Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1998-09-15
2001-05-15
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S246000, C427S343000, C427S406000
Reexamination Certificate
active
06231686
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to treating a metal object having a zinc layer and, in particular, to treating galvanized steel strip to improve its stampability.
BACKGROUND OF THE INVENTION
Metals are often coated to prevent corrosion. Corrosion of steel strips may be inhibited by applying an outer layer of zinc to the strips. The zinc layer is applied by a hot dip galvanizing or electrogalvanizing process, for example. After the zinc layer has been applied to the steel strip, the strip is subjected to a forming process such as stamping. In preparation for the forming process, the strip may be coated with a lubricant such as oil.
Lubricant may be applied prior to or during different stages of the forming process and may be applied by any number of techniques known in the art including electrostatically, by gravity and by “squeegee.” One example of a lubricant that is applied to the steel is referred to as mill oil. Mill oil, which is primarily a rust preventative, is applied at the galvanizing facility to the uncoated galvanized steel prior to coiling. Another lubricant referred to as prelube may be applied by the galvanizing facility to the uncoated galvanized steel. Prelube provides both rust prevention and lubricity to the galvanized steel and is used when more difficult forming is to be carried out. At a stamping plant the steel may be subjected to blanking and the mill oil may be washed off by a blank washer. At the blank washer another lubricant may be applied to the uncoated product. Some stamping lines may stamp the product without applying any lubricant other than the mill oil. Any of the above lubricants may be present on the strip as the strip enters the stamping line and a drawing lubricant may be applied at a press.
During stamping, the steel strip is placed between an upper punch and a lower die of a press where it is held in place and formed into a desired shape. Articles such as automobile hoods and fenders may be formed in this manner. After forming, the article is passed through a bath containing an alkaline cleanser for removing the oil. The article may then be assembled, coated with a phosphate painting preparation and painted.
One factor that is important to the stamping process is the formability of the steel. A metal draw bead may be located adjacent to the die opening for retarding travel of the strip or blank as it is pushed into the die. The amount of frictional resistance at the interfaces between the punch, die and blank is an important factor in the quality of the formed article. Areas of both low and high strip-to-draw-bead speeds may occur during forming. It is desirable for the blank to exhibit a coefficient of friction with as little variation across low and high strip-to-draw bead speeds as possible. The high speed areas may result in a “hydroplaning” effect at a low frictional resistance whereas the low speed areas may suffer from slip stick. Slip stick is a phenomenon that occurs when two metals contact each other at slow relative speeds and high frictional resistance. In effect, a repeated cold welding and breaking-free of the metals occurs during slip stick. Notwithstanding proper die set up and punch operation, if there is excessive frictional resistance during forming, portions of the strip may tear or break, resulting in decreased quality or scrapping of the steel products.
SUMMARY OF THE INVENTION
The present invention is directed to a method of treating a metal object having an outer layer comprised of zinc and, in particular, to treating a galvanized steel strip. Although it is preferred that the object be a strip, any metal object having a coating of zinc that may benefit from improved lubricity may be treated in accordance with the present invention. In its broad aspects, the method includes the step of applying a treating material to the object. The treating material is reacted with the zinc outer layer to form a reaction layer on the outer layer. The reaction layer is comprised of a zinc reaction product, preferably a zinc carbonate compound, even more preferably, a compound known as basic zinc carbonate. The reaction layer is preferably in crystalline form. The reaction layer is present in an amount effective to increase a lubricity of the object, such as during the forming process. A protective coating such as a rust preventative or a lubricant (e.g., oil) is preferably applied to the object. During stamping, the objects exhibit increased lubricity compared to objects having a protective coating but not the reaction layer. This avoids problems such as tearing of the objects during stamping.
A preferred aspect of the invention is directed to using a treating material in the form of a solution comprising a bicarbonate compound. In particular, the solution may be formed by combining a solvent and a bicarbonate or a salt thereof, for example an alkali metal bicarbonate compound. The alkali metal is selected from the group consisting of sodium, potassium and lithium. The treating material may be free of an oxidizing agent, but an oxidizing agent may be used to achieve consistent coverage uniformity, particularly when the metal objects have been oiled and left for an extended period of time.
The time of reaction, temperature of the treating solution and concentration of reactants, are variables that may be adjusted empirically to produce the desired amount of the reaction layer. The treating material is at a temperature not greater than about 100° C. when applied to the object, and may advantageously be applied at about room temperature. The treating solution may be applied by immersing the objects in the solution, spraying, using rollers, or flowing the solution onto the objects. The zinc carbonate compound may also be formed by applying a gaseous treating material to the zinc outer layer. One such suitable treating material comprises carbon dioxide and acetic acid gases.
The treatment time may vary depending upon the intended amount of the reaction layer as well as the method of application. It is preferable to treat metal objects, e.g., steel strips, that have zinc freshly plated on them, since oxidation of the zinc outer layer may inhibit application of the treating material. To this end, in all methods of application it is preferable to treat the objects within 24 hours from when the zinc outer layer has been applied by galvanizing. It is more preferable to treat the objects within about 5 minutes, even more preferably, within about 1 minute, after the zinc outer layer has been applied. Each object is then formed into a shaped article at a reduced frictional resistance compared to an object with a protective coating but not the reaction layer. A ratio of change in coefficient of friction to change in draw bead pulling speed is not greater than about 2.9×10
−4
and, more preferably, not greater than 5×10
−5
, at pulling speeds of 1 and 200 inches/minute. The protective coating, e.g., lubricant, and the reaction layer are then removed from the object after forming. The reaction layer may be removed from the object using an acidic phosphate solution. The article may later be painted.
Another preferred embodiment is directed to a method including the steps of applying to the strip the treating solution comprising the bicarbonate compound. The bicarbonate and the zinc of the outer layer react to form the reaction layer on the outer layer. Lubricant is applied to the strip. The strip is formed into an article of a desired shape by a forming process. The reaction layer is present in an amount effective to increase a lubricity of the strip during the forming process. The treating solution may further comprise an oxidizing agent selected from the group consisting of a peroxide, persulfate and percarbonate compound. A phosphate solution is applied to the strip after the forming process for preparing the strip for painting. The reaction layer may be removed from the strip with the phosphate solution.
The invention is also directed to a metal object having improved formability comprising a body
LTV Steel Company, Inc.
Oltmans Andrew L.
Sheehan John
Watts Hoffman Fisher & Heinke
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