotes the breaking up of the swarf and improves the service life of the cutting tools.  
         [0099]    It has been found that the introduction of oxides based on calcium into a steel with a ferritic structure, in replacement for the existing hard oxides, only very slightly modifies the other characteristics of the ferritic steel in the field of hot deformation, cold forging, corrosion resistance and magnetic properties.  
         [0100]    It has turned out that a steel with a ferritic structure according to the invention, containing no or very little sulfur, has a machining ensuring its industrial use in bar turning, whilst presenting increased corrosion resistance.  
         [0101]    The presence of so-called malleable oxides in a ferritic steel gives rise to advantages in the field of drawing and stretch forming.  
         [0102]    This is because malleable oxides are able to deform in the direction of rolling, whilst the hard oxides which they replace remain in the form of grains.  
         [0103]    In the field of drawing of small-diameter ferritic steel wires, the inclusions chosen according to the invention consequently reduce the rate of breaking of the drawn wire.  
         [0104]    In another field of application, for example in polishing operations, the hard inclusions are encrusted in the ferritic steel and cause furrows on the surface.  
         [0105]    The ferritic steel according to the invention, having malleable inclusions, can be polished with much greater ease in order to obtain an improved polished surface state.  
         [0106]    The steel is produced by electric fusion and then cast continuously in order to form blooms.  
         [0107]    The blooms are then subjected to hot rolling for forming for example machine wire or bars.  
         [0108]    Annealing is necessary to provide the cold conversion operations on the product, for example drawing and stretch forming.  
         [0109]    The steel is subjected to an additional recrystallisation annealing in order to restore and perfect the magnetic properties.  
         [0110]    A surface treatment then follows.  
         [0111]    In one example application, two steels according to the invention were produced, referenced steel 1 and steel 2, as well as two steels of reference A and B, whose compositions are shown in the following Table 1:  
                                                                                                                                     TABLE 1                       %   C   Cr   Si   Mo   Mn   P   N   S   Ni   Cu   Ti   Nb   Ca   C                                Steel   0.010   12.2   1.58   0.48   0.25   0.011   0.009   0.001   0.135   0.04   0.002   0.002   0.0048   0.009       1       Steel   0.011   11.9   1.47   0.49   0.22   0.015   0.007   0.029   0.126   0.06   0.003   0.002   0.0062   0.012       2       Ref A   0.015   17.4   1.25   0.35   0.5   0.02   0.02   0.28   0.3   0.1   0.003   0.002   0.002   0.006       Ref B   0.016   17.5   1.37   1.53   0.38   0.018   0.017   0.277   0.29   0.06   0.003   0.003   0.0017   0.007                  
 
         [0112]    These steels have been converted into 10 mm diameter bars according to the following method:  
         [0113]    hot rolling of a 11 mm round,  
         [0114]    annealing,  
         [0115]    drawing to a diameter of 10 mm,  
         [0116]    final annealing,  
         [0117]    dressing and planing,  
         [0118]    then they were characterised for magnetic properties, machinability, cold forging and corrosion.  
         [0119]    The steels according to the invention have better magnetic characteristics than the reference steels, as presented in Table 2 below.  
                           TABLE 2                                   Steel   Hc(A/m) coercive field                           Steel 1   109           Steel 2   115           Ref A   184           Ref B   177                      
 
         [0120]    These characteristics are due to a low proportion of addition elements, in particular a chromium content of approximately 12%.  
         [0121]    Steel 2 behaves very well in the field of machining by bar turning, in spite of a limited sulfur content. This is explained by the presence of calcium and oxygen.  
         [0122]    Steel 1 has very good suitability for cold forging, because of its low sulfur content. On parts previously forged, the finishing machining by bar turning is effected correctly, without any particular problem.  
         [0123]    Steels 1 and 2 behave very well in the field of corrosion, despite their low chromium content, as can be seen in Table 3 below. This is due, with steel 1, to a low sulfur content and, with steel 2, to a limited sulfur content combined with a low manganese content.  
                                             TABLE 3                                   Potential for               corrosion pitting in   Corrosion in H 2 SO 4             NaCl 0.02 M at 23° C.   2 M at 23° C.                                        Steel 1   220 mV/ECS   10 mA/cm 2             Steel 2   215 mV/ECS   11 mA/cm 2             Ref A   205 mV/ECS   24 mA/cm 2             Ref B   330 mV/ECS    6 mA/cm 2                        
 
         [0124]    The steel according to the invention can be used particularly for the manufacture of ferromagnetic parts such as, for example, solenoid valve parts, injectors for direct petrol injection systems, central door locking in the automobile field and any application requiring parts of the magnetic core or inductor type. In the form of a leaf, they can be used in current transformers or magnetic shielding.