Patent Application
20090007997
The technology described herein relates generally to metal heat treating processes such as annealing, patenting, or tempering. More specifically, the technology described herein relates to a method and system to prevent iron oxide formation and decarburization during strand heat treating of a steel product without the required use of acid pickling. Additionally, this technology relates to placing a coating, such as copper plating, to the surface of a steel wire prior to strand heat treating to avoid both iron oxide formation and decarburization through the surface of the steel wire by preventing interactions between the steel wire and the furnace atmosphere.
Patenting is a heat treatment process for a metal or alloy. Patenting is used to soften a steel metal or alloy and remove any existing brittleness. Control of both time and temperature during the patenting process is critical in order to achieve a final product with appropriate mechanical properties. Patenting is, for example, but not limited to, used for the strand annealing process used during the manufacture of steel tire cord to strengthen the tire cord. For example, during the manufacture of steel tire cord, copper or brass is generally electroplated on the surface of the wire in-line after the heat-treating process in order to promote adhesion of the wire to the rubber of the tire. In general, the process steps known in the art, from the beginning of wire heat treating through the application of electrolytic copper, include: austenitizing, controlled cooling to 500° C. to 650° C., water quenching to room temperature, acid pickling, water rinsing, and electrolytic copper pyrophosphate plating.
The most common method of strand annealing, patenting, or tempering is to heat wire in a direct-fired gas strand-patenting furnace to about 1000° C. This process is termed austenitizing. Austenitizing is immediately followed by controlled cooling to within the range of 500° C. to 650° C., depending upon the specific steel chemistry. While austenitizing with known, current technology, the surface of the steel wire is in direct contact with the atmosphere within the furnace. The atmosphere within the furnace contains oxidizing gases, like oxygen, O2, and carbon dioxide, CO2, and decarburizing gases like water vapor. Decarburization is the decreasing content of carbon, C, in the steel wire. However, it is desired to prevent both the formation of iron oxides and decarburization due to exposure of the wire within the atmosphere of the furnace.
The iron oxide formed in the furnace must subsequently be removed. This is generally accomplished by exposing the steel wire cord to hot, concentrated hydrochloric acid, HCl, or sulfuric acid, H2SO4. Iron oxide may form as FeO, Fe2O3, and/or Fe3O4. It is also common to use electrolytic acid pickling in conjunction with sulfuric acid, H2SO4. Acid pickling is a process used on metallic surfaces to remove impurities and stains, such as rust or iron oxides, before subsequent metal processing, such as plating, drawing, extrusion, or rolling. Formation of iron oxide removes usable material from the surface of the wire, thereby reducing the mass of steel available for subsequent conversion to other products. Pickling acids, such as concentrated hydrochloric acid, HCl, or sulfuric acid, H2SO4, are expensive and environmentally unfriendly. Furthermore, such pickling acids create maintenance and health issues related to disposal, potential spills, and fumes.
When decarburization occurs, generally at the hot surface of the steel wire, and in the presence of water vapor resulting from combustion of gases like natural gas, the wire is unfit for subsequent conversion. Additionally the wire may exhibit property degradation such as low strength, low fatigue life, and/or unsuitable draw ability. Application of a coating to the surface of the wire, like electroplated copper, acts as a barrier between the wire and atmosphere within the furnace. Although copper oxides, CuO and/or Cu2O, will form at the surface of the steel wire, it is possible to apply enough copper to preclude the formation of iron oxides. It is also known in the art that carbon does not mix with or diffuse through copper, preventing the possibility of decarburization. It is also known that the rate of diffusion of copper into steel is slow, making it unlikely for copper-steel alloys to form at the surface of the wire.
It is known in the heat-treating industry to use electroplated copper as a “mask” during carburizing to prevent carbon from entering the surface of certain portions of a part that is intended to be hardened using a carburizing method, e.g. areas that will require subsequent machining. However, copper is not known or used in the art as a “mask” to prevent carbon from leaving the surface of a steel part.
Currently, during direct-fired gas annealing, precautions are taken to help ensure that decarburization is avoided or minimized. In general, furnace atmospheres are set to be oxygen-rich in about the first 20 percent of the furnace when the wires are still relatively cool. This atmosphere is not only conducive to convective heat transfer, the dominant type of heat transfer below a wire temperature of about 700° C., but also it causes a relatively thin layer of tenacious iron oxide to form. Generally, the remaining 80 percent of the atmosphere of the furnace is setup to run gas-rich with increasing amounts of gas located deeper within the furnace.
The amounts of gas present in a furnace are generally determined by measuring the carbon monoxide, CO, or oxygen, O2, level in a specific furnace zone. This measurement is then utilized to back-calculate the amount of excess gas. For example, if a five-zone furnace is used, the first zone would commonly be set slightly oxygen-rich or about 1.5 percent oxygen, the second zone to stoichiometric, the third zone with about 0.9 percent carbon monoxide, the fourth zone with about 1.5 percent carbon monoxide and the fifth zone about 2.2 percent carbon monoxide. The absence of oxygen in the latter portion of the furnace helps to limit the amount of oxide formed deep in the furnace when the wires are the hottest. The reaction between iron, Fe, and oxygen, O2, increases rapidly as the wire temperature increases.
Additionally, tri-atomic and more complex gases, like methane, CH4, help improve radiant heat transfer that is dominant and exponential after the wire temperature exceeds about 700° C. Furthermore, presence of gases containing carbon, such as carbon monoxide, CO, and methane, CH4, help reduce the propensity for decarburization by slightly increasing the carbon potential in the furnace atmosphere. The final precaution known in the art and currently taken during strand annealing of wire products is to carefully monitor the temperature of the steel wire at the exit of the furnace. This is completed directly by measuring individual wire temperatures using an instrument such as a disappearing element optical pyrometer. This is completed indirectly by measuring the ductility of the heat-treated wire by pulling samples in a uniaxial tensile tester and measuring either the elongation during the test or the reduction of area at fracture. It is very difficult to control both the wire temperature and the furnace atmosphere; any shift in the furnace atmosphere will result in a change in the wire temperature.
Furnace atmospheres can shift due to changes in outside barometric pressure which results in a change in the resistance to furnace atmosphere gases leaving the furnace exhaust stack. For example, if the outside barometric pressure decreases, less pressure will be available to hold the furnace atmosphere in the furnace. Generally, furnace stacks are at the furnace entrance. Therefore, the gas-rich portion of the furnace shifts toward the furnace entrance, reducing the oxygen-rich zone and resulting in a decrease in convective heat transfer and a corresponding decrease in wire temperature.
Another method known in the art to control oxidization and decarburization is to use a protective atmosphere in the furnace like hydrogen, carbon monoxide, cracked ammonia, or nitrogen, singularly or in combination. In this case a specialized furnace is constructed. Such a specialized furnace is generally a muffle furnace or a tube furnace. Both of these furnaces, by definition, heat wire indirectly and are relatively energy inefficient. In addition, cost to produce and maintain such a furnace atmosphere in a strand furnace, which is open at both ends, is prohibitively expensive. From an economic standpoint, it is less expensive to use the first method described to help control oxidization and decarburization in a direct-fired gas furnace.
These and other problems exist. Previous attempts to solve these and other problems include the following.
U.S. Pat. No. 4,966,659, issued to Seto et al. on Oct. 30, 1990, discloses a method and apparatus for molten salt electroplating a steel member in which the surface of the steel member is activated by anodic treatment and the molten salt electroplating is performed on the activated surface of said steel member.
U.S. Pat. No. 4,745,002, issued to Vexler et al. on May 17, 1988, discloses a method of making a copper clad conductor by the impinging of copper particles upon a heated steel wire to cause adhesion of the particles to the wire, by coalescence, building up the particles to form a coating and then drawing the coated wire to the required diameter. The coated wire may be heat treated to cause flow of copper to improve the surface finish before the drawing process. The copper particles may be directed at the wire by a spraying technique. Alternatively, the wire is passed over a fluidized bed of the particles and through a cloud of particles thrown up by the bed.
U.S. Pat. No. 4,704,337, issued to Coppens et al. on Nov. 3, 1987, discloses a rubber adherable steel reinforcing element with a composite surface coating. The steel element for reinforcing a rubber article comprises a brass layer and at least one additional outer film of metal or metal alloy selected from the group containing Fe, Ni, Mn, Cr, Mb, Va, Ti, Zi, Nb, Ta, Hf and W.
U.S. Pat. No. 4,686,153, issued to Tominaga et al. on Aug. 11, 1987, discloses an electrode wire for wire electric discharge machining a workpiece at high speed and high accuracy and a process for preparing the same. The electrode wire comprises a core wire made of a copper clad steel wire, 10 to 70% of the sectional area of the copper clad steel wire being occupied by copper, and a copper-zinc alloy layer covering the core wire. The copper-zinc alloy layer is prepared by coating the core wire with zinc by electroplating or hot galvanizing, followed by heating to disperse copper in the zinc layer to convert the same into a copper-zinc alloy layer wherein the concentration of zinc is increased gradually along the radially outward direction. The preferable thickness of the copper-zinc alloy layer ranges from 0.1 to 15 microns and the average concentration of zinc in the copper-zinc alloy layer is preferably less than 50% by weight but not less than 10% by weight.
U.S. Pat. No. 4,155,816, issued to Marencak on May 22, 1979 discloses a method of treating ferrous-based wire comprised of electroplating a negatively charged ferrous based wire in a prescribed aqueous electrolyte solution containing a positively charged stationary anode and, in combination, simultaneously, in the same electrolyte solution, and deplating a similarly electroplated ferrous based wire by passing said plated wire as a supplemental, additional, positively charged, moving anode through said solution to effect a removal of its electroplated outer metal coating.
U.S. Pat. No. 3,630,057, issued to Strohmeier on Dec. 28, 1971 discloses a process and apparatus for manufacturing copper-plated steel wire. Drawing grease is applied to continuously advancing steel wire rod, which is subsequently drawn in succession through a plurality of drawing dies to form wire, which is continuously advanced along a predetermined path. Electric current is passed through the advancing wire along a predetermined portion of the path to heat and anneal the wire. The advancing wire, which has been annealed, is pickled in an electrolytic bath and is subsequently rinsed and thereafter subjected to a chemical copper-plating treatment in a bath consisting mainly of copper sulfate solution. Drawing grease is applied to the advancing copper-plated wire, which is subsequently drawn through at least one drawing die. The advancing wire which has been drawn is finally wound on spools.
The foregoing patent and other information reflect the state of the art of which the inventor is aware and are tendered with a view toward discharging the inventor's acknowledged duty of candor in disclosing information that may be pertinent to the patentability of the technology described herein. It is respectfully stipulated, however, that the foregoing patent and other information do not teach or render obvious, singly or when considered in combination, the inventor's claimed invention.
In various exemplary embodiments, the technology described herein provides systems and methods to prevent iron oxide formation and decarburization during strand heat treating of a steel product without the required use of acid pickling. Additionally, this technology provides placing a coating, such as copper pyrophosphate electrolytic plating, copper sulfate electrolytic plating or hot dip copper plating, to the surface of a steel wire prior to strand heat treating to avoid both the iron oxide formation and decarburization through the surface of the steel wire by preventing interactions between the steel wire and the furnace atmosphere. Other comparable uses are also contemplated herein, as will be obvious to those of ordinary skill in the art.
In one exemplary embodiment, the technology provides a method to prevent iron oxide formation and decarburization during the patenting of steel wire. The method includes coating a steel wire with a metal plating prior to heat treating the steel wire, thereby masking the steel to prevent carbon from leaving a surface of the steel wire. The method includes austenitizing the plated steel wire in a gas furnace, wherein the steel wire is protected against iron oxide formation and decarburization, which would otherwise occur as a result of exposure to the atmosphere of the furnace, because of the plating. The method includes immediately cooling the austenitized and metal plated steel wire in a controlled cooling process environment. The method includes reducing oxides formed by the oxidation of the metal plating in the atmosphere of the furnace. The method also includes cooling the steel wire to room temperature. The coating of the steel wire with the metal plating prior to heat treating prevents iron oxide formation and decarburization during the heat treating, without a required use of acid pickling. The coating used for metal plating a steel wire is an electrolytic copper pyrophosphate plating, and electrolytic copper sulfate coating or hot dip copper plating. The method also includes austenitizing the plated steel wire in a gas furnace to a temperature of 950° C. to 1050° C. and cooling the steel wire to within the range of 500° C. to 650° C.
The method also includes reducing oxides formed by the oxidation of the metal plating in the atmosphere of the furnace by passing the steel wire through a reducing gas, wherein the reducing gas is hydrogen, carbon monoxide, or other reducing gas. Alternatively, the method reduces oxides formed by the oxidation of the metal plating in the atmosphere of the furnace using reverse plating, after the cooling of the steel wire to room temperature, by a copper pyrophosphate electrolytic removal process. Alternatively, the method reduces oxides formed by the oxidation of the metal plating in the atmosphere of the furnace using reverse plating, after the cooling of the steel wire to room temperature, by a copper sulfate electrolytic removal process. The recovered plating is applied to an unplated steel wire prior to its austenization. Alternatively, the method reduces oxides formed by the oxidation of the metal plating in the atmosphere of the furnace by using acid pickling and rinsing with water the steel wire to remove the acid used in the acid pickling. Alternatively, the method removes oxides formed by the oxidation of the metal plating in the atmosphere of the furnace using mechanical descaling of the oxides with mechanical brushes.
The method also includes lightly etching, with a mild acid solution, the steel wire to help improve subsequent electroplating. This etching is subsequent to the reducing of oxides formed. The method also includes, prior to coating a steel wire rod with a metal plating, drawing the steel wire from a steel wire rod taken from a steel coil, descaling the steel wire of any mill scale, or preexisting iron oxides used to prevent further oxidation during shipment, and rinsing the steel wire to remove any extraneous debris from the surface of the wire.
In another exemplary embodiment, the technology provides a system for preventing iron oxide formation and decarburization during steel patenting. The system includes a plating device for coating a steel wire with a metal plating prior to heat treating the steel wire and thereby masking the steel to prevent carbon from leaving a surface of the steel wire. The system includes a tempering device for austenitizing the plated steel wire in a gas furnace, wherein the steel wire is protected against iron oxide formation and decarburization, which would otherwise occur as a result of exposure to the atmosphere of the furnace, because of the plating. The system includes a controlled cooling device for immediately cooling the austenitized and metal plated steel wire. The system also includes a device for reducing oxides formed by the oxidation of the metal plating in the atmosphere of the furnace. The coating of the steel wire with the metal plating prior to heat treating prevents iron oxide formation and decarburization during the heat treating, without a required use of electrolytic acid pickling. In one embodiment, the device for reducing oxides formed by the oxidation of the metal plating in the atmosphere of the furnace is a chamber of hydrogen gas through which the steel wire is passed.
Advantageously, this technology provides a method and system to prevent iron oxide formation and decarburization during strand heat treating of a steel product without the required use of acid pickling. Additionally, this technology provides placing a coating, such as copper pyrophosphate electrolytic plating, to the surface of a steel wire prior to strand heat treating to avoid both the iron oxide formation and decarburization through the surface of the steel wire by preventing interactions between the steel wire and the furnace atmosphere.
There has thus been outlined, rather broadly, the more important features of the technology in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the technology that will be described hereinafter and which will form the subject matter of the claims appended hereto. In this respect, before explaining at least one embodiment of the technology in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The technology described herein is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the technology described herein.
Further objects and advantages of the technology described herein will be apparent from the following detailed description of a presently preferred embodiment which is illustrated schematically in the accompanying drawings.
The technology described herein is illustrated with reference to the various drawings, in which like reference numbers denote like system components and/or method steps, respectively, and in which:
Before describing the disclosed embodiments of this technology in detail, it is to be understood that the technology is not limited in its application to the details of the particular arrangement shown here since the technology described is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.
In various exemplary embodiments, the technology described herein provides systems and methods to prevent iron oxide formation and decarburization during strand heat treating of a steel product without the required use of acid pickling. Additionally, this technology provides placing a coating, such as copper pyrophosphate electrolytic plating, to the surface of a steel wire prior to strand heat treating to avoid both the iron oxide formation and decarburization through the surface of the steel wire by preventing interactions between the steel wire and the furnace atmosphere. Furthermore, the technology prevents iron oxide formation and decarburization without the required use of acid pickling. Other comparable uses are also contemplated herein, as will be obvious to those of ordinary skill in the art.
This technology uses a coating, such as, but not limited to, electroplated copper, to prevent iron oxide formation and decarburization during the strand heat treating of steel product. This coating can help guarantee that decarburization is not possible. Additionally, the coating is easily reduced or removed from the surface of the wire after it is oxidized in the austenization furnace. This process yields less environmental impact and minimal dangers of exposure between humans and harsh chemicals.
When a copper coating is used, the copper oxide formed in the austenization furnace is converted to pure copper though use of a reduction process using a gas such as, but not limited to, hydrogen. The reduction process occurs while the wire is still hot. Additional copper plating can be applied directly to this reduced copper with or without slight low concentration acid etching.
The copper oxide can be converted back into a raw material for future coating applications by reversing the polarity of the electrolytic source used initially to plate the coating onto the wire. When copper is used as a coating, it can be electroplated using a copper pyrophosphate solution that is relatively environmentally friendly and not considered particularly harmful to people during industrial exposure. To plate copper onto the wire in a copper pyrophosphate (or any other) solution, the wire is cathodic. To plate copper oxide from the wire, the wire is anodic, and the same solution of copper pyrophosphate can be used for both operations. Alternately, copper oxide is removed chemically using a mild solution of pickling acid at higher temperature than normal (chemical reaction rates double for every ten degree increase in temperature), an operation that is much less harmful to the environment. Alternately, the oxidized copper coating is also removed mechanically via brushes or other abrasive means.
During the manufacture of steel tire cord copper is generally electroplated on the surface of the wire in-line with the heat-treating process as part of a process to promote adhesion to rubber. In general, the process steps from the beginning of wire heat treating through the application of electrolytic copper are: austenitizing, controlled cooling to 500° C. to 650° C., water quenching to room temperature, acid pickling, water rinsing, electrolytic copper pyrophosphate plating. Using this new technology, the preferred process steps from the beginning of wire heat treating through the application of electrolytic copper are: electrolytic copper pyrophosphate plating, austenitizing, controlled cooling to 500° C. to 650° C., copper oxide reduction in a gas like hydrogen, cooling to room temperature, electrolytic copper pyrophosphate plating. The new process eliminates any requirement for acid pickling.
In addition to the preferred method listed above this technology also includes other methods to utilize a wire coating, like copper, during the heat treatment of product intended for steel tire cord applications.
In one alternative embodiment, the method includes the following steps in order: electrolytic copper pyrophosphate plating, austenitizing, controlled cooling to 500° C. to 650° C., copper oxide reduction in a gas like hydrogen, cooling to room temperature, light etching with a mild acid solution to help improve subsequent electroplating, and electrolytic copper pyrophosphate plating.
In another alternative embodiment, the method includes the following steps in order: electrolytic copper pyrophosphate plating, austenitizing, controlled cooling to 500° C. to 650° C., cooling to room temperature, reverse plating of copper oxide from the wire's surface (wire anodic), and electrolytic copper pyrophosphate plating. In this case, copper oxide plated from the surface of the wire is reused to apply copper to other unplated wire prior to austenization.
In yet another alternative embodiment, the method includes the following steps in order: electrolytic copper pyrophosphate plating, austenitizing, controlled cooling to 500° C. to 650° C., cooling to room temperature, acid pickling to remove copper oxide, water rinsing, and electrolytic copper pyrophosphate plating.
In still yet another alternative embodiment, the method includes the following steps in order: electrolytic copper pyrophosphate plating, austenitizing, controlled cooling to 500° C. to 650° C., cooling to room temperature, brush descaling or other mechanical methods to remove copper oxide, light acid etching to promote subsequent electroplating, water rinsing, and electrolytic copper pyrophosphate plating.
Prior to heat treating during the manufacture of steel tire cord, steel wire rod, which is generally 5.5 mm in diameter, is reduced to certain in-process sizes ranging from about 0.75 mm to 2.5 mm depending upon final product requirements generally using at least one dry drawing process. The as-received 5.5 mm rod is covered with iron oxides referred to as mill scale. Mill scale is intentionally left on the surface of the rod to help protect the steel from further oxidation (rusting) during shipment and to a lesser degree to help prevent handling damage like scratches on the surface of the wire rod.
In general, the operations to prepare an intermediate product for heat treating from 5.5 mm rod are as follows: rod payoff from coils, mill scale removal, drawing precoat application, forced air drying, drawing through tungsten carbide dies, take-up on spools or other in-process carriers for heat treating and plating as described previously. Some steel tire cord manufacturers use two dry-drawing steps, optionally with an intermediate heat-treating process.
Although a coating can be applied to wire or wire rod at any time in the process prior to heat treating, such as immediately before heat treating, it is preferred in this technology to apply electrolytic copper, as described, to protect the steel wire during heat-treating, to 5.5 mm rod subsequent to mill scale removal and prior to drawing precoat application. It is well recognized in the wire drawing industry that copper applied to the surface of steel prior to drawing greatly enhances the draw ability of the steel (improved amount of material processed before changing tungsten carbide die sets due to wear and increased drawing speeds). The preferred process steps for application of a coating like copper prior to heat treating is as follows: 5.5 mm steel rod payoff, mill scale removal, rinse in clean water, electrolytic copper coating, drawing precoat application, forced warm air drying, drawing through tungsten carbide dies, and take-up on spools or other in-process carriers for heat treating and plating as described previously.
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Although this technology has been illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples can perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the invention and are intended to be covered by the following claims.
