Control of Austenite Characteristics and Ferrite Formation Mechanism by Multiple-Cyclic Annealing in Quenching and Partitioning Steel

摘要

Quenching and partitioning(Q&P)treatment is a novel method to produce advanced high strength steel with excellent mechanical properties.In this study,combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention.The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products,while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products.With the increase in cyclic annealing number,both of austenite fraction and austenite carbon content increased,attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel.In traditional Q&P sample,the deformed ferrite was recrystallized by sub-grain coalescence,while the austenite was newly nucleated and grew up during annealing process.As a comparison,the ferrite in multiple-cycle annealing samples was formed by means of three routes:tempered martensite that completely recovered with retention of interior martensite variant,epitaxial ferrite that formed on basis of tempered martensite,ferrite that newly nucleated and grew up during the final annealing process.Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitateεcarbide with C enrichment,Mn enrichment and homogeneous Si distribution.Compared with the traditional cold-rolled Q&P steel,the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation,ascribing to the coarser microstructure and more efficient transformation induced plasticity(TRIP)effect deriving from retained austenite with high carbon content and larger volume fraction.The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%,which efficiently exploit the potential of mechanical performance in low carbon steel.