Experimentelle Studie zur Querrissbildung in mikrolegierten Stählen
Metallurgische, thermische und thermomechanische Behandlung von Werkstoffen
Zusammenfassung der Projektergebnisse
Steels with different Ti and fixed Nb content (A0, A1, A2, A3) were investigated under various process parameters simulating similar conditions during continuous casting. A detailed ductility map could be been drawn for each steel for varied temperatures, strain rates and reheating conditions. No significant effect of change in the reheating rate on the hot ductility was detected at 0.001 s-1. Small differences were measured only at a strain rate of 0.01 s-1. Increasing the strain rate (from 0.001 s-1 towards 0.01 s-1) is likely to improve the hot ductility for each case particularly in the range of ductility trough. For each steel grade, the ferrite formation at the low temperature end of hot ductility (Ar 3) was the main factor causing micro-cracks. The ferrite phase forms even above Ar3 at the strain rate of 0.001 s-1. The formed ferrite between Ar3 to Ae3 was defined as deformation induced ferrite (DIF) which broadens the low ductility plateau. Above Ae3 (~800°C), intergranular micro-crack formation was induced due to the limited DRX and grain boundary sliding. Moreover, brittle micro-regions form as a result of the dispersion of Nb(CN) and TiNb(CN) on GBs with sizes below 1 µmØ. Ti-containing steels show remarkably better hot ductility values than A0 at 900 °C with low strain rates 0.001 s-1 due to the predominance of relative coarser TiNb(CN)s. At the high strain rate (0.01 s-1), a stochiometric value of Ti/N~ 3 at 800 °C and 850 °C can be useful to minimize the possibility of the crack formation under fast cooling rate conditions (600 Kmin-1). Moreover, A2 (Ti/N ~ 3) exhibited a significant ductility increase at low cooling rates (60 Kmin-1) even under the strain rate of 0.001 s-1. It means that Ti/N ~ 3 ensure the best stochiometric ratio in terms of the proportion of Nb in TiNb(CN) and the size of the primary and secondary TiNb(CN) to ensure a remarkable upgrading in ductility trough at a cooling rate of 60 Kmin-1. This behaviour agrees with the observation that larger precipitates (primary precipitates grew from ~ 2 µm up to ~ 4µm, and secondary from ~< 1 µm up to > 1 µm). To summarize, micro-cracks and consequently low hot ductility values were observed in cases where these precipitates smaller than ~1 µm nucleated at GBs. There was no linear relationship between the grain size and hot ductility values at a cooling rate of 60 Kmin-1 except for A0. Intriguingly, a recovery in hot ductility was observed with an increasing grain size of A0 at the cooling rate of 60 Kmin-1. Tis can be explained as follows: the size of Nb(CN) in the absence of Ti (no existence of coarse TiNb(CN)) increases. In other words, there is a relatively smaller amount of precipitates on the GBs in the same phase fraction, and correspondingly, a decrease in the frequency of micro-zones susceptible to the micro-defect formation. As the precipitate size decreases, the phase fraction of Nb in TiNb(CN) mostly increases relatively. The fact that Nb makes carbo-nitrides more brittle caused to micro-defects to be seen more frequently in the Ti containing steels. For this reason, although the grain size of A3 (≤ 200 µm) is likely to be small compared to other steel grades in the parameters causing the hot ductility trough, a sufficient ductility increase was not achieved.
