FR2526814A1 - Steel strip with cured insulating aluminium phosphate coating - formed using nitric acid-contg. phosphate soln. - Google Patents

Abstract

The steel strip has a cured, insulating phosphate coating formed using an aq. soln. comprising 100 vol. pts. of aq. aluminium phosphate, 1-15 vol. pts. of nitric acid and pref. 50-400 vol. pts. water. The aq. aluminium phosphate is pref. a mixt. of phosphoric acid, hydrated alumina and water. The steel strip is useful for magnetic cores of electric motors, generators, transformers, etc. Addn. of nitric acid to the soln. allows steel strip temps. of up to 90 deg.C (compared with 425-650 deg.C in prior art) to be used for curing the phosphate without affecting coating, insulating and space factor properties.

Description

"Elements of steel sheets coated with a phosphate
insulating"
The invention relates to sheet steel elements coated with an insulating phosphate.

 The magnetic cores used in generators, transformers, electric motors, etc., are preferably made of rolled steel cut pieces with a coating of insulating material on each sheet.

As is well known in the art these cores are formed of sheets instead of solid steel so that stray currents are minimized during use of the cores.

 Aluminum orthophosphate, also referred to as aluminum monophosphate, is a mineral coating solution which is used in dilute form to produce an insulating layer between the sheets on the strips and pieces cut from steel sheets. It is used to achieve resistance between the core sheets of the generator and transformer motors and additionally to minimize stray current losses in the machines. The coating solution, at the undiluted concentration, consists of a mixture of phosphoric acid and hydrated alumina and contains about 33% by weight of P 2 O 5, 8% by weight of Al 2 O 3 and 59% by weight of H 2 O. This type of coating solution can be found commercially, for example under the commercial designation Alkophos, and is widely used in the electrical equipment industry. It is applied to steel for electrical purposes at the appliance manufacturer or the steel supplier.

 The aluminum mono-phosphate solution described above is generally further diluted with water before use. The concentrations usually used are about 0.5 to 4 volumes of H 2 O per volume of aluminum monophosphate solution depending on the strength and thickness required, and depending on the coating application of the coating. we use for its application.

This solution is applied with applicators to a roll of felt or rubber on the steel and then baked in a tandem-mounted oven. These types of phosphate coatings are well known in the art, and are used as revexts primarily for hot rolled steel as described in the patent specification.
US Pat. No. 2,743,203 (Steinherz), where steel sheets are baked at 135 ° to 5000 ° C., and the water is removed by heat, leaving a thin layer of phosphate which has reacted with the metal surface the sheet. The coating obtained has good insulating properties and a good space factor. This coating has also demonstrated its ability to withstand the various mechanical, thermal and chemical risks that are encountered in electrical machines or in the stages of operations. during their production.

 In the present practice, however, where cold-rolled steel, the surface of which is readily reactive, is generally used, the temperature of the commercial steel sheets required to bake the unmodified phosphate coating is about 34O0C. In order to achieve this temperature of the steel sheets which will also be the temperature of the phosphate coating, the temperature of the furnace must be set at a much higher level, generally of the order of 425 to 65O0C, depending on the furnace model , the speed of the conveyor and the rolling thickness.

Cooking at such a high temperature is an expensive operation because of the high price of natural gas or electricity for the furnace2 at a high cost of labor due to low coating speeds using unmodified material, at the expense of maintenance of the equipment, plus additional problems, such as significant heat back-pressure on the coating rollers, and an increased risk of insufficient cooking.

 It has long been sought to find a new coating or a process improvement that is simple and inexpensive, while providing a coating between sheets: an insulation and a space factor whose characteristics are equivalent to those of the coating or current process.

Accordingly, the invention consists of an element
of steel having a baked insulating phosphate coating derived from an aqueous solution comprising 100 volumes of aqueous aluminum phosphate and from 1 to 15 volumes of nitric acid.

 This phosphate coating is a mixture of a dilute aluminum phosphate solution, preferably composed of an aluminum monophosphate solution volume / plus 0.5 to 4 additional volumes of dilution water, and 3 to 12 46 by volume, calculated on the solution of aluminum monophosphate, nitric acid.

 Unexpectedly, this addition of nitric acid can lower the temperature at which the commercial steel plate is carried to cook the phosphate coating them to about 900C, while maintaining the coating, insulation and space of the coating obtained without modification. Lowering the cooking temperature also has the additional advantage of significantly reducing handling problems and reducing energy consumption.

 According to the invention, sheets or sheets of different ferrous metal contents may be treated.

It is thus possible to coat ferrosilicon sheets containing up to 7% silicon. Magnetic nickel-1 metal sheets containing up to 85% nickel can be processed. Other magnetic sheets containing a high proportion of iron alloyed with one or more metals can be treated as well. Generally all these sheets are cold rolled continuously. The term "sheet steel sheets" will be used herein including all the others.

 The cooked phosphate coating of the invention is derived from a mixture of dilute aqueous alumina phosphate, and from 1 to 15 volumes%, preferably from 3 to 12 volumes S or better still from 3 to 8 volumes%, based on the volume of dilute alumina phosphate, HN03. Below 1% by volume of HNO3, cooking is not improved. More than 15% by volume dtHN 3 would be too corrosive.

 The dilute aqueous orthophosphate composition of alumniuù consists of 100 volumes of an aqueous solution of alumina phosphate. This alumina phosphate composition is generally a mixture of phosphoric acid and hydrated alumina that can be described in terms of active ingredient content, P205 and Al2O3. The aluminum phosphate composition contains 28 to 38% by weight, preferably 30 to 35% by weight of P 2 O 5, 5 to 12% by weight, preferably 7 to 10% by weight, of Al 2 O 3 and from 50 to 70% by weight of water; and preferably from 50 to 400 volumes, and more preferably from 100 to 50 volumes of additional dilution water. Using less than 50 volumes of additional dilution water would result in coatings that may be difficult to apply even on electrical steel. More than 400 volumes of additional dilution water would yield too thin coatings, which would result in a loss of the insulating effect.

oll R est un groupe assurant la solubilisation à l'eau. As a result, the aqueous mono-phosphate aluminum composition is comprised of 33 to 50% by weight of P205 solids plus Al 2 O 3 where other materials such as zinc, zinc phosphate and nickel are especially excluded. However, an effective proportion of a nonionic wetting agent for steel, usually up to 1.5 volume%, and 0.5 to 1.2% by volume of materials such as those containing a group can be used. medium-length cat-eye attached to a group

oll R is a group that solubilizes with water.

as he is well known in this technique. With less than 0.5 volume% wetting agent the steel would not be wet or adhere to the phosphate coating.

 A typical formula for a phosphate coating modified with nitric acid for use at low temperature consists of: 10 liters of an aluminum phosphate solution made of 33% by weight of P 2 O 5, 8.6 by weight of Al 2 O 3 and 58.4% by weight of H 2 O, 20 liters of additional dilution water, about 0.3 liters of a nonionic wetting agent and 1.5 liters of HNO 3, i.e. 5% HN03 volume based on the combination of the previous ingredients. The term "nitric acid" as defined herein is TONO3 at a concentration greater than 65%, that is, in a reactive degree, usually an HNO3 concentration of 69 to 71%.

 This nitric acid-modified phosphate coating composition is applied to electrical steel, generally of the cold-rolled type, by roller application techniques, using rubber or felt grooved applicator rolls. When this composition is applied to the surface of an underlying steel sheet, and then thereafter this steel sheet and hence the phosphate coating is heated by carrying the steel sheet at a temperature of about 90 to 1300C for sufficient time to remove most of the water, the applied coating cures to a homogeneous mass adhering to the substrate.This layer of adherent coating communicates to the substate a good resistance between sheets which greatly reduces parasitic current losses when separated sheets are stacked for use in an electromagnetic device. This phosphate-enhanced nitric acid phosphate can be used in multiple layers and can be applied to other basic phosphate coatings. The term "temperature of the steel sheet" means the temperature of the steel and the coating measured with a thermo-couple or the like and not the generally much higher temperature of the oven.

 This nitric acid modified phosphate coating gives a post-cure density of between about 1 and about 1.5 grams per cubic centimeter. When it is cooked, it has been found that this coating gives at least 2 ohms per cm of 2 sheet and a value as high as 640 ohms per cm of sheet in the following ASTM A-717 Franklin test. The term "cooked" as used herein means that the phosphate coating is a non-sticky solid. It is believed that, during cooking, HNO3 increases the surface activity of the iron, allowing a much faster reaction of the coating with ferrous steel, for example to form iron phosphate, and thereby cause precipitation of aluminum phosphate.

The invension will be better understood from the following description of an example
Example
A nitric acid-modified phosphate coating composition consisting of a mixture of 10 volumes of a solution of aluminum monophosphate was prepared giving to the analysis 33.1% by weight of 8.6% by weight of A12O3 and 58.4% by weight of H2O, having a viscosity of about 40 centipoise at 250C, a specific gravity of 1.47 and a molecular weight of about 318 (commercially available from Monsanto Chemical Co, under the noncommercial Alkophos C), 20 volumes of additional dilution H2O and 5 volumes of 71% concentrated HNO3, ie 0.05 x 30 volumes = 1.5 volumes of HNO3. It was mixed at 250C, slowly adding the acid as the last component.

About 1% by volume of a phosphorus-containing average length ring alkyl nonionic wetting agent (commercially available from Victor) was added.
Chemical Comp. under the trade name of Victawet 8 12).

 An unmodified phosphate-based coating solution was also prepared in which the same proportions of the same ingredients were included except that nitric acid was excluded. The two roll coatings were then applied to cold rolled electric steel and heated to cook them, and the insulating characteristics were measured. The stamped parts of the reinforcement were then coated and inter-wall strength was examined. The results are given in the tables below.

TABLE 1
Characteristic interleaf insulation on steel
electric M22 0.055 m / m

<tb><SEP> r <SEP> Franklin <SEP>. <September>
<Tb>

<SEP> Temp. <SEP> Thickness <SEP> Franklin <SEP> ohmcm2 / <SEP> Factor
<tb><SEP> e <SEP> cooking <SEP> millet / side <SEP> A. <SEP> (in <SEP> sheet <SEP> of space
<tb><SEP> average) <SEP> (in
<tb> average) <SEP>
<tb> coating <SEP>
<tb> phosphated <SEP> 930C <SEP> 1.8 <SEP><SEP> 0.048 <SEP> 128 <SEP> 98 <SEP>%
<tb> modified <SEP> with
<tb> 3 <SEP>
<tb> coating
<tb> phosphated <SEP> 3400C <SEP> 2.2yU <SEP> 0.042 <SEP> 147 <SEP> 97.9 <SEP>%
<tb> no <SEP> changed
<tb> test tube
TABLE 2
Interfacing Resistance on Stamped Armature Parts
0.062 mm thick and 1.5 in diameter

<tb><SEP> HNO3 <SEP> modified <SEP> with <SEP> coating <SEP> 9.4 <SEP> ohm <SEP> cm / sheet
<tb> phosphated <SEP> (in <SEP> average)
<tb> * <SEP> no <SEP> changed <SEP> with <SEP> 9.4 <SEP> ohm <SEP> cm <SEP> / sheet
<tb><SEP> phosphate <SEP> coating (in <SEP> average)
<tb> test tube
As can be seen, the isolation and space factor characteristics of the HiNO 3 modified phosphate coating according to the invention remain evident, although the cooking temperature has been lowered by about 2500 ° C compared with phosphated coating.standard unmodified.

Claims (8)

 10) Ferrous element made of steel sheet, having a phosphatized, baked, insulating coating, characterized in that the coating is derived from an aqueous solution comprising 109 volumes of aqueous aluminum phosphate and of I to 15 volumes of nitric acid.  20) Element according to claim 1, characterized in that the aqueous aluminum phosphate is a mixture of phosphoric acid, hydrated alumina and water.  30) Element according to one of claims I and 2, characterized in that one uses from 3 to 8 volumes of nitric acid.  40. Element according to any one of claims 1 to 3, characterized in that the aqueous aluminum phosphate consists essentially of 28 to 38% by weight of P205, 5 to 12% by weight of Al203 and 50 to 70% by weight. % by weight of H 2 O.  5) Element according to claim 4, characterized in that the aqueous aluminum phosphate contains 30 to 35% by weight of P2O5 and 7 to 10% by weight of Al2O3.  60) Element according to any one of claims I to 5, characterized in that the aqueous solution contains 50 to 400 volumes of additional dilution water, and 3 to 12 volumes% of nitric acid, based on the volume aqueous aluminum phosphate and dilution water.  70) Element according to claim 6, characterized in that one uses from 3 to 8 volumes% of nitric acid.  80) Element according to one of claims 6 and 7, characterized in that the aqueous solution contains up to 1.5 volumes% of wetting agent.  90) Element according to any one of claims 6, 7 and 8, characterized in that luron uses 100 to 250 volumes of dilution water.