Fast-heating for intercritical annealing of cold-rolled quenching and partitioning steel

In this study, fast-heating (300 °C/s) was applied to achieve the intercritical annealing of a cold-rolled quenching and partitioning (Q&P) steel with a wide range of soaking temperature (770-850 °C) and soaking time (0-120 s) for austenitization. The dilatometry and microstructural analysis revealed that in contrast to the conventional heating rate (5 °C/s), a fast-heating rate led to an accelerated transformation and grain refinement of the prior austenite in the Q&P samples. The microstructures of the Q&P treated samples subjected to different intercritical annealing conditions were studied in detail by various material characterization techniques including electron microscopy, electron probe micro-analysis, and X-ray diffraction. The working hardening behavior and the mechanical stability of the retained austenite were discussed on the basis of the typical stress-strain curves. The statistics of the ultimate tensile strength vs. total elongation of each sample under the orthogonal annealing conditions suggest that, for the investigated steel, the fast-heating process improved the strength with approximately 90 MPa on average within the elongation ranged from 17 to 27%.