Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
  • H01F1/147—Alloys characterised by their composition
  • H01F1/14766—Fe-Si based alloys
  • H01F1/14775—Fe-Si based alloys in the form of sheets
  • H01F1/14783—Fe-Si based alloys in the form of sheets with insulating coating
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
  • C21D1/70—Temporary coatings or embedding materials applied before or during heat treatment while heating or quenching
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C24/00—Coating starting from inorganic powder

Definitions

• the coating compositions comprise a major proportion of magnesium oxide sintered to a specific citric acid activity and pore volume and a minor proportion of: (l) a chloride contributor such as magnesium chloride, barium chloride prchromous EhTorl n fliiitasilicar
• a chloride contributor such as magnesium chloride, barium chloride prchromous EhTorl n fliiitasilicar
• Magnesium oxide is used extensively as a highly heat resistant separator medium and protective coating for metal surfaces. It is also used for forming an electrical insulator coating for metals, as a gatherer for removing impurities, such as sulfur and carbon, from thin metal sheets and particularly for forming a protective base insulation for silicon steel to which other coatings, e.g., phosphate coatings, may be added.
• silicon-containing steel is cold rolled into sheets, decarburized and thereafter coiled into convenient rolls.
• Cold rolling develops in the steel the potential to form a grain oriented structure when the steel is later annealed.”
• the term annealed refers to a process whereby the steel is heated to about l,200C. in an atmosphere of low dewpoint containing hydrogen, or in a vacuum, under programmed conditions with respect to time and temperature. This results in a growth in size of the steel grains and also in a specific grain orientation which provides the desired soft magnetic properties sought. During the annealing process. virtually all of the remaining excess carbon and sulfur content of the steel is lost.
• magnesium oxide serves to reduce impurities such as carbon and sulfur in steel by chemical reaction.
• magnesium oxide provides a major part of an electrically insulating silicate layer by reaction with the steel. For most applications, this silicate insulation is important to form an efficient electrical insulating coating or as the base coating upon which such insulation is formed.
• transformer cores are constructed from thin sheets of soft magnetic steel stacked together to form a laminated body in which each sheet is electrically insulated from its neighbor.
• This construction vastly reduces the generation of eddy currents in the core imposed by an alternating electrical field.
• the average density of soft iron in the core should be as large as possible and consequently the insulation on the plates should be as thin as possible to provide closer stacking of steel plates.
• Serpentine additive For steel containing aluminum nitride, special additives have been proposed which facilitate both the formation of insulation and also proper grain growth for improved magnetic properties.
• U.S. Pat. No. 3,627,594 discloses air oxidation of this type of steel followed by coating with magnesium oxide plus titanium dioxide and manganese oxides.
• U.S. Pat. No. 3,676,227 discloses the use of magnesium oxide plus boron compounds.
• magnesium oxide is applied to steel from an aqueous suspension or slurry.
• magnesium oxide hydrates to magnesium hydroxide to a substantial degree.
• the degree of hydration depends on a number of factors, such as, the surface area of the magnesium oxide, the temperature of the water and the residence time of the magnesium oxide in the water. It is known that the presence of magnesium hydroxide to any substantial degree in a magnesium oxide composition releases H O and, unless removed prior to annealing, can impair electrical properties of the dried and annealed steel coating.
• coating means the dried magnesium oxide composition formed on the steel surface.
• insulation as used herein, means that glassy composition formed when the coating is annealed.
• the aqueous residence time of the magnesium oxide composition In an ordinary steel coating operation, it is therefore important to keep the aqueous residence time of the magnesium oxide composition to a predetermined minimum level.
• the steel is frequently coated while still hot from decarburization. Elevated temperatures imparted to the coating bath accelerate the rate of hydration.
• the residence time For high activity magnesium oxide, the residence time may be about 10 minutes, whereas for low activity magnesia the residence time may be about 30 minutes. This places a burden on a steel coating operation in that the aqueous slurry of magnesium oxide must be used within this relatively short span of time or else become impaired due to excessive hydration.
• Aqueous slurries of the composition can be subjected to a wide range of hydration conditions without altering the quality of the coated and annealed steel. That is, maintaining the magnesia composition in an aqueous slurry at temperatures as high as 130F. for as long as 2 hours will not cause substantial hydration resulting in the formation of excessive deleterious magnesium hydroxide.
• Coatings formed from the present compositions exhibit high density-and adherence to steel.
• the dried and annealed steel insulations display good electrical insulating properties.
• the coatings of the present invention do not give rise to offensive or corrosive fumes.
• the present invention provides a magnesium oxide composition for coating silicon-containing steel sheet to provide the aforesaid benefits, said composition comprising a major proportion of a sintered magnesia having a citric acid activity of from about 30 to 85 seconds and a pore volume of from about 0.02 to about 0.1 cc. per gram, and based on said magnesium oxide:
• a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride, said chloride contributor providing from about 4 X to about 60 X 10' moles of chloride ion per square centimeter of steel surface;
• This invention also provides a process for preparing said magnesium oxide composition for coating onto silicon-containing steel, said process comprising admixing sintered magnesium oxide with sodium metasilicate and a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride, said sintered magnesium oxide having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc./gram, the concentration of said chloride contributor being such as to provide from about 4 X 10' to about 60 X 10" moles of chloride ion per square centimeter of steel surface and the concentration of sodium metasilicate being at least about stoichiometrically equal to said chloride concentration.
• a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride
• said sintered magnesium oxide having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc./gram
• the magnesium oxide steel-coating composition of this invention is conveniently prepared as an aqueous slurry by adding said sintered magnesia to water containing sodium metasilicate and the chloride contributor.
• the sodium metasilicate may be combined with the sintered magnesia to form a homogeneous mass, and then added to water containing the chloride contributor to provide an aqueous slurry for use in a steel-coating operation.
• any sintering agent which will aid in providing the specially sintered magnesium oxide of this invention may be employed. Include those which may be used are lithium chloride, boric oxides, boric acids, and magnesium salts of boric acids.
• boric oxides it is meant to include the oxides of said boric acids.
• magnesium salts of boric acids it is meant to include the various known magnesium borates. such as for example, the magnesium orthoborates, the magnesium metaborates and the magnesium pyroborates.
• the present invention also provides a process for coating silicon-containing steel sheet with an adherent. electrically insulating coating of said magnesium oxide composition, which process comprises in sequence forming an aqueous slurry or suspension of said magnesium oxide composition, coating said slurry to the surface of said steel sheet, heating to remove water therefrom and dry the coating thereon, and thereafter annealing the dried coated steel sheet at a temperature in excess of about 1,000C.
• Citric Acid Activity is a measured of the activity of magnesium oxide and is determined by a method which measures the time required for a given weight of a particular magnesia to provide hydroxyl ions sufficient to neutralize a given weight of citric acid. The test is conducted as follows:
• the stopwatch is stopped the instant the suspension turns pink and the time is noted. This time in seconds is the citric acid activity.
• Pore Volume This is here defined by the following relationship:
• Pore volume as used herein, is taken to be that value (from the above expression) at which the sintered magnesium oxide displays good adherence to steel, as determined by the use of sodium metasilicate in conjunction with the MgO.
• the procedure for determining pore volume is as follows:
• the slurry concentration should be chosen so that the coating weight is less than about 0.08 ounces per square foot of steel.
• the chloride contributor in conjunction with sodium metasilicate, functions to promote the formation of a nonporous insulation coating.
• the porosity of the coating formed by the present invention is determined by means of the Copper Plating Test which is hereinafter more fully described. However, in general, a porous coating will permit copper from a copper sulfate solution to deposit on the steel substrate whereas a nonporous coating will not.
• the concentration of chloride contributor employed is that which will provide from about 4 X to about 60 X 10 moles of chloride ion per square centimeter of steel when the present magnesium oxide composition is coated onto steel.
• the preferred chloride contributor is magnesium chloride. Magnesium chloride may be admixed directly with the sintered magnesia, however, it is preferred to add it to the water used to form the aqueous coating slurry.
• the quantity of sodium metasilicate employed is that which is at least stoichiometrically equal to the quantity of chloride contributor. in view of the reaction of the chloride contibutor. e.g., magnesium chloride, with sodium metasilicate to form magnesium silicate and sodium chloride in situ, and in view of the disclosure of US. Pat. No. 3,265,600 which teaches that an MgO coating containing sodium chloride provides poor insulation, it was surprising to find that the presence of sodium chloride in the present composition does not impair the electrical insulating properties of the dried and annealed coating.
• sintered magnesia be pulverized to a fineness such that less than about 0.5 percent by weight remains on a 325 mesh sieve when subjected to the following screening test:
• Screen Test 1 Place a 3-inch diameter 325 mesh screen in a 6% inch diameter evaporating dish.
• the screen can be moved about and raised from the liquid as desired.
• the screen is then rinsed with an alcohol stream from a wash bottle.
• the sodium metasilicate and chloride contributor may be blended with the pulverized sintered magnesia to form a homogeneous mass, or it may be added to the water used to form the aqueous coating slurry. It appears to be a simpler procedure to add both the sodium metasilicate and the magnesium chloride directly to the water rather than to attempt to form a homogeneous dry blend with the sintered magnesia.
• An important advantage of the present composition is its resistance to excessive hydration when formed into an aqueous slurry.
• excessive hydration it is meant that degree of hydration which results in the formation of red or black iron oxides visible to the naked eye on the surface of the steel.
• ordinary magnesias When ordinary magnesias are placed in water at temperatures up to about F., they hydrate within about 30 minutes to form substantial quantities of magnesium hydroxide.
• the present magnesia compositions are resistant to excessive hydration for as long as two hours. it is recognized in the art that excessive quantities of magnesium hydroxide in a magnesia impairs the electrical insulating properties of that magnesia when coated onto steel plate and thereafter annealed.
• the advantage gained with the present hydration resistant magnesia compositions is that they permit a substantially longer residence time in an aqueous slurry before hydrating to form quantities of magnesium hydroxide which in excessive quantities are deleterious to the electrical insulating properties of the ultimate insulation.
• This feature is an added safety margin in a steel coating operation where, under plant conditions, it is sometimes not possible to complete the steel coating procedure using an aqueous slurry in less than about 30 minutes. Nor is it always possible to completely cool the steel from the decarburization line.
• an aqueous suspension of the above described magnesium oxide composition is prepared by mixing with water to the desired viscosity and leveling and flow-out characteristics. Generally from about 5 to about 20 weight percent of the magnesium oxide composition, based on water, is satisfactory to provide an aqueous slurry having the requisite viscosity and flow properties suitable for coating onto steel sheet.
• the coating slurry may be applied to the magnetic sheet material by any suitable means such as by immersion, brushing, or spraying. It has been found convenient to use an immersion technique whereby the steel sheet is passed through a tank containing the coating slurry. The coated sheet thereafter is heated to drive off water and provide a dried layer of the present composition. The coated metal sheet is then coiled into a roll and placed in a furnace for annealing as previously described. During the annealing process, the coating of this invention forms an adherent, electrically insulating, corrosion-resistant layer which also functions as a separator medium to prevent the coiled metal sheet from sticking to itself.
• the present magnesium oxide coating compositions were coated onto steel sheets having a width of 3 centimeters.
• a steel coating slurry or suspension was prepared from a commercially available magnesium oxide containing no additives and having a Citric Acid Activity of 19.5 to provide a slurry containing 7.2% MgO by weight.
• the resultant suspension was maintained at a temperature of 1 10F. for from about 25 to 50 minutes under constant stirring and was then coated onto a number of silicon steel sheets prepared from the same steel melt, each sheet measuring 3.0 centimeters in width and 12-14 mils in thickness.
• the coatings were leveled and then dried to a steel temperature of about 205C. reached in 15 seconds, to provide a dried coating weight of from about 0.03 to about 0.035 ounces per square foot of steel, the usual coating weight employed in the steel industry.
• the dried coated steel sheets were annealed by heating at l.l77C. for a fixed time interval.
• the annealed sheets were then cooled, and excess or loose magnesium oxide was scrubbed from the surface of each coated sheet by brushing in a stream of flowing water.
• the Conductance of the coated and annealed surface was measured by means of the Franklin Test, and the Porosity of this surface was determined by means of the Copper Plating Test.
• the specimen is heated at l,0OOC. for one hour.
• the Loss on Ignition is a measure of the degree of hydration of MgO, i.e., of the formation of Mg(OH),.
• CONDUCT ANCE FRANKLIN TEST This test is widely accepted and utilized for evaluating the conductance of coated steel sheets. A detailed description is found in ASTM method A-334-52, Standard Methods of Test for Electrical and Mechanical Properties of Magnetic Materials. Briefly, the test is carried out by passing an electric current through brass contacts which cover coated areas 0.1 square inch in area. Current passing through the coating flows through the steel to a contact made directly to the steel by means of a twist drill. The resulting amperage provides a measure of the resistance encountered through the coating. Several hundred contacts are employed in obtaining readings for coating evaluation. A complete short circuit, i.e.. complete Conductance, is indicated by reading of I00 milliamps per 0.1 square inch. Therefore, the lower the reading in milliamps the more insulating the coating. In the present invention, Franklin values of about 50 or less are considered acceptable.
• POROSITY (COPPER PLATING TEST) A steel sheet bearing a dried and annealed coating is immersed in an aqueous solution of copper sulfate. Copper spontaneously plates on the surfaces that are not electrically insulated, and therefore provides an indication of the Porosity of the annealed coating.
• Example IA Result Porosity Completely free of copper plating Good Traces of or partially copper plated Fair Completely copper plated Bad
• the magnesium oxide employed was a sintered MgO containing 0.05 percent by weight of lithium chloride and having a citric acid activity of 50 and a pore volume of 0.05 cc. per gram.
• An 1 1.8 percent suspension of the magnesia in water was prepared containing (on an MgO basis) 0.5 percent magnesium chloride and 1.7 percent by weight of sodium silicate. The suspension so forrned was maintained at a temperature of F. for 60 minutes before coating onto the steel sheets, and then dried so that the steel reached 205C. in 15 seconds.
• the resultant dried and annealed coating therefore was formed from the following constituents:
• Citric acid activity 50 Pore volume 0.05 cc. per gram Lithium chloride 0.05% Magnesium chloride 0.5% Sodium silicate l.7%
• Example 18 The procedure of Example 18 was repeated except that magnesium chloride was not added in forming the steel coating suspension.
• the Adherence, Loss on Ignition, Conductance and Porosity tests were performed as in Example 1A.
• EXAMPLE 2 10 insulation coating was determined to be good. This not 1 Conductance Hydration Loss on (Franklin Example Conditions Ignition Adherence Moles Cl per CmF/Steel Values) Porosity IA -50 Min. 7-] 1% Good-Excellent 62 Good 1B 60 Min. 3% Excellent 4.4 X IO" to 13.1 X lO'" 20-35 Good IF. Not
• Example 3 The procedure of Example 1B was repeated except that the concentration of magnesium chloride was varied to provide from 4 X ID to 158 X 10'" moles chloride per square centimeter of steel and the coating weights varied from 0.047 to 0.066 ounces per square foot of steel. The results are set forth in Table 2.
• Example 18 The procedure of Example 18 was repeated except that barium chloride was substituted for magnesium chloride, the coating weights were varied from 0.067 to 2. There is a significant improvement (decrease) in Conductance of the dried and annealed coating of Example 18 when compared with that of Example I l A.
• Example 5 The procedure of Example 13 was repeated except that chromous chloride was substituted for magnesium chloride, the coating weights were varied from 0.041 to 0.074 ounces per square foot of steel, and the concentration of chromous chloride was varied to provide from 3 X to 120 X 10- moles of chloride per square centimeter of steel.
• the results are set forth in EXAMPLE 7
• the procedure of Example 18 was repeated except that the weight proportions of Na SiO relative to MgCl were varied from 3.4:] .0 to 0.43:1.0 to ascertain the concentration of Na SiO required with respect to MgCl to provide dried, annealed coatings having good insulation (i.e., a Franklin value less than 50) and good la b le A. ,7 Y A l0 porosity.
• Table 6 The results are set forth in Table 6.
• EXAMPLE 6 The foregoing demonstrates that a quantity of sodium silicate substantially less than that stoichiometrically required to react with the magnesium chloride provides coatings which are deficient with respect to insulation (Franklin value) and/or porosity.
• EXAMPLE 8 The procedure of Example 1B was repeated except that the proportions of magnesium chloride and sodium silicate, the coating weights and the number of moles of chloride per square centimeter of steel surface were varied as indicated in Table 5 with the results as set forth therein. The rheology or flow characteristics of tl1e coa ti ng slurries were also evaluated.
• Example 1B The procedure of Example 1B was repeated except lL L Efi l i 2 292093392521??? wei intq sd Table 5
• Example 8A The procedure of Example 8A was repeated except that 1.0 percent by weight of magnesium chloride was used instead of 0.5 percent. Coating Weight Moles Cl 8C Example OzJFt. Per Cm.
• Example 8A The procedure of Example 8A was repeated except 10 9A 0.077 13.7 x 10* that no magnesium chloride was used. 95 0953 95 X 9C 0.046 8.2 10
• the steel used for coat- 9D (1033 X -t ing was a regular grain oriented type steel.
• the results 3? 8-8:: 3-: X obtained were as follows: X
• An aqueous slurry was prepared containing 24 grams of a sintered magnesia having a citric acid activity of 50 seconds and a pore volume of 0.038 cc./gram, 0.05 percent of lithium chloride, 0.5 percent magnesium chloride and 1.7 percent sodium metasilicate. This slurrry was held (hydrated) at a temperature of 130F. for 60 The Adherence and the texture and quality of the coatings was determined. The coating texture was evaluated as follows:
• Example 18 The procedure of Example 18 was repeated with respect to the provision of dried cotaings in a series of experiments in which different percentages of sintered A series of experiments was conducted in which therocedure of Example 18 was repeated except that the following parameters were varied as indicated:
• the coating slurry temperature was maintained at 130F. for a period of time ranging from 30 to minutes. This period of time is the hydration time.
• the moles of chloride per square centimeter of steel ranged from about 10 X 10* to about 15 X 10.
• Drying furnace temperature Temperature of steel after drying Texture of dried coating Adherence Loss on lgnition Moles Cl per Cm.
• EXAMPLE 11 The hydration resistance of the MgO coating composition of the present invention when compared to a commercially available MgO containing no additives, was determined by the following procedure:
• a steel coating was prepared from a 10 percent slurry of a sintered MgO containing 0.05 percent by weight of LiCl and having citric acid activity of 50 and a pore volume of 0.05 cc. per gram. Also added to the water were 0.5 percent by weight of MgCl and 1.7 percent by weight of Na SiO based on the MgO. The resultant slurry was stirred continuously under standard conditions and maintained at a temperature of 130F. for 30 minutes. Thereafter it was coated onto the same steel sheets as used in Example 1A and in the same manner. The coating was removed and its Loss on Ignition at 1,000C. was determined as a measure of its hydration. As a control, a commercially available MgO containing no additives was carried through the same procedure.
• Example 1 1A The procedure of Example 1 1A was repeated except that the time of hydration, i.e., the time that the coating slurry was maintained at 130F. before coating, was 60 minutes.
• the time of hydration i.e., the time that the coating slurry was maintained at 130F. before coating, was 60 minutes.
• a commercially available MgO containing no additives was carried through the same procedure.
• the coatings were leveled and then dried to a steel temperature of about 205C. reached in 15 seconds.
• the dried coated steel sheets were annealed by heating at l,l77C. for a fixed time interval.
• the annealed sheets were then cooled, and excess or loose magnesia was scrubbed from the surface of each coated sheet by brushing in a stream of flowing water.
• the Conductance. Adherence and Porosity were determined as hereinbefore described.
• EXAMPLE 123 The procedure of Example 12A was repeated except that the magnesia employed had a citric acid activity of 39 and a pore volume of 0.053 cc./gram. Thirty-five grams of this magnesia were added to 180 milliliters of water containing 0.73 gram of magnesium chloride and 2.80 grams of sodium metasilicate to form the steelcoating suspension. The Conductance, Adherence and Porosity were determined as hereinbefore described.
• the results are summarized as follows: The foregoing results demonstrate that the concenf y L I U tration of chloride contributor (magnesium chloride) Example .ll l s lrE lrlills (P l' c rl t 15; 0/3?) 65 can be as high as about 60 X 10" moles per square centlmeter of steel in the present invention. NA 30 3.9 Control 30 l3.l
• a magnesium oxide composition for coating sil con-containing steel sheet comprising a major proportion of a sintered magnesia having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc. per gram, and based on said magnesium oxide:
• a chloride contributor selected from the group consisting of magnesium chloride. barium chloride and chromous chloride, said chloride contributor providing from about 4 X to about 60 X 10'" moles of chloride ion per square centimeter of steel chloride, barium chloride and chromous chloride, said sintered magnesium oxide having a citric acid activity of from about 30 to about seconds and a pore volume of from about 0.02 to about 0.1 cc./gram, the concentration of said chloride contributor being such as to provide from about 4 X IO to about 60 X lO moles of chloride ion per square centimeter of steel surface and the concentration of sodium metasilicate being at least about stoichiometrically equal to said chloride concentration.
• a process for coating silicon-containing steel sheet with an electrically insulating coating which comprises in sequence forming an aqueous slurry of a magnesium oxide composition as defined in claim 1, coating said slurry to the surfaces of steel sheet, heating the coated steel to dry said coating thereon and thereafter annealing said dried coated steel at a temperature in excess of about l,0OOC.

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• 1973
  • 1973-09-12 US US00396346A patent/US3841925A/en not_active Expired - Lifetime
• 1974
  • 1974-08-29 GB GB3778274A patent/GB1442730A/en not_active Expired
  • 1974-09-06 RO RO7479931A patent/RO72690A/fr unknown
  • 1974-09-11 DE DE2443531A patent/DE2443531A1/de active Pending
  • 1974-09-12 JP JP49104435A patent/JPS5055608A/ja active Pending
  • 1974-09-12 FR FR7430937A patent/FR2243506B1/fr not_active Expired
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  • 1974-09-12 IT IT27237/74A patent/IT1021332B/it active

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