Nb-microalloyed fire resistant constructional steels

Fire-resistant constructional steels have been commercialized in some parts of the world, and are being examined in the USA. Current activities are focused on development of specifications for testing of elevated temperature properties, and it is envisioned that some material specifications and initially niche applications (e.g. high-rise building columns, structures where friable insulated coatings are undesirable, etc) will follow. Selected metallurgical studies are reviewed, with a focus on Nb-containing steels that are intended to help understand the microstructure/property relationships that control fire-resistant (FR) properties. Specific examples are cited which illustrate the apparent benefit of Mo in suppressing precipitate coarsening rates at elevated temperature, beneficial effects of microstructure refinement, microalloy precipitation, and warm working of ferrite on the FR properties. The concept of “active” fire resistance is illustrated (confirmed with both Cu and Nb containing steels thus far), whereby alloying and processing are designed to allow strengthening precipitates to form in the microstructure as a consequence of the heating encountered during a fire. Some recent ASTM specification-related activities also reviewed briefly. (AU)