Rapid transformation annealing cycles applied to cold rolled steels, can result in substantial grain refinement. The latter is influenced by the interaction between recrystallization and ferrite-to-austenite phase transformation due to a delay of the onset of the recrystallization towards higher temperatures with increasing heating rate and even to temperatures above Ac1. In such conditions the phase transformation can start in a non-recrystallized matrix with a large number of nuclei which will result in remarkable grain refinement. Carbon and alloying elements play an important role in this process by controlling the grain size and austenite enrichment with carbon.
In this work the microstructural evolution of a Quenching and Partitioning (Q&P) steel after annealing with 10°C/s, 500°C/s and 1000°C/s without isothermal soaking (flash annealing) was studied and correlated with the obtained mechanical properties. The shift of the transformation temperatures with increasing heating rates was shown by dilatometric experiments and a non-complete dissolution of carbides at high heating rates was observed as a function of the heating rate. Pearlitic cementite is completely dissolved after heating at rate of 10°C/s, whereas at heating rate of 500°C/s carbides spheroidize in a prior pearlitic cementite morphology and with 1000°C/s, the original pearlite structure was still visible with minor spheroidization. A fine structure of martensite and stabilized due to C-partitioning retained austenite is formed from the significantly refined prior austenite grains due to the high heating rate and the absence of isothermal soaking. The tempered martensite during the partition step is with low C content due to the non-complete dissolution of carbides and at the highest heating rate, fine recovered or partially recrystallized ferrite was present. The ultimate tensile strength increased from 1050MPa in samples heated at 10°C/s prior to partitioning to 1318MPa in samples heated at 1000°C/s with a simultaneous increase in the total elongation of 8.2% (from 6.1% for 10°C/s heated samples to 14.3% for 1000°C/s heated samples). The overall bcc grain size decreased from 2.5µm to 1.4 µm and the fracture behaviour changes from cleaved facets to ductile dimples.
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