Application of Coercive Field Measurements to the Evaluation of Recovery and Recrystallization in Cold Rolled Interstitial Free (IF) Steel

Abstract

The recovery and recrystallization kinetics of a cold rolled interstitial free (IF) steel were studied during isothermal annealing. Magnetic methods based on coercive field measurements, hardness tests and metallography were applied so as to follow the kinetics experimentally. The coercive field measurement technique reveals a higher degree of resolution for monitoring recovery than conventional hardness determination and also allows the recrystallization progress to be monitored. The results obtained are compared to those previously determined for a non-stabilized extra low carbon (ELC) steel. The observed differences are discussed in terms of the presence of microalloying elements, Ti and Nb, which slow down recovery and delay recrystallization.

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

“…This type of interdependence can be directly related to the decrease of the dislocation density due to recovery and has previously been applied to other steels. 1,5,6,11,12) The values of the a and b constants determined by least square fitting of the experimental data to Eq. (5) are plotted in Fig.…”

“…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.…”

“…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. Moreover, NDT methods are applied at the plants as microstructurally blind quality control techniques and some information related to microstructure implicit in the signals is lost.…”

“…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.…”

“…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.…”