Method for treating electrical steel by electroetching and electrical steel having permanent domain refinement

Abstract: Permanent domain refinement of grain oriented electrical steep strip (16) is obtained in a high speed two-stage process. The process removes the glass in narrow regions (17) which just expose the base metal. An electrolytic etch is then used to deepen the regions (17) into the base metal and minimize damage to the remaining glass film. Control of acid concentration and temperature in the electrolytic bath allows a greater increase in productivity. A further feature of the process is the use of permeability measurements to optimize the depth of the etched regions. The improved core loss produced by the process will survive a stress relief anneal.

Description: BACKGROUND OF THE INVENTION The present invention relates to a high speed electroetching method to provide permanent domain refinement for electrical steels to yield improved magnetic properties. The core loss properties of electrical steel may be improved by metallurgical means such as better orientation, thinner gauge, higher volume resistivity and smaller secondary grain sizes. Further improvements in core loss are obtainable by non-metallurgical means which reduce the wall spacing of the 180 degree magnetic domains. High-stress secondary coatings impart tension which decreases the width of the domain. The domain refinement of most interest has been the creation of a substructure which regulates the domain wall spacing. Various means to subdivide the domains have included: 1) narrow grooves or scratches by mechanical means such as shotpeening, cutter or knives 2) high energy irradiation such as a laser beam, radio frequency induction or electron beam and 3) chemical means to act as a grain growth inhibitor diffused or impregnated onto the steel surface such as a slurry or solution of sulfide or nitride compounds. All of these means are generally discussed in U.S. Patent No. 3,990,923. Grooves or scratches have been applied to electrical steels resulting in internal stresses and plastic deformation which subdivides the large domains typically found in large grains into regions of smaller domain sizes. U.S. Patent No. 3,647,575 uses a knife, metal brush or abrasive powder under pressure to form grooves less than 40 x 103 mm deep and spaced between 0.1 and 1 mm. The grooves may be transverse to the rolling direction and are applied subsequent to the final anneal. A stress relief anneal of about 700°C is optional. The March 1979, No.2, 2, Vol. MAG-15, pages 972-981, from IEEE TRANSACTIONS ON MAGNETICS discussed the effects of scratching on grain oriented electrical steel in an article entitled "Effects of Scratching on Losses in 3-Percent Si-Fe Single Crystals with Orientation near (110) [001]" by Tadao Nozawa et al. The optimum spacing between scratches was from 1.25 mm to less than 5 mm. The benefits of tensile stresses were noted. All of the samples were chemically and mechanically polished prior to scratching to obtain bare, uniformly thick and smooth surfaces for good domein observations using the scanning electron microscope. Scratching was conducted after the final anneal using a ball-point pen loaded with a 300 gram weight to produce a groove which was about .1 mm wide and 1 mm deep. U.S. Patent No. 4,123,337 improved the surface insulation of electrical steels having an insulative coating by electrochemical treatment to remove metallic particles which protrude above the insulative coating. U.S. Patent No. 3,644,185 eliminated large surface peaks by electropolishing while avoiding any significant change in average surface roughness. JP-A-61253380 discloses a method of producing permanent domain refining for electrical steel strip comprising the steps of a) subjecting said strip to a final high temperature annealing, b) providing an insulating film on the strip surfaces, c) providing a series of parallel linear regions to at least one of said surfaces which regions have spaced intervals of 2.5 - 10 mm and expose said steel surface to a width of about 0.05 - 0.3 mm, and d) etching said linear regions in an acid, such as hydrochloric acid bath to increase the depth below said insulator film to 0.005 - 0.06 mm. GB-A-2168626 discloses a similar method as the above, with the final addition of filling the recessed parts with a composition having a coefficient of thermal expansion smaller than that of the steel sheet body, and discloses for the etching step immersion, spraying, electrolytic pickling or the like, but gives only etching examples by acid immersion and acid sprays. The prior art has not optimized the groove depth for permanent domain refinement in a manner which avoids damage to the surface conditions. The prior art has been limited regarding line speed to produce the series of grooves for domain refinement. By using a process which combines grooving techniques with an electrolytic etch, the problems with depth control and surface damage may be overcome. The line speed for this combined process becomes commercially attractive. The present invention provides grooves or rows of pits of sufficient depth to penetrate the coating thickness and then electroetches the exposed base metal to a critical depth to obtain permanent domain refinement. BRIEF SUMMARY OF THE INVENTION It is a principal object of the present invention to provide a process which produces permanent domain refinement with improved productivity/lower cost over prior art. It is further object of the present invention to provide an electrical steel with improved magnetic properties which may be given a stress relief anneal while maintaining excellent magnetic properties. It is a still further object to provide a control process for electroetching which monitors the "as-grooved" permeability to optimize the core loss improvement through a feed back control loop. Said objects are achieved, according to the present invention, by a method as claimed in claim 1. Further advantageous features are claimed in the dependent claims. After etching the treated bands, the electrical steel strip is recoated to provide a good surface for an insulative coating which imparts tension. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a schematic illustration of a laser system to produce grooves on moving electrical strip, FIG. 2 shows the effect of groove depth on magnetic improvement (deterioration) in percent for grain oriented electrical steel, FIG. 3 shows the relationship between permeability and optimum core loss improvement by grooving high permeability grain oriented electrical steel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Domain refinement which will survive a stress relief anneal has not been previously obtainable at normal commercial line speeds. The present invention provides 8-10% core loss improvements after stress relief annealing using a process which can operate at line speeds above 100 feet per minute (30 meters per minute) and typically around 300 feet per minute (90 meters per minute). The reason for this is that the invention produces the permanent domain refinement effect in a matter of seconds as opposed to minutes for other processes. The steel may have up to 6.5% silicon and may use any of the known grain growth inhibitors. To obtain permanent domain refinement through the thickness of the strip, it is preferable that the gauge be less than 12 mils (30 mm). Heavier gauges will require a domain refinement treatment on each side. However, this is not a problem since the commercial ranges of interest are normally thinner than 12 mils (30 mm). The first stage of the process is to initiate a series of parallel linear regions in the form of grooves or rows of pits to a depth which just penetrates the glass film and exposes the base metal. U.S. Patent No. 4,468,551 describes an apparatus for developing spots on electrical steel using a laser, rotating mirror and lenses to focus the shape and energy density of the laser beam. The patent, however, was controlling the laser parameters to avoid coating damage. Laser beams may also be focused into lines by using a lens to expand the laser, a lens to collimate the laser beam, and a lens to focus the laser beam. FIG. 1 shows a laser system which can remove the glass film to expose the base metal. In FIG. 1, a laser 10 emits a beam 10a which passes through a beam expander 11 and cylindrical lens 12. Laser beam 10a impinges a rotating scanner or mirror 13 which is reflected through a cylindrical lens 14 and lens assembly 15. Beam 10a contacts strip 16 as a line 17. Line 17 continuously reproduced at spaced intervals of about 5-20 mm. The energy density of laser beam 10a is sufficient to penetrate through the glass coating on strip 16 and expose the electrical steel. Depending on the width of the strip 16, several of these units could be used in combination to produce the grooves in line 17. Other means to produce the initial groove could also be used, such as discs as taught in EP228,157, or cutters as taught in U.S. Patent No. 3,647,575, or any of the means in U.S. Patent No. 3,990,923. It is important to the magnetic properties of the electrical steel that the grooves or rows of pits which initially penetrate the glass film be very shallow. Deep penetration into the base metal will provide permanent domain refinement but will also produce ridges around the penetration and cause metal splatter on the surface of the glass. Both of these have an adverse effect on the glass film properties. Ideally the initial groove or pits should just remove the glass and expose the base metal slightly. While the depth of the affected region should be shallow, the groove width or pit diameter should be about 0.05 to 0.3 mm. The second stage for optimizing the depth of penetration uses an electroetching treatment to increase the depth to about 0.0005-0.003 inches (0.012-0 075 mm). Localized thinning by e!ectroetching improves the domain refinement and does not harm the glass film. The improved magnetic quality does remain after a stress relief anneal which is typically at about 1500-1600°F (815-870°C) for a period of 1 - 2 hours. The electrolytic bath must be selected to not attack the glass film while deepening the groove or pits in the base metal. Nitric acid solutions (5-15%) with water or methanol were the most effective of the solutions evaluated. A 5% nitric solution in water at 160F (70C) with a current of 25 mamps/cm² for 10 seconds attacked the base metal very aggressively without harming the resistivity of the glass. For uniform control, the temperature and acid concentration must be maintained relatively constant. FIG. 2 shows the effect of groove depth on the improvement or deterioration of the magnetic quality of high permeability grain oriented steel. The process of scribing and electroetching does have some scatter in the % improvements to magnetic quality. To reduce the scatter and provide a good improvement in core loss, the process is controlled by monitoring the permeability. A review of FIG. 3 shows the optimum range to be 1870-1890 H-10 permeability (after grooving) to provide minimum scatter in core loss improvement. Before grooving, permeabilities ranged frcm 1910 to 1940. During electroetching, a feedback control system is provided which monitors the permeability of the as-grooved steel. Regardless of the starting permeability, the most uniform core loss improvement will occur as the permeability drops into the range of 1370-1890. The control system continues the electroetching until the material falls within this range. This process is more accurately controlled than using such means as the amount of material removed or depth of groove. This control range is applicable only for high permeability grain oriented electrical steel. To maintain line speed during electroetching, the current may be adjusted using the permeability data to control the permanent domain refinement process. After electroetching, the strip is rinsed and dried. A corrosion inhibitor coating may be applied by roller coating. Potassium silicate mixed in water (about 50 ml/l) could be used. The coating would be cured at 600°F (315°C) and cooled. The width of the scribed line (or spot diameter), time of immersion, current, temperature of the bath, concentration of the acid, initial depth and final depth are all controlled in the process to optimize the permanent domain refinement. The following experiments were conducted to evaluate the process and optimize the conditions for a high permeability grain oriented silicon steel. Slight modifications may further improve the magnetic properties for different chemistries, gauges, glass film and previous process differences. The magnetic characteristics and features of the present invention will be better understood from the following embodiments. Steel having the following nominal composition (in weight %) was used for these studies: %C %Mn %S %Si %Al %N 0.055 0.085 0.025 3.00 0.031 0.007 After conventional processing to obtain cold rolled strip which has been decarburized, given a final high temperature anneal and provided with a glass film and secondary coating, the strip was subjected to the following tests. A YAG laser was used to locally remove the glass in parallel regions perpendicular to the rolling direction. The regions were spaced about 6 mm apart. The data in Table 1 compares the magnetic quality of samp!e blanks with regions of either continuous lines of 0.25 mm in width, or large spots (ellipsoidal in shape) with dimensions 0.4 mm X 0.25mm and 1.2 mm apart, or small spots (also ellipsoid in shape) with dimensions 0.25mm X 0.2 mm and 1.2 mm apart. The major axis of the ellipsoid spots was perpendicular to the rolling direction. The sample blanks were 0.23 mm thick, 75 mm wide and 300 mm long. The data in Table 1 is coded by a) line, b) large spot (0.4 mm x 0.25 mm) and c) small spot (0.25 mm x 0.2 mm). Grooving was done in 5% HNO₃ in water at room temperature for about 1 to 2 minutes at 5 amps. Initial Electroatch Sample Scribe Weight Loss Calculated Groove Depth Core Loss B17 Perm H-10 Core Loss B17 Perm H-10 %Imp. (Det.) (gm) (mm) (w/lb) (w/lb) 1 line 0.2270 0.026 0.559 1922 0.504 1861 9.8 2 line 0.2409 0.028 0.600 1908 0.538 1835 10.3 3 line 0.2045 0.024 0.582 1919 0.497 1866 14.6 4 large spot 0.0903 0.027 0.553 1917 0.513 1908 7.2 5 large spot 0.0724 0.022 0.584 1905 0.552 1901 5.5 6 large spot 0.0988 0.030 0.582 1919 0.527 1908 9.5 7 large spot 0.1440 0.044 0.594 1919 0.518 1896 12.8 8 large spot 0.1833 0.057 0.597 1919 0.508 1883 14.9 9 small spot 0.0570 0.032 0.591 1919 0.546 1918 7.6 10 small spot 0.0835 0.047 0.557 1931 0.496 1923 11.0 The influence of time during electroetching was evaluated on samples of the same chemistry which were mechanically scribed or laser scribed on sample blanks 0.23 mm thick, 75 mm wide and 300 mm long. The scribed lines were spaced apart at 6 mm intervals and were perpendicular to the rolling direction. Results are shown in Table 2. Table 3 shows the improvement in core loss with the samples in Table 2 after electroetching. Magnetic properties were measured before scribing and after electroetching followed by a stress relief anneal (SRA) at 1525°F (830°C). To determine if this process was adaptable to commercial line speeds, a series of tests were conducted with higher acid concentrations (15% HNO₃) and higher bath temperatures. All of the bath temperatures were 170°F (77°C) except sample 19 which was 175°F (80°C). A 5 amp current was used in all cases and the samples were the same size and of the same chemistry as the previous study. Magnetic quality was tested before scribing and after electroetching and stress relief annealing at 1525°F (830°C). Quality Initial Quality After SRA Sample Etch Time Weight Loss Calculated Groove Depth Core Loss B17 Perm. H-10 Core Loss B17 Perm. H-10 %Improvement (Det.) (sec) (gm) (mm) (w/lb) (w/lb) 19 5 0.1657 0.019 0.569 1921 0.500 1893 12.1 20 4 0.1740 0.020 0.611 1912 0.528 1883 13.6 21 3 0.1653 0.019 0.536 1932 0.474 1902 11.6 22 3 0.1582 0.018 0.613 1923 0.512 1898 16.5 23 2 0.1266 0.015 0.577 1915 0.503 1901 12.8 24 2 0.2938 0.034 0.581 1906 0.526. 1833 9.5 A further study was conducted to optimize the quality improvements to core loss after a stress relief anneal. Mechanical scribing was used to evaluate various depths of grooves through the glass film on the surface of the high permeability grain oriented electrical steel. The scribed lines were spaced 6 mm apart and applied perpendicular to the rolling direction. The electrolytic bath was 5% HNO₃ in water at room temperature. As noted previously, higher bath temperatures and higher acid concentrations would allow commercial line speeds but this study was only designed to optimize the depth of the grooves. The samples were the same size, thickness and chemistry as previously stated. Initial Qlty. Electroetch & SRA Sample Etched Wgt. Loss Groove Depth Core Loss B17 Perm. H-10 Core Loss B17 Perm. H-10 %Improvement (Det.) (gm) (mm) (w/lb) (w/lb) 25 0.0891 0.030 0.515 1928 0.495 1894 3.9 26 0.0991 0.033 0.518 1929 0.489 1885 5.6 27 0.1328 0.043 0.523 1930 0.501 1862 4.2 28 0.1852 0.074 0.520 1931 0.519 1811 0.2 29 0.3245 0.107 0.516 1926 0.533 1749 (3.3) 30 0.3570 0.117 0.526 1929 0.515 1648 2.0 Various electrolyte etchants and conditions were evaluated in Table 6 for their effect on the glass film quality of the samples. Scribe lines were made mechanically and aligned perpendicular to the rolling direction at 6 mm intervals. Basically, the damage to the glass film is minimized by keeping times for etching under 10 seconds and using higher currents or bath temperatures to minimize the times. Generally, the preferred composition would be a nitric acid of 5% to 15% concentration in water at 160°F (70°C). The present 2-stage process for permanent domain refinement thus provides improved core loss which remains after a stress relief anneal. The process provides an improved glass surface over the other domain refinement processes which rely on grooves, scratches or rows of spots. The process also provides a unique means of controlling the etching process by monitoring the permeability level. The resultant electrical steel has improved magnetic properties which will survive a stress relief anneal as a result of the 2-stage process which provides a better glass surface. The embodiments of the invention in which an exclusive property is claimed are defined as follows:

Claims: A method of producing permanent domain refinement for electrical steel strip (16) containing up to 6.5% silicon which comprises the steps of: a. subjecting said strip (16) to a final high temperature annealing step, b. providing a glass film on the surfaces of said strip (16), c. providing a series of parallel linear regions (17) to at least one of said surfaces which have spaced intervals of about 5 to 20 mm, said regions exposing said steel surface to a width of about 0.05 to 0.3 mm, d. electroetching said linear regions (17) in a bath to increase the depth below said glass film to about 0.012 to 0.075 mm, and e. controlling said electrolytic etching of step d. by measuring the permeability to control the amount of metal removal in said parallel linear regions (17) and thereby to provide improved core loss properties with reduced scatter in values. The method of claim 1, wherein the electroetching step uses a bath of nitric acid at a concentration of 5-20% in solution with water. The method of claim 1, wherein the electroetching step uses a bath of nitric acid at a concentration of 5-20% in solution with methanol. The method of claim 1, wherein said bath is heated above 40°C. The method of claim 1, wherein the current is from 0.1 to 0.5 amp per square centimeter of said exposed base metal in the linear regions (17). The method of claim 1, wherein a rinsing and drying step is used after electroetching. The method of claim 1, wherein a rust inhibitor coating is applied to said strip (16) after eletroetching. The method of claim 1, wherein said steel is given a stress relief anneal. The method of claim 1, wherein said parallel linear regions (17) are produced using a laser (10). The method of claim 9, wherein said laser (10) produces parallel linear regions (17) which are grooves. The method of claim 9, wherein said laser (10) produces parallel linear regions (17) which are rows of spots. The method of claim 6, wherein a coating for inhibiting corrosion is applied after said rinsing and drying step. The method of claim 1, wherein the current is adjusted to the permeability value to control the depth of electroetching and provide uniform core loss improvement. The method of claim 1, wherein the electrolytic etching is completed when the H-10 permeability measurement indicates a value of 1870-1890. The method of claims 1 to 6 and 9, wherein the electroetching bath has a nitric acid concentration of 5 to 15 %, and the electroetching time is under 10 seconds.

Inventors: BLOCK WAYNE F, BLOCK, WAYNE F.
Applicants: ARMCO INC, ARMCO INC.
Application year: 1989
Main classification: C21D8
IPC codes: C21D8, H01F1, C25F3