Coated silicon-iron product and process therefor
The present invention relates generally to the art of producing electrical steel and is more particularly concerned with a novel method of electrolytically depositing on a boron-containing silicon-iron magnetic sheet a coating of boron-containing electrically-insulating material in which the boron is concentrated in close proximity to the metal surface. This invention is also particularly concerned with the new and useful product of that process. 
     CROSS REFERENCES 
     This invention is related to the invention disclosed and claimed in U.S. patent application Ser. No. 774,808 filed of even date herewith in the names of Ronald H. Arendt and Patrick F. Aubourg entitled, &#34;Silicon-Iron Production and Composition and Process Therefor&#34; and assigned to the assignee hereof and directed to the novel concept of electrolytically codepositing Mg(BO 2 ) 2  and Mg(OH) 2  on boron-containing electrical steel strip or sheet material. 
     This invention is also related to the invention disclosed and claimed in U.S. patent application Ser. No. 774,806 filed of even date herewith in the names of Ronald H. Arendt and Matthew J. Curran entitled, &#34;Coated Silicon-Iron Product and Process Therefor&#34; and assigned to the assignee hereof and directed to the novel concept of providing a calcium metaborate coating on silicon-iron sheet or strip material by an electrolytic deposition process. 
     BACKGROUND OF THE INVENTION 
     Following the discovery by Grenoble (U.S. Pat. No. 3,905,842 granted Sept. 16, 1975, and assigned to the assignee hereof) that boron is effective in small but critical amount in critical proportion to nitrogen in silicon-iron to promote secondary recrystallization during the final texture-developing anneal, Maucione (U.S. patent application Ser. No. 677,147, filed Apr. 15, 1976, and also assigned to the assignee hereof) found that the presence of a very small amount of boron in the coating on such a boron-containing steel further promotes secondary recrystallization and development of still better magnetic properties in the ultimate product. Maucione further found that the presence of boron in the coating can cause secondary recrystallization to take place when it otherwise would not, and also discovered that the presence of boron in the insulating coating was not effective in causing or promoting secondary recrystallization in the absence of boron in the metal itself at the outset of the final anneal. 
     In the practice of Maucione&#39;s teachings, boron has been incorporated in the refractory oxide coating, usually magnesium hydroxide  Mg(OH) 2  !, provided in accordance with the process disclosed and claimed by McQuade in U.S. Pat. No. 3,054,732 by a dipping operation or by brushing a solution of a suitable boron compound on the coating, or even spraying it on. 
     Then, through the discovery by Arendt and Aubourg that a boron-containing compound and Mg(OH) 2  can be electrolytically codeposited as described and claimed in their patent application referred to above, it became possible to exercise better control over the amount of boron incorporated in the insulating coating and also to distribute the boron more uniformly throughout the coating. As another important advantage of this codeposition method, the resulting product has a surface which is more amenable to fabrication operations customarily involved in the use of electrical steel. 
     SUMMARY OF THE INVENTION 
     In accordance with our discoveries to be described, it is possible to enhance substantially the beneficial effect on the ultimate sheet product of boron in the coating without losing the foregoing advantages of the Maucione and the Arendt and Aubourg inventions. In particular, we have found how to achieve at once the heretofore conflicting objectives of using coating boron to prevent premature egress of boron from the silicon-iron sheet during the final anneal while limiting the boron content of the coating to a somewhat lower level to avoid detrimental effects of boron on the ultimate product. Specifically, we have discovered that the boron necessary to block loss of boron from the metal substrate too early in the final anneal can be provided in the form of a relatively thin primary coating. Further, we have discovered that by providing a somewhat heavier or thicker overcoat containing little or no boron, the boron of the primary coating will be retained in place of proximity to the metal surface long enough to insure development of the desired secondary recrystallization texture in the silicon-iron sheet. Consequently, both the boron concentration requirement and the total coating boron content requirement are met through these discoveries and the invention based thereon. 
     We have further found that while the overall thickness of the duplex or combination coating of this invention is not sharply critical to the attainment of our new results, it is important that the boron-containing primary coating be of thickness sufficient to provide the requisite amount of boron for the egress-blocking purpose. Consequently, the thinner the method, it is not feasible in production operations as a practical matter for the reasons indicated above concerning subsequent fabrication operations. Still further, we have found that while Mg(OH) 2  is preferred for this purpose, other refractory oxides may be used instead, as set forth in U.S. Pat. No. 3,054,732 -- McQuade. 
     Briefly described, then, the new method of this invention comprises the steps of providing a boron-containing electrical steel, electrolyzing a solid MgO-buffered aqueous solution of magnesium acetate and magnesium metaborate of pH 8.0-9.0 with the silicon-iron sheet material being arranged as the cathode in the solution and with the solution being at a temperature of at least about 65° C and thereby covering the sheet with a boron-containing adherent electrically-insulating but relatively thin coating, and then electrolyzing a solid MgO-buffered aqueous solution consisting essentially of magnesium acetate with the resulting coated sheet as the cathode in the magnesium acetate solution and thereby covering the boron-containing Mg(OH) 2  coating with a substantially thicker coating of Mg(OH) 2 , and thereafter subjecting the resulting double-coated sheet to a final heat treatment to develop (110) 001! secondary recrystallization texture in the silicon-iron sheet. 
     Similarly described, the article of this invention is the double-coated primary recrystallized product of this process having a primary boron-containing coating 0.02 to 0.07 mil thick and a secondary substantially boron-free coating about 0.10 to 0.18 mil thick. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated by the drawing accompanying and forming a part of this specification, this invention is carried out using a boron-containing electrical steel sheet substrate and applying thereto a comparatively thin primary coating of suitable refractory material having electrically-insulating characteristics and containing magnesium metaborate substantially uniformly distributed throughout the coating. Then a substantially thicker coating of suitable refractory material is applied to the coated electrical steel sheet. As the initial step in the process, the substrate metal sheet is provided by preparing a silicon-iron melt of the required chemistry and then casting and hot rolling to intermediate thickness. Thus, the melt on pouring will contain from 2.2 to 4.5 percent silicon, manganese and sulfur in amounts in a ratio of manganese to sulfur less than 2.3, from about three to 50 parts per million boron and about 15 to 95 ppm nitrogen in the ratio range to boron of one and 15 parts to one, the remainder being iron and small amounts of incidental impurities including carbon, aluminum, copper and oxygen. Following anneal, the hot band is cold rolled with or without intermediate anneal to final gauge thickness and then decarburized. 
     The resulting fine-grained, primary recrystallized, silicon-iron sheet material in whatever manner produced is processed to provide the essential boron-containing primary coating and the secondary coating of this invention in preparation for the final texture-developing anneal. Processing at this point involves electrolytically codepositing Mg(OH) 2  and a boron compound source as disclosed and claimed in referenced patent application Ser. No. 774,801.  With the sheet material connected as a cathode and the circuit as described in referenced U.S. Pat. No. 3,054,732 and immersed in an electrolyte as described above, a uniform thickness coating (suitably about 0.02 to 0.07 and preferably about 0.05 mil) of Mg(OH) 2  ·XMg(BO 2 ) 2  ·YH 2  O, X = ≦1, Y = 0-15, is formed over that part of the sheet surface in contact with the electrolyte. 
     The electrolyte employed in this process is preferably prepared by adding boric acid to an aqueous magnesium acetate solution containing magnesia as a dispersed solid second phase. This magnesium acetate solution is suitably of 0.05 to 1.0 molar concentration and preferably about 0.2 molar strength. The pH of the electrolyte so produced will be between 8.0 and 9.0, reflecting the presence of excess magnesia. The amount of boric acid added is that which will provide the requisite boron content of the ultimate coating, which is preferably between 10 and 70 parts per million on the basis of the silicon-iron substrate, as disclosed and claimed in copending U.S. patent application Ser. No. 749,117 of Howard C. Fiedler, filed Dec. 9, 1976, as a continuation-in-part of U.S. patent application Ser. No. 677,146, filed Apr. 15, 1976 now abandoned, and assigned to the assignee hereof. At the outset of the electrolytic codeposition step, the electrolyte is at a temperature above about 65° C, preferably about 90°-95° C, and throughout the period that codeposition is conducted the electrolyte is maintained at such elevated temperature. 
     As the next step of this invention method, the resulting coated sheet material is immersed in a solid MgO-buffered aqueous magnesium acetate solution as a cathode and an electric current is applied across the terminals to provide an electrolytic deposit of Mg(OH) 2  over boron-containing primary coating. This step is suitably carried out as disclosed and claimed in U.S. Pat. No. 3,054,732 so that the thickness of the duplex coating is in the range stated above. Additionally, in the preferred practice of this invention the secondary coating will contain little or no boron, it being a purpose of this invention to confine the boron to the primary coating so that it is present for its essential boron egress-blocking function and is not present in excess of that amount particularly in the secondary coating. Such excess boron at locations relatively removed from the sheet material surface will become comparatively mobile at the temperatures prevailing in the latter stages of the usual final or texture developing anneal. 
     As the final step of the process of this invention, the resulting duplex or combination coated sheet is heated in hydrogen or a mixture of nitrogen and hydrogen to cause secondary grain growth which begins at about 950° C. As the temperature is raised at about 50° C per hour to 1000° C, the recrystallization process is completed and heating may be carried on to up to 1175° C if desired to insure complete removal of residual carbon, sulfur and nitrogen. 
    
    
     The following illustrative, but not limiting, examples of our novel process as actually carried out with the new results indicated above will further inform those skilled in the art of the nature and special utility of this invention: 
     EXAMPLE I 
     Eleven-mil strips of silicon-iron of the following composition were prepared as described in U.S. Pat. No. 3,905,843, referred to above: 
     
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Carbon              0.030%                                                
Manganese           0.035%                                                
Sulfur              0.031%                                                
Boron               0.0010%                                               
Nitrogen            0.0050%                                               
Copper              0.24%                                                 
Aluminum            0.005%                                                
Iron                Remainder                                             
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     From this melt composition, 10.2 mil sheets were produced in a series of hot rolling passes followed by pickling and annealing of the intermediate thickness sheet material (about 100 mils) and cold rolling to 60 mils thickness, whereupon the material was reheated and cold rolled again to final thickness and the coldworked sheet was given a decarburizing heat treatment at 800° C for eight minutes in hydrogen (room temperature dew point). 
     Epstein strips were cut from the strip to provide seven packs for ultimate magnetic properties test in the usual way. The strips comprising one such pack were electrolytically coated with Mg(OH) 2  through the use of a magnesium acetate electrolyte as described in U.S. Pat. No. 3,054,732, the strips being cathodes in electric circuits and eight volts being applied across the terminals at current density of 90 amperes per square foot until the coating mass on each strip was about 83.00 milligrams, i.e., about 0.0285 ounce per square foot which is equivalent to about 0.22 mil average thickness over the entire surface area of the strip. Franklin insulation values for these strips as well as those of the other six packs of this experiment prepared as described below (allowance being made for minor thickness variations) were uniformly about 0.2 ampere following annealing in hydrogen at about 1175° C for eight hours. 
     The strips of the six other packs were likewise electrolytically coated except that in each instance two separate coatings were provided, the first being a Mg(BO 2 ) 2  coating and the second being a Mg(OH) 2  coating like that of the first pack described just above. The electrolyte used for deposition of the primary coat was prepared by adding boric acid to a slurry of magnesium acetate and magnesia in distilled water. The amount of boric acid added to the slurry was 0.4317 mole per liter calculated as set forth in referenced copending patent application Ser. No. 774,808. 
     As shown in Table I, the thickness of the primary and secondary coatings of the strips of the six duplexcoated Epstein packs were varied in this experiment to determine the effects of relative thickness upon magnetic properties of the finished electrical steel product, total coating thickness being maintained reasonably constant through the individual strips of all seven packs. Also, as indicated in Table I, the first coating mass value refers in each instance to the primary Mg(BO 2 ) 2  coating, while the second designates the mass (i.e., relative thickness) of the secondary coating. 
     
                       TABLE I                                                     
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                            Coating                                       
Loss, Watts Per Pound       Masses                                        
Pack  15 kG     17 kG      μ10 Oe                                      
                                  Mg Per Strip                            
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1     0.556     0.930      1755    0.00                                   
                                  83.00                                   
2     0.498     0.689      1888   20.57                                   
                                  74.59                                   
3     0.621     0.1000     1720   32.67                                   
                                  51.37                                   
4     0.517     0.761      1837   52.15                                   
                                  36.47                                   
5     0.492     0.694      1893   16.03                                   
                                  70.20                                   
6     0.500     0.707      1872   37.04                                   
                                  49.15                                   
7     0.518     0.727      1852   48.60                                   
                                  37.33                                   
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     The ratio of MgO to B 2  O 3  was from 2.5 to 3.0 in the primary coatings on the strips of Packs 2, 3 and 4, but only 1.5 to 2.0 in the corresponding borate coatings of Packs 5, 6 and 7. 
     EXAMPLE II 
     In another experimental test of this invention process 10.0 mil Epstein strips like those of Example I (i.e., of the same composition and processing history) were coated by the procedures just described just above with the results set out in Tables II and III: 
     
                       TABLE II                                                    
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                        2.5 MgO                                           
Loss, Watts/Pound       B.sub.2 O.sub.3                                   
                                  Mg(OH).sub.2                            
Pack 15 kG     17 kG    μ10 Oe                                         
                              Mg/Strip                                    
                                      Mg/Strip                            
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1    0.494     0.740    1852  3.70    87.70                               
2    0.476     0.677    1897  7.20    77.00                               
3    0.475     0.675    1888  13.90   69.10                               
4    0.483     0.674    1895  16.20   64.40                               
5    0.498     0.689    1888  20.57   74.59                               
6    0.468     0.656    1894  21.90   63.20                               
7    0.464     0.661    1887  23.70   63.70                               
8    0.621     1.000    1720  32.67   51.37                               
9    0.517     0.761    1837  52.15   36.47                               
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                       TABLE III                                                   
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                        1.5 MgO                                           
Loss, Watts/Pound       B.sub.2 O.sub.3                                   
                                   Mg(OH).sub.2                           
Pack 15 kG     17 kG    μ10 Oe                                         
                              (Mg/Strip)                                  
                                       Mg/Strip                           
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1    0.465     0.675    1883  4.20     86.00                              
2    0.467     0.663    1897  7.10     76.30                              
3    0.472     0.665    1900  11.40    65.50                              
4    0.467     0.655    1904  12.70    62.30                              
5    0.467     0.664    1891  15.10    62.80                              
6    0.492     0.694    1893  16.03    70.20                              
7    0.462     0.649    1906  19.90    66.60                              
8    0.500     0.707    1872  37.04    49.15                              
9    0.518     0.727    1852  48.60    37.33                              
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     EXAMPLE III 
     The process of this invention was again tested in a manner similar to that described in Examples I and II except that all thirty-one samples from different steel heat/coil combinations were processed in parallel by the present invention process (1.5 MgO.B 2  O 3  primary coat) and by the procedure set forth in Example III of the referenced Maucione patent application Ser. No. 677,147. While the steel samples were of variable magnetic quality the average results were as follows: 
     
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              Loss at 17 kG                                               
              Watts × 10.sup.-3 /Pound                              
                            μ10 Oe                                     
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Maucione EX. III Process                                                  
                704             1870                                      
Present Invention Process                                                 
                697             1881                                      
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     Those skilled in the art will recognize that coating weight or thickness is commonly expressed in terms of density in ounces per square foot of steel strip surface and that 0.0275 oz/ft 2  .tbd.77 milligrams per Epstein strip. Further, it is understood generally that 77 mg/Epstein strip corresponds to a uniform coating thickness of 0.05 mil.