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SMALLER GRAINS FORMED BY VANADIUM AND NITROGEN

Austenitic-grain refinement in high-strength steels is achieved by hot rolling in the recrystallization temperature range. The coarse grains found at the beginning of the rolling process are refined by repeated deformation and recrystallization during hot rolling. Vanadium allows this multiple recrystallization to take place at normal rolling temperatures, yielding a very-fine austenitic-grain structure after the thermal-mechanical-controlled process (TMCP) has been completed.

Optimum properties are obtained when the fine-austenite grains are transformed to a very-fine ferritic structure. This fine ferrite is achieved by a proper combination of accelerated cooling after rolling as well as through alloy additions (like manganese) that lower the transformation temperature. Such a practice enhances ferrite nucleation and slows grain growth, providing the desirable balance of strength and toughness that is characteristic of high-strength, vanadium-nitrogen steels.

Data Sheet (PDF): How Heating and Cooling Practices Affect
the Properties of Flat-Rolled Microalloyed Steel

Intergranular Ferrite Nucleation in Heavy Sections

In heavy sections where accelerated cooling cannot be achieved, the vanadium-nitride precipitates produced during deformation promote the nucleation of ferrite grains within the grain boundaries. The combined nucleating effect within the grains and grain boundaries produces the desirable fine-ferritic structure in the finished steel.

The smallest grains are found in the vanadium-nitrogen steel at the bottom. Therefore, this steel has the best combination of strength and toughness.