Effect of Cyclic Annealing on Microstructure and Mechanical Properties of Medium Carbon Steel

The microstructure and mechanical properties of medium carbon steel after cyclic heat treatment were in- vestigated. The effects of cyclic numbers and long time annealing on the microstructure and mechanical properties of the experimental steel were compared. A short duration （5 min） holding at 1023 K （above A1 temperature） and a short-duration （3 min） holding at 893 K are adopted in each cyclic heat treatment. The spheroidization is accelerated during cyclic heat treatment, and the spheroidizing ratio grows with cyclic numbers. After 12-cycle heat treatments, there are few incompletely spheroidized regions in the specimens, and cementite lamellae mostly change into cement- ite particles. The morphological character of cementite for 12 cycles is similar to that undergoing annealing for 10 h at 973 K. The strength of the experimental steel after 5-cycle heat treatment is the lowest in the following cyclic heat treatment, but it is still higher than that of specimens with subcritical annealing over a long period （10 h）. After 12- cycle heat treatment, the strength of the experimental steel is close to that of the normalized steel, and the plasticity is the best in all heat treated specimens.

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

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.