Special Low-Carbon Steels

These are the steels that are not classified in the aforementioned SAE table or listed in the aforementioned ASM table. As mentioned earlier, carbon is not always beneficial in steels. These are special steels with carbon contents below the lower level of the SAE/AISI 10xx steels. There are a number of steels that are produced with very low carbon levels (less than 0.002% C), and all the remaining free carbon in the steel is tied up as carbides. These steels are known as IF steels, which means that the interstitial elements of carbon and nitrogen are no longer present in elemental form in the iron lattice but are combined with elements such as titanium or niobium as carbides and nitrides (carbonitrides). Interstitial-free steels are required for exceptional formability, especially in applications requiring deep drawability. 

Drawability is a property that allows the steel to be uniformly stretched (or drawn) in thickness in a closed die without localized thinning and necking (cracking or breaking). An example of a deep-drawn part would be a compressor housing for a refrigerator. With proper heat treatment, IF steels develop a preferred crystallographic orientation that favors a high plastic anisotropy ratio or r value. High r-value steels have excellent deep-drawing ability and these steels can form difficult parts. Another type of low-carbon steel is a special class called DQSK steel. This type of aluminum-treated steel also has a preferred orientation and

high r value. The preferred orientation is produced by hot rolling the steel on a hot strip mill followed by rapid cooling. The rapid cooling keeps the aluminum and interstitial elements from forming aluminum nitride particles (i.e., the Al and N atoms are in solid solution in the iron lattice). After rolling, the steel is annealed to allow aluminum nitride to precipitate.

The aluminum nitride plays an important role in the development of the optimum crystallographic texture. The DQSK steel is used in deep-drawing applications that are not as demanding as those requiring IF steel.

A new family of steels called bake-hardening steels also have a low, but controlled carbon content. These steels gain strength during the paint–bake cycle of automotive production. 

Controlled amounts of both carbon and nitrogen combine with carbonitride-forming elements such as titanium and niobium during the baking cycle (generally 175 C for 30 min). The precipitation of these carbonitrides during the paint–bake cycle strengthens the steel by a process called aging.

Enameling steel is produced with as little carbon as possible because during the enameling process, carbon in the form of carbides can react with the frit (the particles of glasslike material that melts to produce the enamel coating) to cause defects in the coating. 

Thus, steels to be used for enameling are generally decarburized in a special reducing atmosphere during batch annealing. In this process, the carbon dissipates from the steel. After decarburization, the sheet steel is essentially pure iron. Enamel coatings are used for many household appliances such as washers and dryers, stovetops, ovens, and refrigerators. Also, steel tanks in most hot-water heaters have a glass (or enameled) inside coating. 

Electrical steels and motor lamination steels are also produced with as low a carbon content as possible. Dissolved carbon and carbides in these steels are avoided because the magnetic properties are degraded. The carbides, if present in the steel, inhibit the movement of the magnetic domains and lower the electrical efficiency. These steels are used in applications employing alternating current (AC) in transformers and electric motors. Most electric motors for appliances and other applications have sheet steel stacked in layers (called laminations) that are wound in copper wire. Electrical steels used for transformers contain silicon, which is added to enhance the development of a specific crystallographic orientation that favors electrical efficiency.

READ MORE.......

Bruce L. Bramfitt

International Steel Group, Inc.

Research Laboratories

Bethlehem, Pennsylvania

Reprinted from Handbook of Materials Selection, Wiley, New York, 2002, by permission of the publisher.

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz

Copyright  2006 by John Wiley & Sons, Inc.

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