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Intercritical annealing, flash process and tempering were innovatively combined to obtain high strengthhigh ductility combination in 0.12Ce4.89Mn-1.57Al steel. The process referred as multi-step partitioning (MSP) was designed to accomplish the following objectives: (a) enrichment of austenite with Mn to enhance the stability of retained austenite, (b) transformation hardening during quenching in the flash process and (c) stress relaxation and carbon enrichment of retained austenite. The tensile strength of steel increased from~667 MPa in intercritically annealed steel to~986 MPa in flash processed steel. The product of strength and elongation of flash steel and tempered steel were 23.2 GPa% and 24.9 GPa%, respectively and higher than the intercritically annealed steel (21.3 GPa%). The high ductility, especially the uniform elongation of flash steel (16.2%) and tempered steel (19.4%) is attributed to~15e19% by volume of Mn-rich stable retained austenite and efficient TRIP (transformation induced plasticity) effect. Thermodynamic calculations enabled us to understand the partitioning behavior of alloying elements in MSP. C, Mn and Al reverse partitioning during the flash process led to increased stability of retained austenite. The unique distribution of chemical constituents contributed to two types of martensitic transformation during the flash process: (a) austenite / a 0 -martensite transformation dominated at high temperature and contributed to the formation of stacking faults and ε-martensite transformation and (b) austenite / ε-martensite / a 0 -martensite phase transformation dominated at lower temperature. The stability of retained austenite and interaction with stress concentration contributed to highly efficient TRIP effect in flash processed and tempered steel. The experiment findings were consistent with the diffusion-controlled transformation simulation analysis.
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Metals
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Materials, 2014
A medium Mn steel has been designed to achieve an excellent combination of strength and ductility based on the TRIP (Transformation Induced Plasticity) concept for automotive applications. Following six passes of hot rolling at 850 °C, the Fe-7.9Mn-0.14Si-0.05Al-0.07C (wt.%) steel was warm-rolled at 630 °C for seven passes and subsequently air cooled to room temperature. The sample was subsequently intercritically annealed at various temperatures for 30 min to promote the reverse transformation of martensite into austenite. The obtained results show that the highest volume fraction of austenite is 39% for the sample annealed at 600 °C. This specimen exhibits a yield stress of 910 MPa and a high ultimate tensile stress of 1600 MPa, with an elongation-to-failure of 0.29 at a strain rate of 1 × 10 −3 /s. The enhanced work-hardening ability of the investigated steel is closely related to martensitic transformation and the interaction of dislocations. Especially, the alternate arrangement of acicular ferrite (soft phase) and ultrafine austenite lamellae (50-200 nm, strong and ductile phase) is the key factor contributing to the excellent combination of strength and ductility. On the other
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