Niobium - Tantalum - Boron - Calcium

Niobium (Columbium).
Niobium is also important in microalloyed (HSLA) steels for its precipitation strengthening through the formation of niobium carbonitrides. Some microalloyed
steels employ both vanadium and niobium. Because of its affinity for both carbon and
nitrogen, niobium is an element found in some IF steels. Niobium is also added as a carbide stabilizer (prevents carbides from dissolving and re-forming in undesirable locations) in some austenitic stainless steels (AISI type 347, 348, and 384), ferritic stainless steels (AISI type 436 and 444), and precipitation hardening stainless steels (AISI type 630).

Because of its affinity for carbon, tantalum, like niobium, is added as a carbide
stabilizer to some austenitic stainless steels (AISI type 347 and 348).

On a weight percent basis, boron is the most powerful hardenability element in steel. A minute quantity of boron, e.g., 0.003%, is sufficient to provide ample hardenability in a low-alloy steel. However, boron is a strong nitride former and can only achieve its hardenability capability if in elemental form. Thus, boron must be protected from forming nitrides by adding a sufficient amount of titanium to first combine with the nitrogen in the steel.

Calcium is a strong deoxidizer in steel but is not used for that purpose. In an
aluminum-deoxidized (killed) steel, calcium combines with sulfur to form calcium sulfide particles. This form of sulfide remains as spherical particles as compared with manganese sulfide, which is soft and elongates into stringers upon hot rolling. Thus, steels properly treated with calcium do not have the characteristics associated with MnS stringers, i.e., property directionality or anisotropy.

Bruce L. Bramfitt
International Steel Group, Inc.
Research Laboratories
Bethlehem, Pennsylvania

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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