The effect of chemistry on the formation of ultrafine ferrite in steel

A thermomechanical process has been developed to produce ultrafine (1-2 mu m) ferrite grains in hot rolled steel strip. This process involves rolling the steel at or just above the austenite to ferrite transformation temperat ure, and was applied to a variety of steel chemistries, including plain low and high carbon grades and steels containing microalloying elements such a s Nb, Ti, B and Mo. Significant volume fractions of ultrafine grains were o bserved in all samples. However, it was found that the chemical composition of the steel slightly influenced the morphology and volume fraction of ult rafine grains formed in the surface layers of the strip, and significantly altered the microstructure formed in the core of the strip. In plain carbon grades, the level of ferrite refinement increased slightly as carbon conte nt increased. In the case of the microalloyed steels, those containing Nb a nd Ti additions produced the greatest level of refinement. The volume fract ion and level of refinement of the ultrafine ferrite grains also influenced the strengths of the steels.