Coating processes – Immersion or partial immersion – Molten metal or fused salt bath
Reexamination Certificate
1998-11-23
2001-01-23
Beck, Shrive (Department: 1762)
Coating processes
Immersion or partial immersion
Molten metal or fused salt bath
C427S431000, C427S321000, C427S434500
Reexamination Certificate
active
06177140
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to methods for galvannealing and galvanizing steel. More particularly, the present invention is directed to methods for continuous hot-dip galvannealing and galvanizing of steel employing a bath of molten zinc and aluminum.
BACKGROUND OF THE INVENTION
In continuous hot-dip galvanizing and galvannealing of steel strip, a bath of molten zinc is employed. Prior to entering the bath, the strip typically undergoes a heat treatment in a furnace. An end portion of the furnace that extends into the bath, called a snout, seals the furnace from the surrounding air. As the strip passes through the snout, the strip becomes immersed in the bath. Typically, two or more rolls are disposed in the molten bath. A sink roll reverses the travel direction of the strip in the bath, and a pair of stabilizing rolls in the bath stabilize and guide the strip through the coating knives.
In the production of galvanized and galvannealed products, aluminum is typically present in the molten zinc bath for controlling zinc-iron alloy growth. Interfacial zinc-iron alloy on galvanized steel is undesirable because it causes low adherence of the zinc coating to the strip. Typically, a relatively low aluminum content is used for galvannealing (e.g., 0.13-0.15wt. %), and a relatively high aluminum content is used for galvanizing (e.g., 0.16-0.2wt. %).
In some conventional processes, two baths are used in a production line in order to produce both galvanized and galvannealed steel. In those processes, one bath is needed to provide a relatively low aluminum content for galvannealing, and a second bath is needed to provide a relatively high aluminum content for galvanizing. However, two baths are disadvantageous because the line must be stopped in order to switch from one bath to the other bath. Also, two baths reduce scheduling flexibility for the production of galvannealed and galvanized steel. Further, a second bath is an extra equipment expense.
In conventional production lines which employ a single bath, the aluminum content is ramped up gradually between galvannealing and galvanizing. This can result in the production of low quality galvanized steel during the transition from galvannealing to galvanizing because, during the transition, the aluminum content may be too low for galvanizing. For example, products with critical surface quality requirements generally cannot be made during the transition, nor can vacuum degassed ultra low carbon steels, which are highly reactive, nor can high strength steels. Moreover, conventional methods generally have poor bath circulation, which results in relatively high variation in composition and temperature in the bath. Such poor circulation can exacerbate the problems encountered during the transition from galvannealing to galvanizing in conventional processes that employ a single bath.
In conventional hot-dip galvanizing processes, an undesirable intermetallic iron-zinc or iron-zinc-aluminum compound, called dross, can form. Dross pick-up on the rolls in the bath, and subsequent transfer to the surface of the strip where it produces pimples and print-through defects, is a major problem with galvanneal products and exposed galvanized products. Surface blemishes caused by dross particles are particularly visible when high gloss paint finishes are applied to the coated steel, which is common in the automotive and appliance industries. Use of cemented carbide-coated rolls in the bath reduces, but does not completely eliminate, these defects.
In addition to causing surface defects, dross formation can directly increase the cost of production. Zinc is one of the most expensive raw materials used in galvanized and galvannealed steel production. Because the weight of the dross generally averages about 8-10% of the zinc consumed during production, production costs are increased.
Conventional methods generally employ baths with high aluminum content for galvanizing and low aluminum content for galvannealing. The low aluminum content of the bath during galvannealing can lead to excessive dross formation and dross pick-up by the strip during galvannealing. Furthermore, accumulation of dross at the bottom of the bath can limit the length of a galvanneal production run and a transition to galvanizing may be required to remove the bottom dross through chemical conversion with a high aluminum addition. If the bottom dross build-up is very heavy, the production line may be shut down for mechanical dross removal.
The high aluminum content of the bath during galvanizing can lead to excessively high aluminum in the coating during galvanizing. High aluminum content for galvanizing is also detrimental to the transition from galvanizing to galvannealing as well as to the reverse transition, because several hours may be required to complete the transition from one aluminum content to another. The transition from galvannealing to galvanizing and vice versa is costly because the change in aluminum content in the bath causes poor quality products during the transition from galvannealing to galvanizing and vice versa. Thus, using conventional methods, it is difficult to make exposed quality coated steel products or vacuum degassed ultra low carbon steels or high strength steels using a single bath for both galvannealing and galvanizing. A reason for the poor surface quality during the transition is that the bottom dross is being converted to top or floating dross as the aluminum content increases during the transition to galvanizing resulting in dross pick-up by the strip.
Although aluminum generally is required in the bath to control iron-zinc alloy growth during galvanizing and galvannealing and to reduce the amount of dross, excess aluminum is not desirable. For instance, too much aluminum in the coating can decrease the spot weldability of the product.
A high temperature in the bath increases the solubility of iron in the bath, which ruins the contents of the bath by causing a formation of both top and bottom dross attributed to iron saturation. In a zinc bath that is saturated with iron, even a small change in the bath temperature causes a precipitation of dross compounds. Thus, it is advantageous to (a) lower the iron content in the zinc bath from a saturated state by using a low and constant galvanizing bath temperature and (b) maintain iron content close to the solubility limit, and thus minimize the precipitation of dross particles from the molten zinc. These particles are a combination of bottom dross (FeZn
7
) and top dross (Fe
2
Al
5
). These particles are discussed in greater detail in the publication by Kato et al., entitled
Dross Formation and Flow Phenomenon in Molten Zinc Bath
, Galvatech '95 conference proceedings, Chicago, 1995, pages 801-806. This publication is incorporated herein by reference as background material elaborating upon the types of dross particles that are formed in the environment in which the present invention operates.
If the strip is hotter than the bath when the strip is immersed in the bath, the bath can overheat, which causes increased dissolution of iron from the strip into the bath. The strip is hotter than the bath at the snout (i.e., near the point of immersion) unless the strip is sufficiently cooled following the heat treatment that occurs prior to immersion in the bath. In conventional processes, the temperature of the bath is relatively high (e.g., about 460° C.) to avoid freezing of zinc at the bath surface whether a single bath or two baths are employed for galvannealing and galvanizing. Use of a significantly cooler bath or baths, however, can cause zinc to freeze at the bath surface because of poor circulation in conventional baths and because the small difference between the strip immersion temperature and bath temperature.
Both high bath temperatures and dross formation can decrease roll life by increasing abrasion and erosion. Also, other components in the bath, such as bearings and sleeves, have decreased lives because of high bath temperatures and dross formation. The decreased li
Patil Ramchandra S.
Sippola Pertti
Barr Michael
Beck Shrive
Ispat Inland Inc.
Marshall O'Toole Gerstein Murray & Borun
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