Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase （DP） steels with almost constant martensite contents of 20vo1% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature （1043 K, FH1） with fast-heating （300 K/s） show banded ferrite/martensite structure, whereas those soaked at high temperature （1103 K, FH2） with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated （5 K/s） reference material （SH0）. The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard-Jaoul technique demonstrate two stages of work hardening for all samples.

Kinetic transitions and Mn partitioning during austenite growth from a mixture of partitioned cementite and ferrite:Role of heating rate

Austenite formation from a ferrite-cementite mixture is a crucial step during the processing of advanced high strength steels(AHSS).The ferrite-cementite mixture is usually inhomogeneous in both structure and composition,which makes the mechanism of austenite formation very complex.In this contribution,austenite formation upon continuous heating from a designed spheroidized cementite structure in a model Fe-C-Mn alloy was investigated with an emphasis on the role of heating rate in kinetic transitions and element partitioning during austenite formation.Based on partition/non-partition local equilibrium(PLE/NPLE)assumption,austenite growth was found alternately contribute by PLE,NPLE and PLE controlled interfaces migration during slow-heating,while NPLE mode predominately controlled the austenitization by a synchronous dissolution of ferrite and cementite upon fast-heating.It was both experimentally and theoretically found that there is a long-distance diffusion of Mn within austenite of the slow-heated sample,while a sharp Mn gradient was retained within austenite of the fast-heated sample.Such a strong heterogeneous distribution of Mn within austenite cause a large difference in driving force for ferrite or martensite formation during subsequent cooling process,which could lead to various final microstructures.The current study indicates that fast-heating could lead to unique microstructures which could hardly be obtained via the conventional annealing process.