Steels for Porcelain Enameling

There are a number of flat-rolled carbon steel products available today for porcelain enameling. The general categories are:

• Low-carbon enameling steels (enameling iron replacements)

• Decarburized steels

• Interstitial free steels

• Titanium-stabilized steels

• Common cold-rolled steels

• Common hot-rolled steels

Within each of these categories are products differing in chemical composition and/or method of production.

Aluminum killing and the steel industry's movement from ingot casting to continuous casting have added new sheet steel products for the porcelain enamel industry. At the same time, these industry changes have resulted in the gradual withdrawal of some long-time enameling-grade products from the market.

Low-Carbon Enameling Steels (Enameling Iron Replacements). For many years, enameling iron, a rimmed-ingot poured product with a low metalloid content, was used extensively for porcelain enameling. The enameling characteristics were excellent, with the caveat that the product required a separate ground coat before applying a cover coat. Because enameling iron is a rimmed steel, it cannot be continuously cast. The steel industry's movement to continuous casting has resulted in the recent withdrawal of the product from the market.

A number of highly serviceable enameling iron replacement products are now available for porcelain enameling. These aluminum killed steels have low carbon (0.02 to 0.04 wt%) and low manganese (0.15 to 0.30 wt%), along with 0.015 wt% P max, 0.015 wt% S max, 0.015 wt% Si max, and 0.03 to 0.07 wt% Al. Some contain a boron addition (0.006 wt% max) to aid in grain size control and to help improve resistance to enamel fishscale. The yield strength is 172 to 221 MPa (25 to 32 ksi), the tensile strength is 303 to 338 MPa (44 to 49 ksi), and the elongation is 38 to 46%.

Unlike the old enameling iron, these products do not require normalizing; they are either box annealed or continuously annealed. These processing changes result in steels that are more formable than their predecessors. Experience to date indicates that the enameling characteristics are similar to those of enameling iron, and these steels are being used in the same applications. They perform well with both ground-coat and two-coat/two-fire systems.

Decarburized Steels. The manufacture of fully decarburized sheet for direct-on cover coat enameling became practical following the development of the open coil annealing process. Different melting and teeming practices may be used to produce decarburized sheets: ladle aluminum killed-ingot poured, and ladle aluminum killed-continuously cast. These aluminum killed products are not subject to the return of yield point elongation following temper rolling. As killed steels, the hot coiling off the hot strip mill eliminates the possibility of having deep drawability as measured by the R value (plastic strain ratio), but in stretch and plane strain conditions, the decarburized products are excellent performers.

Enameling characteristics for decarburized steels are excellent, and these steels are being used in all types of today's enamel systems. The low carbon content eliminates primary boiling and consequent defects such as black specks, pullthrough, and dimples caused by the evolution of carbon monoxide and carbon dioxide through the porcelain enamel coating during firing.

These steels have excellent resistance to warpage. They also exhibit good resistance to defects such as "fishscale" and ground-coat reboiling during firing of the cover coat, both of which are caused by the evolution of hydrogen gas. If overpickled using the acid etch/nickel deposition metal preparation method, ladle killed steels will tend to have a "gassy" enamel surface, and enamel adherence is likely to be substandard. Ladle killing with aluminum and continuously casting is becoming the predominant way to produce these steels. It is expected that all decarburized steels will soon be manufactured by this method.

Interstitial-free steels are products in which all of the carbon and nitrogen contained in them are combined with an alloying element. Titanium or niobium (columbium), or a combination of titanium and niobium additions, are used to fully stabilize the steel. Domestically produced interstitial-free steels contain from <0.008 to 0.020 wt% C, 0.15 to 0.25 wt% Mn, 0.06 to 0.15 wt% Ti, and 0 to 0.10 wt% Nb. Vacuum degassing is used to produce the very-low-carbon versions, whereas the higher-carbon version is controlled by the basic oxygen shop. The yield strength is 138 to 165 MPa (20 to 24 ksi), the tensile strength is 290 to 317 MPa (42 to 46 ksi), and the elongation is 41 to 45%.

These steels have superior formability, particularly deep drawability as measured by the R value, which is in the range of 1.8 to 2.0. This permits extremely deep draws to be made successfully.

These steels exhibit excellent strength retention after strain and firing, as well as good base metal sag properties. They do not exhibit stretcher strain and are nonaging and nonfluting. Because titanium can effect the development of ground-coat adherence, enamel systems should be optimized to ensure satisfactory adherence with these steels.

There are many specific applications in the appliance and plumbingware sector where these steels have proved to be valuable, especially where deep drawing is required. Also, these products can be produced by either ingot pouring or continuous casting, thereby ensuring their future availability.

Titanium-stabilized steels are a variation of interstitial free steels. These steels are popular for large panels where maximum flatness can be achieved because of the product's resistance to warpage. Not being vacuum degassed, these steels are somewhat higher in carbon content than interstitial free steels.

This carbon is stabilized as titanium carbide by the addition of titanium. Stabilization provides carbon boil resistance, a high after-fire strength, and excellent deep drawability as measured by the R value. The higher titanium carbide content increases yield strength and reduces overall ductility. These steels offer high-temperature strength and resulting warp resistance, and they are now being produced as continuous-cast products.

Common cold-rolled steels have been used for many years in the porcelain enamel industry. They have been typically rimmed-ingot poured steels, but they are now being produced as continuous-cast products. They typically contain 0.04 to 0.07 wt% C and 0.25 to 0.040 wt% Mn. For cold-rolled drawing-quality steels, yield strength is 193 to 220 MPa (28 to 32 ksi), tensile strength is 295 to 330 MPa (43 to 48 ksi), and elongation is 40 to 44%. Enameling of cold-rolled steels should be restricted to ground coats and to noncritical appearance components.

Cold-rolled steels may also be used for two-coat enameling, where appearance is less important than in such components as range tops and other exterior parts for major appliances. Because of the high tendency of cold-rolled steel to warp and to carbon boil during porcelain enameling, the use of this material is generally restricted to components for which appreciable distortion during firing can be tolerated, applications using a low enamel firing temperature, or parts where a greater metal thickness is provided to minimize warpage. Most authorities recommend that parts fabricated from cold-rolled steels be porcelain enameled on one side only to minimize fishscale defects.

Common Hot-Rolled Steels. Hot-rolled sheets have a high susceptibility to fishscaling when porcelain enameled, especially if both sides of the steel are coated. When the use of hot-rolled steels is necessitated by the thickness and/or strength requirements for a given application, such as for water heater tanks, porcelain enameling should be limited to only one side of the sheet. This permits hydrogen gas, which produces the fishscaling, to escape from the unenameled side of the sheet.

Steel plate, tubes, pipes, and rolled sections may be porcelain enameled for specific applications. Examples include low-alloy steel pipe and tubing that is fabricated and porcelain enameled (i.e., glass-lined) for chemical pressure vessels and for sewage treatment plant piping systems, and low-carbon hot-rolled steel sections that are enameled for use as architectural components. The sheet should be thick enough so that sagging is reduced to an acceptable level.

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