Abstract:
Iron/nickel alloy having a low coefficient of expansion, the chemical composition of which comprises iron, nickel, manganese, silicon, calcium, magnesium, less than 0.005% of aluminum, less than 0.001% of sulfur, less than 0.01% of oxygen, less than 0.005% of nitrogen, and less than 0.003% of phosphorus, the chemical composition furthermore satisfying the relationship: 
       S ≦0.02×Mn+0.8×Ca+0.6×Mg

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an iron/nickel alloy having a low coefficient of linear expansion and to a product, preferably a strip, comprising this alloy and obtained by hot rolling and then, optionally, cold rolling.  
           [0003]    2. Discussion of the Background  
           [0004]    Iron/nickel-containing alloys having a low coefficient of expansion, containing between 35% and 40% by weight of nickel are well known. However, in addition to nickel and iron, these alloys always contain impurities such as sulfur, phosphorus, oxygen, nitrogen and carbon, typically resulting from smelting. The alloys may also contain elements such as cobalt, chromium, copper, molybdenum, vanadium, niobium and silicon which are added to adjust mechanical properties. Unfortunately, and because of the presence of impurities, these alloys are difficult to hot-deform by rolling or by forging. In order to improve the hot deformability, it is known to add manganese in amounts greater than 0.2%. However such addition has the drawback of appreciably increasing the coefficient of expansion of the alloy.  
           [0005]    In U.S. Pat. No. 2,050,387, a process has been proposed for manufacturing an iron/nickel alloy having a low coefficient of expansion and good hot forgeability, in which the alloy is smelted by annealing high-purity raw materials under hydrogen above 1150° C. for 20 h and then by casting and solidifying under a hydrogen atmosphere. This process, however, has the drawback of being complicated to implement and being very expensive.  
         OBJECTS OF THE INVENTION  
         [0006]    One object of the present invention is to provide an iron/nickel alloy with a low manganese content and a low coefficient of linear expansion, which has good hot ductility and which can be smelted using conventional processes.  
           [0007]    Another object of the invention is to provide a product, such as a strip, comprising the above-described iron/nickel alloy, preferably by hot rolling and then, optionally, by cold rolling the alloy.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0008]    The above objects are provided by an iron/nickel alloy whose chemical composition comprises, by weight:  
           [0009]    35.5%≦Ni≦37%  
           [0010]    Mn≦0.1%  
           [0011]    0.03%≦Si≦0.15%  
           [0012]    0.0001%≦Ca≦0.002%  
           [0013]    0.0001%≦Mg≦0.002%  
           [0014]    Al≦0.005%  
           [0015]    S≦0.001%  
           [0016]    O≦0.01%  
           [0017]    N≦0.005%  
           [0018]    P≦0.003%  
           [0019]    In addition, the chemical composition satisfys the following relationship: 
           S≦0.02×Mn+0.8×Ca+0.6×Mg 
           [0020]    and has, preferably, a balance of iron.  
           [0021]    Preferably, the chemical composition is, in addition to the above, such that:  
           [0022]    O≦0.005%  
           [0023]    N≦0.003%  
           [0024]    S≦0.0005%  
           [0025]    In order to obtain good deep-drawability and a good surface finish after heat treatment, it is preferable that the chemical composition described initially or the preferable composition immediately above further meet the following conditions:  
           [0026]    C≦0.005%  
           [0027]    B≦0.0004%  
           [0028]    Preferably, the nickel content of the invention alloy is between 35.8% and 36.3% and more preferably between 35.9% and 36.2%, and Cr≦0.1%, Cu≦0.1%, Mo≦0.1%, V≦0.1%, and Nb≦0.1%.  
           [0029]    In order to obtain a very low coefficient of expansion, it is preferable that the chemical composition of the invention alloy meet the following condition: Cr+Cu+Mo+V+Nb+Si≦0.15%.  
           [0030]    The invention also relates to a product, preferably a strip, made of the iron/nickel alloy according to the invention, obtained by hot rolling and then, optionally, cold rolling. The strip preferably has a width greater than 400 mm and a thickness as desired, including 0.01-1 mm, the strip having a coefficient of linear expansion between 20° C. and 100° C. which is less than 0.9×10 −6 /K, preferably less than 0.82×10 −6 /K. The strip may be of any desired length.  
           [0031]    The alloy according to the invention is an alloy based on iron and nickel, containing iron and from 35.5% to 37% of nickel, as well as, optionally, complementary alloy elements such as cobalt, chromium, copper, molybdenum, vanadium or niobium in amounts less than 3% by weight, these being intended to adjust the mechanical properties depending on the intended uses.  
           [0032]    The invention alloy contains less than 0.1% by weight and preferably less than 0.05% by weight of manganese, since this element increases the coefficient of thermal expansion of the alloy.  
           [0033]    The low manganese content of the invention alloy has the drawback of adversely affecting the hot deformability (hot ductility) of the alloy, and in order to obtain good hot ductility it is necessary for the alloy to contain in combination:  
           [0034]    less than 0.001%; and preferably less than 0.0005% of sulfur where “%” here and everywhere herein means % by weight,  
           [0035]    less than 0.003% of phosphorus,  
           [0036]    between 0.0001% and 0.002% of calcium,  
           [0037]    between 0.0001% and 0.002% of magnesium,  
           [0038]    between 0.03% and 0.015% of silicon.  
           [0039]    The manganese, calcium, magnesium and sulfur contents also satisfy the relation: 
           S≦0.02×Mn+0.8×Ca+0.6×Mg 
           [0040]    The calcium and magnesium contents are limited herein to 0.002% in order to prevent the formation of large inclusions liable to cause pitting during chemical cutting, as is particularly the case when the invention alloy is used in the form of thin steets for the manufacture of shadow masks for cathode ray tubes.  
           [0041]    It is also necessary, in order to obtain good hot ductility, for the invention alloy to contain less than 0.005% of aluminum and less than 0.005% and preferably less than 0.003% of nitrogen. It also contains less than 0.01% and preferably less than 0.005% of oxygen.  
           [0042]    In order to obtain the lowest coefficient of expansion possible, the nickel content herein is preferably between 35.8% and 36.3% and, even better, between 35.9% and 36.2%, the optimum content being 36.05%.  
           [0043]    The alloy elements chromium, copper, molybdenum, vanadium, niobium and silicon have an unfavorable effect on the coefficient of expansion. In addition, when searching for the lowest possible coefficient of expansion, the contents of each of these elements must remain less than 0.1% and preferably less than 0.05%. Even better, it is desirable that: 
           Cr+Cu+Mo+V+Nb+Si≦0.15% 
           [0044]    Cobalt is an element which may be partially substituted for nickel up to a level of 10%, but this element has drawbacks when the alloy has to undergo chemical attack, for example in order to perform chemical etching; in addition, its content is preferably limited to 0.5%, especially when the alloy is intended for manufacturing shadow masks for cathode ray tubes.  
           [0045]    When the invention alloy is used to manufacture thin sheets intended to be creep drawn, it preferably contains less than 0.02% and more preferably less than 0.010%, and even better less than 0.005%, of carbon so as to have the lowest possible yield stress, which is favorable to good deep-drawability.  
           [0046]    The hydrogen content of the invention alloy is preferably less than 0.001% and preferably less than 0.0005% in order to prevent the formation of blisters. The boron content is preferably less than 0.01% and more preferably less than 0.005%, and even better less than 0.0004%, so as to prevent the formation of boron nitrides at the surface of the products during heat treatments. These nitrides form a pulverulent layer which leads to poor adhesion of the blacking layer produced on shadow masks.  
           [0047]    The invention alloy makes it possible to manufacture products such as ingots or slabs by hot rolling and to then produce, e.g., thin strips with thicknesses less than 0.3 mm and width greater than 400 mm, by cold rolling, these strips being used especially for the manufacture of shadow masks for cathode ray tubes.  
           [0048]    The invention will now be further described by way of an example. This example is nonlimiting, however. 
       
    
    
     EXAMPLE  
       [0049]    A 600 mm wide and 0.15 mm thick strip was manufactured by hot-rolling and then cold-rolling an ingot of iron/nickel alloy whose chemical composition was, by weight,:  
         [0050]    Ni: 35.97%  
         [0051]    Co: 0.019%  
         [0052]    Cr: 0.025%  
         [0053]    Cu: &lt;0.01%  
         [0054]    Mo: 0.0058%  
         [0055]    V: &lt;0.005%  
         [0056]    Nb: &lt;0.005%  
         [0057]    Mn: 0.041%  
         [0058]    Si: 0.048%  
         [0059]    S: &lt;0.0005%  
         [0060]    Ca: 0.0002%  
         [0061]    Mg: 0.0003%  
         [0062]    Al: &lt;0.005%  
         [0063]    O: 0.0045%  
         [0064]    C: 0.0033%  
         [0065]    N: 0.0032%  
         [0066]    P: &lt;0.003%  
         [0067]    H: &lt;0.001%  
         [0068]    B: &lt;0.0004%  
         [0069]    When the contents of elements above are indicated as being “less than” a certain value, this means that they are contained in amounts less than the limit of sensitivity of the analytical methods used.  
         [0070]    Hot rolling was carried out without forming crazes. The coefficient of expansion was 0.81×10 −6 /K. After heat treatment for 30 minutes at 800° C., the yield stress E 0.2  was 270 MPa and the coercive field was 0.45 Oe. The strip exhibited very good chemical etching behavior. All these properties make it particularly suitable for the manufacture of shadow masks for cathode ray tubes. The alloy according to the invention may also be used, for example, in metrology or for the manufacture of bimetallic strips.  
         [0071]    This application is based on French patent application 94 14 011 filed Nov. 23, 1994, incorporated herein by reference. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.