High strength Nb microalloyed hot rolled steel coils (cut to length plates) for structural applications

Hot rolled strip products are generally used for direct processing for various applications in structural and engineering industries, wherein there is strong demand for steel with high strength levels, balanced formability and good impact toughness. This combination of steel properties enables the construction and engineering industries to meet the stringent demands of the heavy structures and hence has direct impact on the total life of the structures. To meet these requirements, at Essar Steel the development of hot rolled high strength steel grades is in principle focused on two different concepts: on one hand the high strength micro-alloyed and thermo-mechanically rolled steel grades and on other hand the transformation hardened (ferritic bainitic, multiphase) steel grades. The present paper focus on the newly developed very high strength Nb micro-alloyed steel grade with minimum yield strength of 600MPa and 650MPa coupled with good impact toughness at sub-zero temperatures. Some aspects of the alloy design and the industrial production process for these steel grades are also highlighted. The specific mechanical and mechanic-technological properties of these high strength steel grades are derived from the complex micro-structure consisting of ferrite, irregular ferrite, bainite, martensite and micro-alloying constituents. (AU)