Investigation On Morphological Control And Spherodization Behavior Of Cementite In Fine-blanking Steel

Fine-blanking is a high-precision and efficient forming method,and has been widely used in the production of automotive parts in recent years.In addition to the equipment and processing process,the uniform "ferrite+spherical cementite"annealed microstructure is an important guarantee to obtain a smooth,high precision punching surface.Hot rolled microstructure,as the initial condition for spheroidizing annealing,significantly affects the final annealed microstructure.This work was based on the Fe-C-Mn-Cr/Nb alloy design.Investigation on hot-rolled microstructure control and its spheroidizing annealing behavior was carried out under new generation thermo-mechanical control process(TMCP)which includes ultra-fast cooling(UFC)as its key technology.The main works are as follows.(1)Using isothermal phase transformation simulation and hot rolling experiments,the control mechanism of hot-rolled microstructure was investigated based on the calculation of phase diagram with Themo-Calc and the bainite transformation temperature.Results showed that,a mixture of ferrite and pearlite was obtained and cementite was in the form of lamellar shape when the ultra-fast cooling(UFC)stop temperature was high.In contrast,bainite microstructure was achieved and the dispersed or short-rod cementite located in between bainitic ferrite plate for a low UFC stop temperature.(2)Cold rolling and spheroidizing annealing was conducted to study the effect of hot-rolled microstructure and cold rolling on spheroidizing annealing behavior of cementite,i.e.the speed of spheroidizing and uniformity of cementite.It's found that,due to the late start of cementite precipitation,the formation of proeutectoid ferrite was the main reason which was responsible for the inhomogeneous size and distribution of cementite.The cold rolling sped up the spheroidizing process and promoted the uniform distribution of cementite.(3)The underlying mechanism that how the Cr and Nb would act on the recrystallization of deformed ferrite during annealing process was analyzed.It's suggested that,during spheroidizing process in Fe-C-Mn-Cr steel,the partition of Cr and Mn from ferrite to cementite slowed down the growth kinetics of cementite and the resulted small and dispersed cementite impeded the recrystallization behavior of ferrite.In Fe-C-Mn-Nb steel,there are large-sized cementites and fully precipitated and grown NbC,which are the main factors leading to the fast recrystallization of ferrite matrix.