Patent Grant
7156926
The present invention relates generally to methods for removing iron oxide scale from processed sheet metal and inhibiting further oxidation in the processed sheet metal. More particularly, the present invention relates to methods for removing iron oxide scale from the surfaces of processed sheet metal using a mechanical surface conditioning apparatus in a manner to inhibit further oxidation on the conditioned surfaces and to reduce surface roughness.
Processed sheet metal has a wide variety of applications. For example, aircraft, automobiles, file cabinets and household appliances, to name only a few, contain sheet metal bodies or shells. The sheet metal is typically purchased directly from steel mills and/or steel service centers, but may be passed through intermediate processors (sometimes referred to as “toll” processors) before it is received by an original equipment manufacturer. Sheet metal is typically formed by hot rolling process and, if the gauge is thin enough, it is coiled for convenient transport and storage. During the hot rolling process, carbon steel typically reaches finishing temperatures well in excess of 1500° F. (815° C.). Once the hot rolling process is completed, the hot rolled steel is reduced to ambient temperature, typically by quenching in water, oil or polymer, as is well known in the art. As a result of reactions with oxygen in the air and moisture, an iron oxide layer (or “scale”) is formed on the surface of hot rolled carbon steel while the steel is cooled. The rate at which the product is cooled, and the total temperature drop, will affect the amount and composition of scale that forms on the surface during the cooling process.
Iron has a complex oxide structure with FeO (“wustite”) mechanically bonded to the base metal substrate, followed by a layer of Fe3O4 (“magnetite”) chemically bonded to the wustite, and then a layer of Fe2O3 (“hematite”) chemically bonded to the magnetite and which is exposed to the air. Oxidation tends to progress more rapidly at higher temperatures, such as those reached in a typical hot rolling process, resulting in the formation of wustite. The relative thickness of each of the distinct wustite, magnetite and hematite layers is related to the availability of free oxygen and iron as the hot rolled substrate cools. When cooled from finishing temperatures above 1058° F. (570° C.), the oxide layer will typically comprise at least 50% wustite, and will also comprise magnetite and hematite in layers, formed in that order from the substrate. Though a number of factors (e.g., quenching rate, base steel chemistry, available free oxygen, etc.) affect the relative thicknesses of wustite, magnetite and hematite, as well as the overall thickness of the oxide layer, research has shown that the overall thickness of the oxide layer (inclusive of all three of these layers) in hot rolled carbon steel will typically be about 0.5% of the total thickness of the steel sheet. Thus, for example, in ⅜″ hot rolled carbon steel, the overall thickness of the oxide layer will be about 0.002″.
Various methods exist for flattening sheet metal and for conditioning the surfaces thereof. Flatness of sheet metal is important because virtually all stamping and blanking operations require a flat sheet. Good surface conditions are also important, especially in applications where the top and/or bottom surfaces of the metal sheet will be painted or otherwise coated. For processed sheet metal that is to be painted or galvanized, current industry practice is to remove all evidence of oxide from the surface to be painted or galvanized. With respect to painted surfaces, removing all evidence of oxide before painting ensures optimum adhesion, flexibility, and corrosion resistance of the intended paint coating layer. With respect to galvanizing, removing all evidence of oxide before coating allows a sufficient chemical bond of zinc to base metal.
The most common method of removing all oxide from the surface of hot rolled sheet metal before coating is a process known as “pickle and oil.” In this process, the steel (already cooled to ambient temperature) is uncoiled and pulled through a bath of hydrochloric acid (typically about 30% hydrochloric acid and 70% water) to chemically remove the scale. Then, after the scale has been removed, the steel is washed, dried, and immediately “oiled” to protect it from rust damage. The oil provides an air barrier to shield the bare metal from exposure to air and moisture. It is critical that the metal be oiled immediately after the pickling process, as the bare metal will begin to oxidize very quickly when exposed to air and moisture. The “pickle and oil” process is effective in removing substantially all of the oxide layer, including the tightly bonded wustite layer, and results in a surface that is suitable for most coating applications. However, the “pickle and oil” process has a number of disadvantages. For example, the oil applied to the metal after pickling must be removed before coating, which is time consuming. Also, hydrochloric acid is an environmentally hazardous chemical, which has special storage and disposal restrictions. In addition, the oil coating interferes with some manufacturing processes, such as welding, causes stacked sheets to stick together, and gets into machine parts during manufacturing processes. Also, while the pickling process is effective at removing substantially all of the oxide layer, resulting in a surface that is suitable for most coating applications, the pickling agent (hydrochloric acid) tends to leave a clean but slightly coarse surface.
Thus, there is a need for an improved method of surface conditioning processed sheet metal, which removes enough scale from the surface to ensure optimum conditions for accepting coatings, which results in a smooth surface that is suitable for virtually all coating applications, which includes a means for inhibiting further oxidation prior to coating, and which is less expensive and troublesome than standard pickling and oiling.
It is therefore an object of the present invention to provide an improved method of removing iron oxide scale from processed sheet metal in a manner to ensure optimum surface conditions for accepting paint, galvanizing, or other coating. A related object is to provide an improved method of removing iron oxide scale from processed sheet metal, which results in a smooth surface that is suitable for virtually all coating applications. Another object is to provide an improved method of removing iron oxide scale from processed sheet metal in a manner that will inhibit further oxidation without the need to coat with oil. Still another general object is to provide an improved method of removing iron oxide scale from processed sheet metal, which is less expensive and troublesome than standard pickling and oiling.
The present invention includes methods of removing iron oxide scale from processed sheet metal, wherein the iron oxide scale generally comprises three layers: a wustite layer, a magnetite layer, and a hematite layer. The wustite layer is bonded to a base metal substrate of the processed sheet metal. The magnetite layer is bonded to the wustite layer, and the hematite layer is bonded to the magnetite layer. In general, the methods comprise the steps of: providing a surface conditioning apparatus; and conditioning a surface of the processed sheet metal with the surface conditioning apparatus. The surface conditioning apparatus has at least one surface conditioning member. The step of conditioning the surface of the processed sheet metal includes bringing the at least one surface conditioning member into engagement with the surface of the sheet metal. The surface conditioning member is brought into engagement with the surface in a manner to remove substantially all of the hematite layer and magnetite layer from the surface. Additionally, the surface conditioning member is brought into engagement with the surface in a manner to remove some but not all of the wustite layer from the surface, so that a portion of the wustite layer remains bonded to the base metal substrate of the processed sheet metal.
In another aspect of the invention, methods of removing iron oxide scale from processed sheet metal comprise the steps of: providing a surface conditioning apparatus having at least one rotating conditioning member; and conditioning a surface of the processed sheet metal with the surface conditioning apparatus. The step of conditioning the surface of the processed sheet metal includes bringing the at least one rotating conditioning member into engagement with the surface of the sheet metal. The rotating conditioning member is brought into engagement with the surface in a manner to remove some, but less than substantially all of the iron oxide scale from the surface so that a layer of oxide scale remains bonded to a base metal substrate of the processed sheet metal. Additionally, the rotating conditioning member is brought into engagement with the surface in a manner to reduce an arithmetic mean of distances of departure of peaks and valleys on the surface, measured from a mean center line, to less than 50 micro inches.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding and appreciation of the invention may be attained by referring to the Figures and detailed description of the preferred embodiments, which follow.
The accompanying Figures, which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and, together with the description, serve to explain the principles of the invention.
Reference characters shown in these Figures correspond to reference characters used throughout the following detailed description of the preferred embodiments.
In performing the methods of the present invention, a surface conditioning apparatus, which will be described in detail hereinafter, may be used in conjunction with a number of different machines for flattening and leveling sheet metal, without departing from the scope of the present invention.
A surface conditioning apparatus of the type used in practicing the methods of the present invention is represented generally in
It has been found that cleaning brushes manufactured by Minnesota Mining and Manufacturing (3M) under the name Scotch-Brite®, or their equivalent, are suitable for use in the surface conditioner 10 of the present invention. In these brushes, abrasive particles are bonded to resilient synthetic (e.g., nylon) fibers of the brush with a resin adhesive. The resilient brush fibers of the Scotch-Brite® product are of an open-web construction, which gives the fibers a spring-like action that conforms to irregular surfaces and prevents surface gouging. Scotch-Brite® brand cleaning brushes are available in a variety of grades of coarseness and fiber density, though suitable abrasive and non-abrasive cleaning brushes manufactured by others could be used without departing from the scope of the present invention. The inventor has determined that 3M's Scotch-Brite® brand finishing-cleaning brushes identified by 3M item number #048011-90626-3, SPR 22293A are suitable for use in practicing the methods of the present invention, though other brushes with other grades of coarseness and fiber density may also be suitable. The selection of other suitable brushes would be within the skill of one of ordinary skill in the art.
As shown in
Preferably, a spray bar 80 having a plurality of sprayer nozzles 72 is positioned just downstream of the cleaning brush 70, with the sprayer nozzles 72 aimed generally toward the point of engagement of the cleaning brush 70 and the surface of the strip 46. The sprayer nozzles 72 apply a coolant/lubricant, such as water, to the cleaning brush 70 during operation of the surface conditioner 10. Preferably, the coolant/lubricant is applied at the rate of about 4 to 6 gallons per minute per 12″ length of the cleaning brush 70. This enhances performance of the surface conditioner 10 by producing a cooler running operation, while washing away cleaning by-products (scale and smut removed by the abrasive surface of the brush), and by extending the life of the cleaning brush 70. As shown in
For optimum performance, the surface conditioner 10 requires a very flat surface. This is why the stretcher leveling machine 12 and tension leveling machines 40 shown in
Preferably, the various apparatus an environments described above are used to practice the present invention, which includes methods of removing iron oxide scale from processed sheet metal.
In general, a method of the present invention comprises conditioning a surface of the processed sheet metal 46 with the surface conditioning apparatus 10 by bringing the generally cylindrical conditioning surface 76 of the rotating cleaning brush 70 into engagement with the surface of the sheet metal 46. As the sheet metal 46 is advanced through the surface conditioning apparatus 10, the rotating cleaning brush 70 is rotated in the upstream direction against the downstream advancement of the length of sheet metal 46. This engagement of the brush 70 against the surface of the sheet metal 46 removes substantially all of the hematite layer 92 and magnetite layer 90 from the surface. In addition, the engagement of the brush 70 against the surface of the sheet metal 46 removes some (but not all) of the wustite layer 88 from the surface, so that a portion of the wustite layer 88 remains bonded to the base metal substrate 94 of the processed sheet metal, as shown in
The hematite layer 92 and magnetite layer 90 are rather brittle, so the above-described mechanical brushing is very effective at removing all or substantially all of these layers. The removal of these layers has been confirmed by a napkin wipe test (e.g., wiping a napkin across the surface), which is considered standard process control. Once the surface has been conditioned in accordance with the methods of the present invention, a napkin wiped across the surface should not pick up any visually perceptible scale or smut. Also, as indicated above, this mechanical brushing also preferably removes about 30% of the tightly adhered wustite layer 88 from the surface of the sheet metal 46, leaving a layer of wustite bonded to the base metal substrate 94. It has been found that the remaining layer of wustite 88 is beneficial because it allows the conditioned surface of the sheet metal to withstand further oxidation. Limited research by the inventors herein has shown that this benefit occurs at least in part as a result of the mechanical brushing removing all or substantially all of the magnetite and hematite composition layers. With these layers removed, there is less available free iron to form a “red rust” oxide. Magnetite (chemically known as Fe3O4) and hematite (chemically known as Fe2O3) contain much more available iron atoms than the remaining wustite layer (chemically known as FeO). It is also theorized that the process of mechanical brushing has a “smearing” effect on the remaining wustite layer, which may contribute to the sheet metal's ability to withstand further oxidation by making the remaining wustite layer more uniform and thereby reducing the likelihood of ambient oxygen and moisture reaching the base metal substrate 94. However, this theory has not been confirmed.
In another aspect of the present invention, a method of removing iron oxide scale from processed sheet metal comprises the steps of: providing a surface conditioning apparatus 10 having at least one rotating conditioning brush 70; and conditioning a surface of the processed sheet metal 46 by bringing the rotating conditioning brush 70 into engagement with the surface of the sheet metal 46 in a manner to remove some, but less than substantially all of the iron oxide scale from the surface so that a layer of wustite 88 remains bonded to a base metal substrate 94, and in a manner to smooth the surface. Preferably, the “smoothing” achieved by engagement of the rotating conditioning brush 70 with the surface of the sheet metal 46 is sufficient to reduce an arithmetic mean of distances of departure of peaks and valleys on the surface, measured from a mean center line, to less than 50 micro inches. More preferably, the smoothing achieved by the rotating conditioning brush 70 is sufficient to reduce the arithmetic mean of the distances of departure of peaks and valleys on the surface, measured from the mean center line, to between about 35 and 45 micro inches.
Surface roughness is measured with a profilometer, as is well known in the art, and is usually expressed as an “Ra” value in micro meters or micro inches. This Ra value represents the arithmetic mean of the departure of the peaks and valleys of the surface profile from a mean center line over several sampling lengths, and is therefore also sometimes referred to as a “center line average” (CLA). The lower the Ra value, the smoother the surface finish. Limited quantitative evidence exists demonstrating that hot rolled sheet metal surface conditioned in accordance with the methods of the present invention, as measured with a profilometer, has a lower (i.e., better) Ra value than that of typical hot rolled steel which has been pickled. In fact, limited research has shown that hot rolled sheet metal surface conditioned in accordance with the methods of the present invention has an Ra value that is comparable to or better than cold roll regular matte finish (which typically has an Ra value of between 40 and 60 micro inches).
The inventors herein have found that the surface of the remaining wustite layer 88 left by mechanical brushing in accordance with the present invention is relatively smooth (as indicated by the Ra values noted above) and requires minimal or no additional surface preparation prior to painting or other coating. It has been found that the painting characteristics of material surface conditioned in accordance with the present invention are as good or better than pickled material. To the eye, the surfaces are virtually indistinguishable, as both appear to be free of oxide scale. However, testing has shown that, over time, material surface conditioned in accordance with the present invention is better suited to resist further oxidation than similar material that has been picked and oiled. Independent “salt spray tests” (which are standard in the industry) were conducted by Valspar Corporation, a reputable industrial paint manufacturer, and material that was stretcher leveled and then surface conditioned in accordance with the present invention was found to be substantially corrosion free after as long as 1000 hours of salt spray testing, whereas hot rolled steel that was pickled and oiled showed signs of further corrosion after as little as 144 hours of salt spray testing.
Again, it has been found that the layer of wustite 88 remaining after mechanical brushing in accordance with the methods of the present invention is beneficial because it inhibits further oxidation, due at least in part to the removal of all or substantially all of the magnetite and hematite composition layers, which leaves less available free iron to form “red rust” oxide. But in addition to this, and in addition to the smoothness benefits described above, mechanical brushing in accordance with the methods of the present invention is preferable to pickling and oiling because there is no need to remove the oil before coating; hydrochloric acid (an environmentally hazardous chemical that has special storage and disposal restrictions) is not used; and there is no oil to interfere with manufacturing processes, such as welding.
In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. However, as various modifications could be made in the invention described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying Figures shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.
This is a divisional patent application from patent application Ser. No. 10/408,732, filed Apr. 7, 2003 now U.S. Pat. No. 6,814,815.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2318432 | Stanier | May 1943 | A |
| 2449507 | Pope | Sep 1948 | A |
| 2907151 | Peterson | Oct 1959 | A |
| 3686921 | Roper | Aug 1972 | A |
| 3722251 | Withrow | Mar 1973 | A |
| 3753522 | Voges | Aug 1973 | A |
| 3924428 | Noe | Dec 1975 | A |
| 4312325 | Voges et al. | Jan 1982 | A |
| 4539830 | Noe et al. | Sep 1985 | A |
| 4751838 | Voges | Jun 1988 | A |
| 4872245 | Kawasaki et al. | Oct 1989 | A |
| 4887502 | Voges | Dec 1989 | A |
| 4982593 | Holloway | Jan 1991 | A |
| 5077887 | Holloway | Jan 1992 | A |
| 5181411 | Holloway | Jan 1993 | A |
| 5579658 | Noe et al. | Dec 1996 | A |
| 5759307 | Berger et al. | Jun 1998 | A |
| 5820704 | Veyer et al. | Oct 1998 | A |
| 5829287 | Noe et al. | Nov 1998 | A |
| 6088895 | Nelson et al. | Jul 2000 | A |
| 6089063 | Noe et al. | Jul 2000 | A |
| 6205830 | Voges | Mar 2001 | B1 |
| 6210501 | Okano et al. | Apr 2001 | B1 |
| 6537388 | Wynns et al. | Mar 2003 | B1 |
| 6732561 | Voges | May 2004 | B1 |
| Number | Date | Country |
|---|---|---|
| 1452957 | Apr 1969 | DE |
| 0498038 | Aug 1992 | EP |
| 58-116902 | Jul 1983 | JP |
| 0158862 | Nov 1963 | SU |
| Number | Date | Country | |
|---|---|---|---|
| 20050016566 A1 | Jan 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10408732 | Apr 2003 | US |
| Child | 10915928 | US |
