Abstract:
The present invention relates to a process for producing permanent domain refinement continuously and at very high line speeds in grain oriented electrical steel having an aluminum nitride inhibitor system. After the final high temperature anneal, the glass film and insulative coating on the surface is removed in narrow bands (grooves or rows of spots). The steel is electroetched to increase the depth of the bands, coated with aluminum by electrophoresis and given a stress relief anneal to bond the aluminum coating to the base metal by diffusion. A localized stress field is induced during cooling which causes domain refinement due to the differential thermal contraction between the aluminum and the base metal.

Description:
This is a continuation of copending application Ser. No. 07/173,697 filed on Mar. 25, 1988 now abandoned. 
    
    
     The present invention relates to a method which produces a permanent domain refinement effect in oriented electrical steels using continuous line speeds which are above previous methods. The productivity increases in this process makes this a commercially viable process. Permanent domain refinement is the refinement of magnetic domains capable of surviving a stress relief anneal for improving the magnetic properties. 
     One of the main factors in electrical steel which must be controlled for optimum core loss properties is eddy-current loss. Some of the factors that influence eddy-current loss are electrical resistivity (e.g. silicon content) stress which causes tension (e.g. surface coatings) and the size of the magnetic domain (e.g. grain size). 
     During the processing of grain oriented electrical steel to obtain the desired texture, a high temperature final anneal is required to allow the growth of (110) [001] grains at the expense of primary recrystallized grains. Essential to this operation are grain growth inhibitors such as aluminum nitride or manganese sulfide. The secondary recrystallization develops excellent orientation but results in large grain sizes. A larger grain size typically provides a wider domain wall spacing. 
     To reduce the losses due to magnetic domain size, many attempts have been made to reduce the width of the 180 magnetic domains. Mechanical means to produce grooves or scratches have included shot peening, cutters and knives. High energy irradiation means have included laser beams, electron beams, radio frequency induction or resistance heating. Chemical means to act as grain growth inhibitors have been diffused or impregnated onto the surface prior to the final high temperature anneal. The treatments to produce artificial boundaries to subdivide the domains are typically applied perpendicular to the rolling direction and have a controlled width and spacing between the boundaries. 
     The domain refinement techniques are generally broken down into two categories. Most of the above systems fall into the first category in which the benefits are erased if given a stress relief anneal. The other category includes permanent domain refinement which survives the stress relief anneal and is sometimes conducted after the final high temperature anneal. 
     Patents which are typical of domain refinements that won&#39;t survive a stress relief anneal include U.S. Pat. Nos. 3,990,923; 4,468,551; 4,545,828 and 4,535,218. 
     Examples of patents which permanently refine the domain structure after the final high temperature anneal include U.S. Pat. Nos. 4,293,350; 4,363,677; 4,554,029 and 3,647,575. 
     One of the patents which discusses chemical treatments for domain refinement is the previously mentioned U.S. Pat. No. 3,990,923 which diffuses or impregnates the surface of the steel with a sulfide, oxide, nitride, selenide or antimonide during the final high temperature anneal. A solution or slurry is painted on the strip to prevent secondary recrystallization. Thus, normal grain growth occurs outside the local chemical treatment which prevents the growth of secondary recrystallization into the treated regions. By diffusely injecting a resistant to secondary grain growth, a finer grain size results. The treated regions must be properly spaced to ensure an appropriate degree of recrystallization is attained. The painted bands of annealing separation agent produces lower core losses and higher permeabilities. 
     One other known patent for chemical treatments to improve the magnetic properties of grain oriented electrical steel is U.S. Pat. No. 4,698,272. This patent teaches the application of a thin coating after the final anneal to the entire surface after the glass has been removed and the surface has been polished. The thin coating of Al 2  O 3  or TiN was applied by physical vapor deposition or chemical vapor deposition to a thickness of 0.005-2 mm to provide increased tension. Since there is no plastic microstain, the properties are not influenced by a stress relief anneal. 
     A domain refinement technique that produces supplemental domains which will survive a stress relief anneal at about 1500° F. (815° C.) is very difficult to obtain at existing line speeds used in the production of grain oriented electrical steel. Chemical means have been used for grain growth control during the final anneal and for improved tension to the entire strip. However, chemical means to provide permanent domain refinement which could be applied at commercial line speeds have not been used or suggested by the prior art. 
     The present invention uses a process which overcomes the problems in providing permanent domain refinement at commercial operating speeds. 
     It is an object of the present invention to provide a process which can be utilized at commercial line speeds above 300 feet per minute to form localized lines on secondary metal coatings which create regions of stressed base metal. 
     It is also an object of the present invention to provide a grain oriented electrical steel strip having improved magnetic properties after a stress relief anneal as a result of a localized secondary metal coating in addition to the general secondary coating applied for tension and insulation. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to localized stress by surface alloying to produce permanent domain refinement in grain oriented electrical steel. The electrical steel strip is subjected to a high temperature final anneal and provided with a mill glass on the surfaces of the strip. The strip then has a secondary insulative coating applied to it. Narrow regions of the surface films are removed by means such as a laser, cutting disc, shot peening or the like to expose the base metal beneath the glass. The bands of exposed metal are electrolytically treated to deepen the grooves which are applied perpendicular to the rolling direction. The strip is preferably rinsed and dried. 
     A metal such as aluminum is deposited into the grooves by flame spraying, slurry coating or electrophoresis. The coating is then flash sintered by means such as induction heating to a temperature of 1200° F. (650° C.) in about 10 seconds or less. The metal deposits resulted in a core loss improvement of 8-12% at B-17 for high permeability grain oriented electrical steel after a stress relief anneal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Grain oriented electrical steels are known to develop large domain wall spacings during the final high temperature anneal. Applying a metal, such as aluminum, in lines modifies this domain spacing by introducing a secondary metal coating after the final high temperature anneal in localized regions where the glass has been removed. The differences in thermal expansion will cause localized stress which reduces domain wall spacing and improves magnetic properties. The improvements in magnetic properties are permanent and will survive a stress relief anneal. The objective of the present invention is to apply this technology at commercial line speeds. 
     The starting material of the present invention may be regular grain oriented electrical steel or high permeability grain oriented electrical steel. The steels may contain up to 6.5% silicon although a range of 2.8 to 3.5% silicon is generally employed. The steels may contain additions of manganese, sulfur, selenium, antimony, aluminum, nitrogen, carbon, boron, tungsten, molybdenum, copper or the like in various well known combinations to provide the metallurgical means to control grain size and texture. The melt composition for the steels evaluated had the following composition in weight percent: 
     
         ______________________________________  Carbon  0.055%  Manganese          0.085%  Sulfur  0.025%  Silicon  2.97%  Aluminum          0.031%  Nitrogen          0.007%  Tin     0.045%  Iron    Balance______________________________________ 
    
     The electrical steel is fabricated into cold rolled strip by any of the well known processes and provided with a decarburizing anneal if needed prior to the final high temperature anneal. The strip is subjected to a final high temperature and provided with a glass film on the strip surfaces and a secondary insulative coating is applied. 
     According to the present invention, the glass film must be removed in narrow regions spaced about 5 to about 10 mm apart. The locally treated regions could be produced using any of scribing means listed in the domain refinement patents previously which cause surface removal. The selection of a laser, shot peening or scratching means is based on the line speed limitations to accomplish the removal of the glass. For an in-line operation, the process requires a short treatment time and a laser is the preferred choice. The laser could be a continuous wave, pulsed or Q-switched to deliver the energy required to remove the glass in a short dwell time. U.S. Pat. No. 4,468,551 discusses the various laser parameters which control the depth of penetration and energy per unit area. The patent teaches the level at which coating damage occurs and can be controlled by selecting the proper power, dwell time and beam shape. For an insulative coating such as taught in U.S. Pat. No. 3,996,073, the laser energy per unit of vertical area is multiplied by a constant related to the thermal diffusivity (about 0.48 for silicon steel) and should exceed a value of about 40 for coating degradation. The coating removal may be in the form of a groove or row of spots and should have a width (or spot diameter) of about 0.05 to 3 mm and a depth of about 0.0025 to 0.0125 mm. Obviously these values are related to the thickness of the mill glass surface. 
     The CO 2  laser was selected for removing the glass and deepening the grooves or spots. However, the thermal effect from the laser caused the samples to curl. A significant amount of molten metal was splattered around the ridges. The laser must be controlled to remove the glass and expose the base for electroetching to develop the desired depths for the secondary metal coating. The following CO 2  laser conditions were used for a laboratory trial: 
     
         ______________________________________Focal Length  pulsePulse Rate    5 inches (12.7 cm)Pulse Width   139-1000 pulses/secondAverage Power 100-420 wattsSpot Spacing  0.025-0.06 inches (0.63-1.5 mm)Spot Diameter 0.01-0.014 inches (0.25-0.35 mm)Line Speed    40 feet/minute (12 meters/minute)______________________________________ 
    
     The desired groove (or spot) depth is preferably obtained using a 2-stage process. Once the glass surface is removed in the localized regions, an electrolyte process is used to obtain the desired depth. This process is covered by a copending application filed in the name of W. F. Block and assigned to the assignee of the present invention. 
     Electroetching enables the base metal to be removed rapidly and avoids the damage caused by other processes. Other means to generate the same groove will cause ridges around the groove (or spots) and cause base metal splatter during the removal process to be deposited on the glass film. The localized thinning by electroetching increased the depth up to about 0.025 mm. 
     The electrolytic etch preferably uses a nitric acid of 5-15% concentration in water or methanol to etch the groove in less than about 10 seconds. Preferably water at a temperature of about 65° C.-80° C. is used to increase the rate of etching. A current of 0.5-1.0 amp/cm 2  of exposed base metal in the scribe line region. The strip is then rinsed with water and dried prior to depositing a secondary metal coating. 
     The metal deposit must be applied using a process which confines the metal to the grooves or rows of spots where the surface films have been removed on the strip. 
     One technique which was studied was to apply aluminum rapidly by flame spraying. The magnetic results of flame spraying aluminum onto 0.23 mm samples of high permeability grain oriented electrical steel are reported in Table 1. The samples were masked to leave 1 mm wide lines, spaced 10 mm apart, exposed for coating. An argon-hydrogen atmosphere was used. The samples were given a stress relief anneal at 1500° F. (815° C.) and tested for magnetic properties and domain refinement. The results indicated that diffusion and alloying did occur during the anneal which resulted in domain refinement. However, the large drop in permeability indicated the size of the deposit was too great. Smaller deposits should result in greater improvement. Also, further consideration of the flame spray method showed that directing the aluminum to well defined areas of the strip could not be accomplished rapidly enough for commercial feasibility. 
     
                       TABLE 1______________________________________Line Speed Limitation        Quality As-SprayedInitial Quality        and SRA&#39;d      % ImprovementB15   B17            B15   B17        (Deterioration)(w/lb) (w/lb)  H-10   (w/lb)                      (w/lb)                            H-10 B15    B17______________________________________.398  .534    1939   .388  .528  1914 2.5    1.1.405  .566    1960   .384  .541  1905 7.5    4.4.388  .527    1935   .387  .530  1902 0.3    (0.6).384  .536    1927   .371  .507  1876 3.4    5.4.386  .537    1921   .389  .529  1865 0      1.5.382  .531    1925   .373  .513  1884 2.4    3.4.381  .554    1931   .367  .502  1886 3.7    9.4.392  .535    1928   .377  .514  1854 3.8    3.9______________________________________ 
    
     A second technique considered for rapid aluminum deposition was slurry coating. The magnetic results of slurry deposition are shown in Table 2. Similar samples were masked to give different deposit thicknesses and a range of line spacings. 
     A slurry of 12% polyvinylacetate in water and 1 gm/ml aluminum was used for coating. Only one side was coated onto the masked samples. The coating was cured in air at 200° F. (95° C.) for 5 minutes. After curing, the samples were stress relief annealed at 1500° F. (815° F.) and tested for magnetic properties and domain refinement. The thinner deposits clearly provided the greatest core loss improvements. The deposits were clearly smaller than with flame spraying. The results indicate the process can provide improvements in magnetic properties equivalent to laser irradiation and the benefits would survive stress relief annealing. However, similar limitations in commercial feasibility resulted. Masking was a necessary part in correctly locating the lines of aluminum deposit. This technique would be undesirable for in-line processing. 
     
                                           TABLE 2__________________________________________________________________________Aluminum Slurry CoatingInitial Quality      Quality As-coated and SRA&#39;d                     Deposit LineB15 B17    B15  B17  H-10 Height                         Spacing                              % Improvement(w/lb)    (w/lb)   H-10      (w/lb)           (w/lb)                (mm) (mm)                         (mm) B15  B17__________________________________________________________________________.421    .574   1945      .372 .490 1947 .012                         11   11.6 14.6.400    .548   1938      .372 .494 1931 .012                         11   7.0  10.0.400    .544   1936      .394 .524 1909 .050                         11   1.5  3.7.391    .522   1944      .379 .499 1920 .050                         11   3.4  6.3__________________________________________________________________________ 
    
     A third technique was tried based on an electrophoretic coating which is deposited by an electric discharge of particles from a colloidal solution onto a conductive substrate. In this case, however, the goal was to only coat the aluminum powder onto lines running perpendicular to the rolling direction and spaced approximately 6 mm apart. The magnetic results from electrophoretic deposition are given in Table 3. The bath composition which appears to provide the best control for aluminum deposition has the following conditions: 
     
         ______________________________________Bath     methanol; 0.25 gm/l AlCl3; .035 gm/l Tannic AcidPowder   atomized aluminumTemperature    room temperatureAgitation    sufficient to suspend particlesVoltage  0.1 volts (dc)/cm of scribe line______________________________________ 
    
     The samples prior to deposition were the same as the previous studies. During deposition, electrical contact was made at the edge of the sample. The samples were dried in heated air to remove the methanol and then subjected to a stress relief anneal. Testing for magnetic properties and domain refinement was then conducted. The results indicate the process generates a substantial quality improvement, survives a stress relief anneal and may be accomplished within 10 seconds which makes it a commercially attractive process for use with existing line speeds. The process is further optimized when the aluminum deposit does not form a ridge. Deeper grooves would alleviate this problem which adversely influences the stacking factor and surface resistivity. 
     
                                           TABLE 3__________________________________________________________________________Electrophoretic Deposition of Aluminum      QualityInitial Quality      Deposited and SRA&#39;d                  Deposit Size                           % ImprovementB15 B17    B15 B17 H-10                  Wt./Scribe Line                           (Deterioration)(w/lb)    (w/lb)   H-10      (w/lb)          (w/lb)              (mm)                  (mgm/cm) B15  B17__________________________________________________________________________.397    .534   1929      .387          .517              1925                  12       2.5  3.2.392    .527   1926      .391          .518              1922                  13       0.0  1.7.399    .531   1928      .387          .513              1922                  27       3.0  3.4.397    .540   1937      .376          .500              1931                  36       5.3  7.4.401    .535   1926      .371          .493              1926                  37       7.5  7.8.404    .545   1929      .360          .480              1918                  53       10.9 11.9.378    .511   1926      .347          .464              1904                  78       8.2  9.2__________________________________________________________________________ 
    
     The beneficial effect of aluminum deposition by electrophoresis on magnetic quality has been determined. The processing requires a means to remove the glass film and provide scribed regions where the aluminum may be deposited for permanent domain refinement. To be commercially attractive, the combination of laser scribing, electroetching and electrophoretic deposition of aluminum appears to have the highest line speed capabilities. As other techniques to remove the glass film, or prevent its formation, are developed, the benefits from this type of metal coating for permanent domain refinement would still exist. 
     It will be understood that various modifications may be made to the invention without departing from the spirit and scope of it. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows in the appended claims.