Role of Nb on Microstructural Evolution during Intercritical Annealing in Low-carbon Steels

Abstract

We investigated the microstructural evolution during intercritical annealing in Nb-added low-carbon steels, focusing on the synergistic effects of the addition of Nb and the ferrite (α) to austenite (γ) phase transformation on the recrystallization behavior of α. Two kinds of specimens, containing 0.02 and 0.05 mass% Nb, were prepared and annealed at the intercritical temperature (750°C) for a long time. The progress of recovery and α recrystallization was retarded by increasing the amount of Nb addition during intercritical annealing. Moreover, the progress of α recrystallization during intercritical annealing was mainly attributed to continuous recrystallization due to subgrain growth. The fraction of γ that formed during intercritical annealing increased owing to Nb addition, but it did not increase with increasing amount of Nb addition. These results suggest that the progress of recovery and α recrystallization was retarded during intercritical annealing by the addition of Nb, thereby causing the increase in γ fraction. Furthermore, the increase in γ fraction led to further retardation of α recrystallization and α refinement with the addition of Nb.

Effect of Initial Microstructures on Austenite Formation Behavior during Intercritical Annealing in Low-Carbon Steel

“…11) In addition, the recrystallization of ferrite was completed when the heating rate was slow, while non-recrystallized ferrite grains were observed when the heating rate was rapid. 4) We have reported that the number of austenite nucleation sites increases with the fraction of non-recrystallized ferrite grains when the annealing temperature reaches the Ac 1 temperature. 4) Therefore, it is likely that austenite growth tends to proceed under local equilibrium when the number of austenite nucleation sites is small, whereas austenite growth tends to proceed under paraequilibrium when the number of austenite nucleation sites is large.…”

Research Progress of Desulfurization Refining Slag: A Review

“…Furthermore, several metallurgical researchers have conducted laboratory studies and successfully applied their developed refining slag systems in industrial production, achieving cost savings and obtaining favorable metallurgical results. [47][48][49][50]…”

Advances in research on deformation and recrystallization for the development of high-functional steels

“…This resulted in a relatively coarse microstructure consisting of elongated α and γ (αʹ) structures (Figure 11(a,b,d,e,g,h)). When 0.02% and 0.05%Nb is added to such a steel, three phenomena occur simultaneously: α recrystallization, α→γ reverse phase transformation, and NbC precipitation [30]. First, the formation of NbC clusters occurs during heating, which significantly retards the recovery and recrystallization of α. Consequently, α→γ reverse phase transformations occur at the unrecrystallized α subgrain boundaries, which leads to the formation of a very fine reversely transformed γ.…”