Aging Properties of High Strength Cold Rolled Steels with Dual Phase Microstructure

Abstract

Aging propcrties of cold rolled high strength steels produced with WQ CAL process (continuous annealing line equipped with a water quenching device) were studied with regard to the factors characterizing these steels such as the second phase (martensite) and a considerable amount of solute C, N atoms in ferrite phase. The results are as follows;
1. The second phase suppresses the recovery of yield point elongation (YP-El). The extent of this suppression depends on the volume fraction and the hardness of the second phase.
2. Because of this suppression, no recovery of YP-El is observed until the aging advances to such high degree that the activation energy of aging, determined from the increase in yield stress (ΔYS) or YP-El, is 26 to 30 kcal/mol.
3. Following consecutive cycles of temper-rolling and subsequent aging (at 170°C for 20 min), discontinuous yielding repeats appearing and disappearing. The solutes C and N decrease distinctively during the first aging treatment and no appreciable change is detected in subsequent treatments.
On the basis of these results, the reason and the condition for the remarkable feature of these steels, as characterized by the large ΔYS after the BH-treatment (an aging treatment at 170°C for 20 min) and no YP-El after the room temperature aging (simulated by storage at 38°C for 30 days), are discussed. Discussion on the difference in the influence of carbon and nitrogen and the related aging mechanism is also made. Finally overall picture of the aging behaviour of those dual phase steels is proposed.