SMALLER GRAINS FORMED BY VANADIUM AND NITROGEN

Austenitic grain refinement in high-strength steels is achieved by hot rolling in the recrystallization-temperature region. The coarse austenitic grains found at the beginning of the rolling process are refined by repeated deformation and recrystallization during hot rolling. Although grain growth between these rolling passes would normally be expected, grain growth is retarded by the presence of the vanadium and nitrogen in the steel.

The accelerated cooling that takes place after the final rolling pass helps produce a very-fine ferritic structure after the austenitic grains transform to ferrite. This rapid cooling enhances ferrite nucleation and slows grain growth, providing the desirable balance of strength and toughness that is characteristic of high-strength, vanadium-nitrogen steels.

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 the grain boundaries produces a fine ferritic structure in the finished steel.

No Vanadium, 44 Parts per Million of Nitrogen

0.15% Vanadium, 227 Parts per Million of Nitrogen

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

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