DE2031323A1 - Process for coating steel sheets - Google Patents

Description

Process for coating steel sheets.

The invention relates to a method for coating Steel sheets.

Purpose of the invention is especially to provide a process for coating steel sheets, in particular sheet metal transformer, including silicone steel plates to a similar substantial improvement to achieve their Stanzbarke.it.

For the production of cores for electrical equipment from transformer sheets or silicon steel sheets, for example ' of anchor cores of electric motors or generators or Cores for smaller transformers, transformer sheets are punched out in the desired shape and several in this way punched sheets stacked or laminated.

If the burrs are too high when punching, join these possibly neighboring sheets are des'laminated Package electrical, from which there is a deterioration in the electromagnetic properties of the core and others

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adverse 'effects result. Will the ridges after repeated punching processes too high, the punching die is usually expanded in the manufacturing plant and reduced reground the burr formation. The expansion, grinding and reinstalling the punching tools require additional labor and cause accordingly high costs. read especially the complicated forms for Punching the sheets for cores of engines have an intricate profile, so that their cut is special Skill required. Overall, the formation of the burrs worsens the economy, i.e. the The cost of the punching process is too great.

The punchability of transformer sheets is therefore an important factor Factor in the manufacture of laminated cores for electrical equipment. For their improvement have already been made numerous suggestions. One way of doing this is to extend the life of the dies or dies, i.e. increasing the number of punching operations possible without regrinding.

The critical permissible height of the burrs when punching transformer sheets was empirically determined to be 4-0 to 80 µm. Common steel tools cause burrs of this critical height after about 30,000 to 70,000 punching operations. Thanks to the use of tungsten carbide alloys, such as WC-Co alloys, the service life of the Stamping tools to 300,000 to 700,000 in recent times Punching processes, i.e. increased tenfold compared to conventional tools. ·

The machinability of sheet iron and steel to be punched has been investigated many times in the recent past, and there have been numerous reports of

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Stamping beneficial mechanical i £ L gense sticks of iron ' and steel sheets such as hardness, elongation limit and tensile strength released. jJarin is estimated to be the service life of the steel tools through improvement the punchability of iron and steel sheets can be doubled at most. This is especially true for transformer sheets. Extent of a chemical or thermal treatment of the Metal sheets to improve their mechanical properties are generally limited by the fact that at the same time the. ■■ '. must achieve necessary electrical and magnetic properties. Thus there is little scope for, f

there is an improvement in the punchability of transformer sheets. For example, Japanese patent application 24,74-4- / 1967 discloses an improvement in the punchability of steel sheets, which, however, is at most 40 to 50 / fr. Subsequent baking of the coating, as can be seen, for example, in the article "Fiat üolled Electrical Steel" in the specialist publication AISI Proceedings, July 1968. Such a paint application can increase the number of punching processes up to regrinding to, for example, 60,000

150,000 to double. On the other hand, there are such λ

However, coatings from organic substances and only tolerate relatively low temperatures, for example JOG C, but not the to the heat treatment of iron and steel plates, for example, when starting the voltage reduction, the required high temperatures of about 800 ⁰ C. In the production of a core for a stator Inert gas tungsten arc welding (CMcgonarc welding) burns the paint and causes a large number of bubbles, which greatly reduce the mechanical strength of the welds. Such a weakening of the welding parts represents a serious impairment.

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In order to avoid these disadvantages, Japanese Patent Publication No. 527,632 (Japanese Patent Publication 7 850/1968) the use of a water-soluble, inorganic, chromic anhydride as the main component and others Coating containing substances such as magnesium oxide and adipic acid. By heat treatment or sintering of with steel sheets provided with such an inorganic coating ' the punchability can be increased by 1.5 to 2 times compared to sheet metal provided with conventional coatings to enhance. On the other hand, a coating of the type mentioned last has chromic anhydride as its main component Chromate or a bichromate has the disadvantage that it causes exceptionally heavy wear on the tungsten carbide alloy cutting dies or tools and thus their service life reduced to 200,000 to 500 punching operations. So when cutting dies out Tungsten carbide alloys for processing steel sheets with the above-mentioned inorganic lacquer coating are used, their service life is reduced to about half to a fifth of the value, which " when processing uncoated steel sheets or steel sheets provided with known phosphate coatings is achievable. An inorganic paint according to the last-cited Japanese patent is thus hardly suitable for cutting dies made of tungsten carbide alloys.

Both with the well-known organic as well as with the known inorganic coatings, the permissible fit between punch and die of the form is very much small amount. Even if an experimental one Die with certain dimensions prove to be useful proved to bring manufacturing stamping tools for stamping parts of intricate form, for example for runners of Electric motors, not satisfactory results.

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883/1920.

It is an important object of the invention that in the known punching processes of iron and steel sheets occurring To reduce or eliminate difficulties and to provide an improved coating process.

The method according to the invention is characterized in that a first layer with a thickness of 0.5 to 2.5 µm is produced for the coating by applying a solution containing chromic acid, chromates or bichromates or several of these substances, that a second, opaque, pn is applied lacquer varnish existing layer in a thickness of 0.1 to 2.5 and that the films are baked at 250 to 600 ⁰ C.

Practical tests have confirmed that such a combined coating of a first thin layer of chromic acid compounds and a second thin layer The varnish after baking is suitable for the emergence to effectively prevent burrs and the inevitable increase in play between punch and die Compensate the shape in the course of the stamping operation. As a result, by means of the method according to the invention largely suppress the inevitable formation of burrs during punching and the service life of the molds extend by several tens factors.

According to the invention is the combined by the Auitrag Coating on the steel sheets to be punched increases the service life of punching tools made of steel to 800,000 and that of punching tools made of tungsten carbide alloys can be extended to 5,000,000 punching processes. The combined The coating consists of a first thin layer of chromic acid compounds and a second thin layer of varnish. In the method according to the invention it is not necessary to specify the chemical "composition, mechanical properties or electromagnetic

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Change characteristics of the steel sheets themselves. the The punch can be used in a conventional manner and is required for punching with the inventive combined coating provided steel sheets no special precautions.

The method according to the invention can also be used for transformer sheets with an Si content of 4% or less. The combined coating applied to such transformer sheets for electromagnetic cores has excellent properties with regard to the insulation of the sheets, in particular a "high fill factor, high corrosion resistance, good adhesiveness, one high resistance between the lamellas and high heat resistance to guarantee fully-fledged welded joints and to enable relaxation tempering.

The first layer of the combined coating according to the invention is reddish-brown ^{because it contains hexavalent chromium (Cr +).} The reddish brown color is retained even if the layer contains a chromic acid-reducing substance. When the second layer is applied by applying varnish to the first layer and then baked, the reddish brown color of the first layer, which is typical for hexavalent chromium, tends to fade. Sometimes the finished combined coating has a greenish-brown color. When bending steel sheet provided with the combined coating to peel off the combined coating, the first and second layers of the combined coating peel off in one piece. This suggests that the first, chromic acid compounds. containing S seal chemically reacts with the second layer of lacquer, so that at least a small part of the six ™ advertising chromium (Cr) is reduced ^{to trivalent chromium (Cr + ^).} That is, the chromic acid compounds

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the first layer and the varnish of the second layer the inventive combined coating not only through physical adhesion, but possibly also through mutual chemical bonding firmly to one another are connected. The presence of such a chemical bond is also likely because Photomicrographs of sections of the combined coating show no boundary lines between the first and second layers showed.

In the known single-layer coating of steel sheets with chromic acid compounds, a corresponding amount of a reducing agent is added to them and the coating is rather heavily baked in order to reduce the hygroscopic properties of the coating. In the method according to the invention, it is no longer necessary to add such reducing agents when applying the first layer containing chramic acid compounds. Even if reducing substances should be added in special cases, their amounts can be small. The heating of the first layer according to the invention is only used for drying. Therefore, the heating takes place at a

low temperatures and for a short time. The first layer the combined coating according to the invention therefore does not need to be stoved.

The amount of reducing agent to be added to the first layer The substances and the conditions for heating are determined by the period from the application of the first layer to for applying the second layer of laek varnish. if For example, if the first and second layers are applied immediately one after the other, it is enough Drying the first layer. . ■ »■

The compounds contained in the first layer of the water-soluble coating according to the invention can be any

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water-soluble compounds of chromic acid, chromates and Be bichromats.

Chromic acid compounds which can preferably be used in the context of the invention are: -

^ Magnesium chromate, magnesium dichromate, or a Mix of themj

(2) zinc dichromate or a mixture of zinc dichromate and chromic acid;

(3) Calcium dichromate or a mixture of calcium dichromate and chromic acid »■

As a reducing agent for the chromic acid compounds, if should such be required, one or more of the following inexpensive organic polyhydric alcohols are usable with satisfactory result; Ethylene glycol1, Diethylene glycol, triethylene glycol, glycerine, sucrose, Glucose and the like.

It should be noted that with a thickness of the first layer of less than 0.5 pm the desired improvement, the Punchability of sheet steel regardless of the one for the second layer selected thickness is not achievable. On the other hand, if the thickness of the first layer is more than 2.5 µm, it was found that the adhesiveness the combined coating produced therewith on sheet steel is reduced. With such a reduced The combined coating tends to adhere Peeling off the steel sheets, being the interlaminare Resistance in transformer sheets drops below the permissible value. In addition, there is a risk that the the edges of the steel sheets detached from the combined coating accumulate on the surface of the molds and thus deteriorate the punching performance. The area determined between 0.8 and 1.5 µm.

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For the second layer of the combined according to the invention Coating, synthetic resin varnishes are preferable to natural resin varnishes. According to the invention, the following synthetic resin varnishes, among others, can be used: phenolic resin varnishes, amino resin varnishes, "" Epoxy resin varnishes, silicone resin varnishes, polyamide resin varnishes, acrylic acid resin varnishes, melamine resin varnishes, polyester resin varnishes, Vinyl resin varnishes and coumarone resin varnishes. To determine their satisfactory usability for the inventive Various attempts were made with the above-mentioned insulating varnishes.

The insulating varnish is usually dried at temperatures between room temperature and 180 .0. In the event of If the paint dries too much at excessive temperatures, burns appear in the paint and weaken it these, so that the adhesion of the paint to the steel sheet and its insulating properties suffer.

Wires and the like coated with enamel lacquer are exceptionally ^{dried at relatively high temperatures, for example 350 °} C. for times of several tens of seconds after the lacquer has been applied to the bare conductor. This improves the drying efficiency. In such cases, however, both the drying temperature and the duration of the heat treatment at such elevated temperature must be carefully monitored in order to avoid impairment of flexibility and other mechanical properties due to excessive baking.

According to the inventive method, the baking is carried out to achieve the chemical bonding of the paint varnish to the first, chromic acid compounds containing layer at relatively high temperatures of 250 am b00 ⁰ C, preferably 280 to 400 ⁰ C, at temperatures so that were previously avoided in the paint industry, . By baking at temperatures below 2 ^ 0 G is

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the desired improvement in punchability and the lubricating effect that occurs at the same time cannot be achieved, although acceptable adhesion of the coating can be achieved. At temperatures above 600 ^° C., the stoving time would have to be extremely short, the control of such very short stoving times in economically constructed systems being extremely difficult.

The rapid stoving of the second layer of the combined coating at such high temperatures not only affects the lacquer layer, but also causes the first layer, which contains chromic acid compounds, to be stoved. Accordingly, the heat treatment of the first layer only needs to be very brief before the application of the second layer covering it. ^ ^{1 For} their thickness, a range of 0.2 to 1.5 µm has proven to be optimal. The second layer of lacquer should be at least 0.1 μm thick. Lacquer layers with a thickness of less than 0.1 μm lack the lubricating effect, which is decisive for improving the punchability. On the other hand, if the thickness of the varnish layer exceeds 2.5 µm, its adhesiveness to steel sheets deteriorates so that the edges are peeled off during punching. The paint peeled off at the edges collects on the surface of the die of the die and causes deformations in the punched steel or iron sheets. If the lacquer used consists of organic compounds, such thick layers of lacquer also tend to form blisters in inert gas tungsten arc welding (argon arc welding),

Further advantages and features of the invention emerge from the following description of exemplary embodiments based on the drawing. In this is;

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Fig. 1 and Pig. 2 each have a diagram to illustrate the according to the invention achievable improvement in Punchability and *

Fig. 3 and Fig. Λ each a schematic sectional view the wear and tear of cutting dies and tools.

To explain the effect that can be achieved according to the invention are in the following embodiments of the invention in terms of punchability compared to some conventional ones Procedure described. "

3 and 4 show the essential part of the punching device used to investigate the punchability of various steel strips or sheets in accordance with the examples described below. The punching device has a punch 1 fastened to a movable punch holder 3 by means of screws 5 and a die 1 fastened to a fixed table 4 by means of screws 6. After aligning the pin holes 7 j 7a, a guide pin 8 in this einge- M is hit, the stamp 1 immovably holds during repeated stamping operations with respect to the punch holder. 3 In the same way, tenon holes 9 and 9a are drilled through the fixed die table 4 and the die 2. A further guide pin 10 is driven into this after alignment. The individual steel sheets to be examined are placed on the upper surface of the die 2. The die 2 has a cavity R into which the punch 1 can be inserted in a form-fitting manner. When the punch 1 is lowered through the upper level of the die 2, the steel sheet applied to the die 2 is punched by the interaction of the punching edges F and Q of the punch and die.

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example 1

1. Pretreatment of the steel strips.

Steel strip samples with a content of 0.018 '> carbon, 0.30 % silicone, 0.029% manganese and 0.01 % phosphorus, 0.5 mm thick and a Vickers hardness of "Ή = 128 were cleaned.

2. Coating and treatment conditions. A. Coating According to the Invention; A solution was prepared by adding 1.5 liters of ethylene glycol to 100 liters of a 17% calcium dichromate solution and then applied uniformly to the samples described above by means of a pair of corrugated rubber rollers. By heating the steel strip samples to 200 ^° C. for 60 s in an electric furnace, a first layer with a thickness of 1.2 μm was produced and the steel strip was then cooled to room temperature by air cooling. A phenolic resin varnish was thinned by adding a suitable thinner so that in the viscosity test its processing time for a Mr. 4 Ford cup was still 15 s. The thinned paint was applied to the first layer on the steel strip in a thickness of 0.5 μm by means of a reversing application roller. The lacquer layer was then baked in a hot air oven at 350 G for 80 s.

B. Coating with chromic acid compounds only: By adding 2.0 1 ethylene glycol to 100 1 a 17% solution of calcium dichromate made a solution. By applying this solution au! the steel strip samples listed above by means of a pair of corrugated rollers coatings were 1.2 µm thick

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generated and for 100 s at 300 ° .ϋ in one Burned-in electric furnace. ·

C. Coating with paint only:

A phenolic resin varnish was diluted by adding a suitable thinner so that its throughput time for a Ford cup Ur. 4 was still 40 s. The diluted paint was applied evenly to the steel strip samples listed above and then ^{baked for 80 s at 350 °} C. in a hot air oven. A lacquer layer with a thickness of 5 »5 ρ was created.

3. Punch attempt.

A steel mold made from SKD-1 steel was used for the punching tests with all-round play between punch and die of 10 μm and a tungsten carbide mold from one WG-Co alloy with 16 wt .-% Go with all-round ■ play between punch and die of 15 µm used. When using a light oil as Lubricants were punched disks with a diameter of 20 mm. At every 10 QOO this way Three specimens were taken from the punched disks and the height of the ridges at four locations on each specimen measured by micrometer. The average values of the greatest ridge height of the three Specimens determined.

4. Results.

Pig. 1 shows the punchability of steel sheets with the above-described layers A ₅ B or C when using a steel mold, and FIG. 2 shows the corresponding punchability when using the tungsten carbide mold. '·

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As can be seen from FIG. 1, the burr height increased in the case of conventional coating with chromic acid compounds or with lacquer after 130,000 or 190,000 punching operations to 50 μm. In the case of the coating according to the invention, this burr height was only achieved after 800,000 punching operations, that is to say the punchability was improved four to six times over conventional methods by the method according to the invention. The curve A ¹ in FIG. 1 also shows that the improvement in punchability by the method according to the invention is not significantly influenced by the 10 .mu.m to 30 .mu.m mentioned above when the clearance between punch and die is increased. The method according to the invention thus enables a wide tolerance range for the play between punch and die.

As can be seen from Fig. 2, the tungsten carbide form can be used Perform 1,200 die cuts with the conventional lacquer coating, with the conventional one However, coating with chromic acid compounds only 120,000 before a burr height of 50 µm occurs. In contrast, the burr height exceeded the coating of the same test sheets according to the invention with the same coating materials, even after 2,400,000 punching operations, not yet 20 μm.

The wear of the tungsten carbide form used in the tests was determined by placing a thin sheet of copper on it with a thickness of 0.5 mm on the edges of the punch and the die (P and Q, Fig. 3) according to Completion of the punching tests determined, the wear corresponding to the edges impressions on the copper sheet left behind. ' Figs. 3 and 4 show the state of the edges P and Q of the punch and die before and after

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repeated punching cuts with the above-described coatings A and B. In the steel sheet samples with coating B, the tungsten carbide form was worn to a considerable extent after 200,000 punching cuts, namely by about 0.015 mm horizontally, as with X. and X ~ in! '' ig. 4, and by about 0.05 mm in the vertical direction, ivie at 1, and Xp shown in FIG. In contrast, with the coating according to the invention, after 2,400,000 punching operations, only slight wear from the usual rounded shape was noticed.

The test results with & r Stahlform show that ■ the lubricating effect of the coating according to the invention that of the conventional coating with only chromic acid compounds or is superior only with varnish. Leave the test results with the tungsten carbide mold recognize .that in the coating according to the invention. the first layer containing chromic acid compounds covering the second layer of lacquer a contact between the mold and the first layer effectively prevented.

Around. the effects of the viscosity of lubricating oils are increasing Investigate various lubricating oils, starting from volatile lubricants with low viscosity to non-volatile lubricants with high Viscosity investigated. It was found that the coating according to the invention is independent of the predictable variability in the viscosity of the lubricating oil provides excellent results.

Example 2

1 of a 15% solution of magnesium chromate 1.2 l of glycerine and 0.7 kg of chromic anhydride were added. By evenly applying the prepared solution on

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Sheet steel / samples according to Example 1 using a pair of corrugated rubber rollers, a first layer with a thickness of 1.8 μm was produced and dried in an electric oven ^{at 220 ° C. for 50 s.} The first layer was then cooled.

An amino resin varnish was thinned using a suitable thinner until, in the viscosity test, its throughput time for a Ford cup No. 4 was still 12 seconds. The thinned varnish was applied to the .aufgebracht first layer and then baked s in a hot air oven 70 at 320 ⁰ G.

The steel sheet specimens coated in this way were tested using a steel mold in which the clearance between punch and die 15 pm, punched into disks with a diameter of 20 mm » During 950,000 punching operations, the burr height reached 50 µm «

Example 3

A 100 1 of a 20% Zinkbichromatlösung and 1.5 glycerol existing solution was applied by means of a pair of grooved rubber rollers evenly on steel sheet sample according to Example 1 and dried for 30 s at x 300 ⁰ G in an electric furnace Here, a layer was formed in a thickness of 1.1 'p, which was then cooled »

Burcli the addition of a suitable thinner, a terephthalate lacquer was diluted to such an extent that in the viscosity test its flow rate for a JiOrcl cup No. ₀ 4 was still 20 seconds .8 µm applied to the first layer and then baked in a hot air oven for 60 s »

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The steel sheet samples with this coating according to the invention were made using a tungsten carbide mold with a game punched between punch and die of 12.5 µm. To The height of the burr was 25 μm.

As can be seen from the above description, the The coating according to the invention has a special lubricating effect and thus improves the punchability of transformer sheets in considerable measure. In addition, the coating provides excellent insulation. The interlaminar Resistance of the Trafobleehe invention was after the second JIS-C2550 method (JIS = Japanese Industrial Standard) with 30 to 60 ohm-cm per shift. These Values are comparable to those of conventional transformer sheets.

After the steel sheets according to the invention had been laminated, the laminated edge was welded using the inert gas tungsten arc method (Argonarc method). In the process, high-quality welds were achieved without the formation of bubbles. The steel sheets welded in this way were ^{tempered for 2 h at 750 °} C. to compensate for tensions, the first layer containing chromic acid compounds adhering firmly to the steel sheet, while the second layer on the outside only peeled off to a small extent. The steel sheets ■ thus proved to be satisfactory for all practical requirements.

The excellent heat resistance of the invention Coating appears to be based on the first layer containing chromic acid compounds being thicker than the second, made of lacquer, so that the very thin second layer of lacquer spreading the decomposition underneath Prevents heat exposure beyond the layer itself while holding the first and second layers together as a unitary coating. ·

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Thus the invention provides a method of improvement the punchability of sheet steel, especially transformer sheet en with compliance with the required mechanical and electromagnetic properties. Furthermore, the inventive Coating is extremely heat-resistant and therefore tolerates welding and other heat treatments. In addition, the combination coating according to the invention reduces the wear and tear of the punching tools and thus extend their service life or service life. Another An important advantage of the invention is that the improvement in punchability is independent of the inevitable Change or increase in the game between punch and die can be achieved. This is according to the invention no excessive care is required in the assembly of the punching tools. Overall, the Invention therefore to a large extent to improve the punching ability of steel sheets and the efficiency during punching at.

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Claims (8)

Claims MJ method for coating steel sheets, characterized in that a first layer in a thickness of 0.5 "to 2.5 pa is produced for the coating by applying a solution containing chromic acid, chromates or bichromates or several of these substances second, opaque, made of lacquer varnish layer in a thickness of 0.1 to 2.5 microns is applied, and that the layers are baked at 250 to 600 ⁰ C. 2. The method according to claim 1, characterized in that g e k e η η ζ e i c h η et that the first layer has a thickness between 0.8 and 1.5 µm. ■ 3 · Method according to claim 1 or 2, characterized in that ge k e η η * draws that the second layer has a thickness between 0.2 and 1.5 4. The method according to any one of claims 1 to 5? thereby geke η η ζ eich η et, dsiB .the first layer one or more compounds from the'vors magnesium chromate, magnesium dichromate, zinc chromate, zinc dichromate, calcium chromate, calcium dichromate mxä. Contains chromic acid formed group. 5 «Method according to at least -this of claims 1 to 4, characterized geke η zi ζ eic Ii aet ₅ that the first layer further comprises a fieduktionsmitt-el from of BATH ORIGINAL Ethylene glycol, diethylene glycol, triethylene glycol, glycerine, Contains sucrose and glucose group formed. 6. The method according to at least one of claims 1 to 5, characterized in that the second Layer consists of a varnish, the one or several resins from the range of phenolic resins, amino resins, ijpoxy resins, silicone resins, vinyl resins and coumarone resins formed group contains. · 7. Method at least according to claim 1, characterized characterized in that the first layer before Application of the second layer is dried. 8. The method at least according to claim 1, characterized in that the baking of the layers takes place at a temperature between 280 and 400 ⁰ G. 9. Method according to at least one of the preceding claims, characterized in that the steel sheets are dynamo or transformer sheets " 3ÄD ORSGI ^ ÄL 009883/19? Π Blank page