Recovery during annealing in a cold rolled low carbon steel. Part I: Kinetics and microstructural characterization

Sensitivity of Conventional and Non-destructive Characterization Techniques to Recovery and Recrystallization

“…As theoretical considerations demonstrate that the coercive field, H c , is proportional to the square root of the dislocation density, r this parameter has been the most used among several magnetic parameters to monitor recovery in different materials. [1][2][3][11][12][13] During recovery, the grain structure remains constant and microstructural changes only occur in the cold rolling dislocation substructure inside the grains. The reduction in the dislocation density during recovery decreases the density of pinning sites for the motion of magnetic domain walls in the matrix, which is normally reflected as a decrease in H c values from the cold rolled state.…”

“…The reduction in the dislocation density during recovery decreases the density of pinning sites for the motion of magnetic domain walls in the matrix, which is normally reflected as a decrease in H c values from the cold rolled state. [1][2][3][11][12][13] As a fast, although minimally invasive technique, hardness measurements have been shown not to have enough sensitivity to characterize the recovery processes in a heavily cold rolled low carbon steel 3) and in an interstitial free (IF) steel to have small resolution at temperatures slightly below the onset of recrystallization, but to be absolutely insensitive at lower temperatures. 12) Recrystallization produces microstructure regeneration through the nucleation of substructure free volumes and by the recrystallization front migration through the material which results in a new grain microstructure producing the mechanical 14) and magnetic softening of steel.…”

“…Quantification of dislocation densities in materials can be achieved through transmission electron microscopy (TEM). However, as heavily cold rolled steels develop a highly complex substructure, such evaluation cannot be accurately achieved and TEM is replaced by indirect methods like magnetic techniques, [1][2][3] X-ray line broadening, 4) X-ray peak resolution, 5) tensile stress 6,7) or Vickers hardness measurements. 8,9) Among these techniques, magnetic measurements have the advantage of being non-destructive in nature.…”

“…16) However, in spite of the importance that NDT techniques have from an industrial point of view this subject has received little attention until recently. 3,12,[17][18][19] Recrystallized fraction is generally determined in the laboratory by metallographic observations, which are time consuming and often rather subjective, or by hardness measurements. Although hardness has provided good estimations of the recrystallized fractions in some extra low carbon (ELC) steels, 3,20,21) in others (including IF steels) the mechanical softening significantly deviated from the recrystallized fraction.…”

“…3,12,[17][18][19] Recrystallized fraction is generally determined in the laboratory by metallographic observations, which are time consuming and often rather subjective, or by hardness measurements. Although hardness has provided good estimations of the recrystallized fractions in some extra low carbon (ELC) steels, 3,20,21) in others (including IF steels) the mechanical softening significantly deviated from the recrystallized fraction. 12,20,22,23) Higher softening fractions than recrystallized fractions are generally attributed to recovery concurrent with recrystallization.…”