Alloys or metallic compositions – Containing over 50 per cent metal but no base metal – Iron containing
Utility Patent
1997-08-11
2001-01-02
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Containing over 50 per cent metal but no base metal
Iron containing
C420S583000, C420S586000, C420S585000
Utility Patent
active
06168757
ABSTRACT:
BACKGROUND OF THE INVENTION
Galvanized steel utilized in the automotive, construction and appliance industries is formed in very thin strips (0.015 to 0.060 inch thick), which is passed through a molten bath of either zinc (galvanizing), aluminum (aluminizing), or aluminum/zinc (galvanneal, galfan, galvalume, etc.), in which the levels of aluminum vary from a fraction of a percent to as much as 100 percent.
The “hot dip” metalizing coating process requires equipment that runs submerged in the molten metal. The molten metal temperature usually varies from as low as 820° F. to as high as 1300° F.
A heated metal pot contains a bath of molten zinc/aluminum. A continuous moving strip of low carbon steel is introduced into the bath from a furnace in the conventional manner. The strip passes around a sink roll and tensor rolls while submerged in the bath, so the surface of the strip picks up a zinc/aluminum coating. The strip is delivered to the bath through a conventional tubular snout. The interior of the snout housing contains an inert gas such as nitrogen or a mix of nitrogen and hydrogen. This procedure, as is well known to those skilled in the art, is useful in preventing the steel strip from oxidizing.
Because of the extremely large dimensions of the equipment and in spite of efforts to prevent all possible air leaks into the furnace, small leaks do occur, generating ferrous oxides (Fe
2
O
3
FeO, etc.) When the steel strip enters the bath, a chemical process occurs in which the melt in the bath reacts with the iron in the steel strip (inducing the coating) but also reacts with the oxides to form dross that contains ZnFe, ZnAlFe, ZnFe+Al
2
O
3
, etc. The free iron settles to the bottom of the molten metal pot. Because of the slightly or nearly identical density to the molten metal, the oxides (Al
2
O
3
, ZnO) and the intermetallics formed (ZnFe, ZnAlFe, etc.) remain in suspension or float to the surface in the form of dross. The dross increases its concentration by being nearly entrapped in the zone comprised by the snout, the strip, the sink roll and the tensor rolls, where it gradually forms deposits on top of the sink roll and the strip being processed.
Outline of Requirements for Galvanizing Equipment
Standard rolls and equipment used in the hot dip metalizing process, when the alloy melt is zinc or zinc/aluminum with aluminum concentrations of less than 60%, are 316-L stainless steel. The rolls and bearings, in particular, require continuous maintenance of their surfaces. The rolls are removed weekly from the pot and their surfaces machined to remove accumulated dross, to smooth the roll surfaces as well as to return them to a round and straight condition. The main reason for this continuous maintenance is because 316-L stainless steel is not a material formulated specifically for this application and, consequently, it lacks the properties to meet the operational needs.
In order of importance, although all requirement must be met to a minimum degree, the properties required for a proper roll material that meets the operational needs are as follows:
1. Very low solubility in molten zinc or zinc/aluminum alloys. In other words, 0=S<4×10
−5
in/hr Where S=the amount of roll radial loss due to molten metal dissolution.
2. Low adhesion (non-wettable) to zinc/iron and zinc/iron/aluminum dross. Wetting plays the main role in the bonding of solid-liquid state metals.
3. High surface hardness (R
c
larger than 40). Abrasive wear contributes nearly half of the loss of roll life in metalizing applications.
4. Dimensional stability at operating temperatures up to 1300° F., for straightness and roundness. This property is necessary because of the difficulties encountered when the lines operate at over 100 RPM, generating excessive vibration and damage to the holding equipment.
5. Thermal shock resistance. The roll should be capable of withstanding a thermal shock of no less than 500° F. when going from air to the molten metal, and 1300° F. when going from the molten metal to air.
6. Good impact and notch resistance strength. This is importance due to the severity of the application.
7. Centrifugally castable and machinable by standard procedures in order to provide simple and available maintenance.
8. Economic viability.
The following expands, in corresponding order, each of the material properties required to meet the listed operational needs.
Evaluation of Specification Requirements
In order to obtain a material formulation that is capable of having a dissolution rate of
O=S<4×10
−5
inches/hour
It is important to understand the interaction of dissimilar metals in solid-liquid states. The joining of dissimilar metals in a solid-liquid state is governed by their physico-chemical properties and by the interaction between them; or, in the case of more complex systems, such as super alloys, by their interaction with all other alloying elements and impurities. When the melting point of the corrosive metal (the coating alloy in our case) is much lower than that of the metal being attacked (the roll material), the roll material may remain in a solid state throughout the process. In this case, a strong metallic bond between the atoms of the coating metal and the roll material occurs in the wetting process. It is true, however, that other associated processes can significantly influence the attack rate and kinetics of solubility, i.e., dissolution, interdiffusion and formation of intermetallics that have a significant effect on the bonding properties of the intermetallic layers being formed.
Experimental as well as theoretical findings have shown that the attack on a solid metal by zinc and zinc/aluminum alloys is a topochemical reaction in which a two-stage formation of strong bonds between atoms of the two materials is a characteristic feature.
In the first stage, a physical contact is established by the close proximity of the two metals allowing interaction between the atoms. The electrostatic interaction between the surface atoms is of great importance at this stage.
In the second stage, the chemical interaction takes place and the formation of a strong bond is completed. At this stage, quantum processes between the electrons prevail. Thus, the occurrence of electron interaction of different types of materials requires a definite quantity of energy for surface activation. This energy, in the case of “hot dip” metalizing is imparted in the form of heat retained in the molten metal that is maintained at temperatures well above their melting temperature in order to improve the coating capability of the melt alloy in accelerated production. In other words, the lower the temperature of the melt in the pot, the slower the two basic stages of alloying formation.
During galvanizing and aluminizing, both stages as well as the subsequent diffusion take place so fast that it is difficult to join zinc/aluminum to steel without the formation of brittle intermetallic layers at the contact zone. Zinc/aluminum alloys are so active that adhesion and diffusion into steel is achieved even in the presence of a passive film of iron oxides, as long as the oxide layer is no thicker than 100 Å (see FIG.
1
).
In order to improve the resistance of ferrous alloys to molten aluminum, it is necessary to study the dissolution process that follows wetting in detail. The dissolution of solid ferrous alloys into molten aluminum has been studied by M. Kosaka and S. Minowa (Transactions Iron & Steel Institute of Japan, Vol. 50 and 52, 1964.) It is also theoretically described by Nernst-Shchukarev's equation
dc/dt=K
s
A/V.(C
s
−C
i
) (1)
where C
i
=the instantaneous concentration of the dissolved metal in the melt (weight percent)
C
s
=the saturation concentration at operational temperature (weight percent)
K
s
=the dissolution rate constant (or mass transfer coefficient)
A=the surface exposed to the Zn/Al melt
V=the volume of the melt
From this equation and assuming the dissolution of the solid m
Alphatech, Inc.
Chandler Charles W.
Yee Deborah
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