| This thesis is based on the program supported by National Natural Science Foundation of China:"Control of Nano Precipitates in Low Carbon Steels under Ultra Fast Cooling(UFC)and Comprehensive Strengthening Mechanisms".In this investigation Ti was added into the common C-Mn steels and the continuous cooling transformation behavior,ferrite transformation behavior and precipitation behavior of micro-alloy carbonitride of this steel were investigated by thermal simulation experiment and hot rolling.The strengthening and toughening mechanism of Ti-microalloyed steel under ultra-fast cooling process was also discussed.The main contents are as follows:(1)The continuous cooling transformation behavior during the continuous cooling process was investigated.The continuous cooling transformation(CCT)curves of the experimental steels were established,and the effects of Ti micro-alloy,cooling rate and deformation on the transformation behavior were also investigated.The results showed that deformation increased the start and finish temperature of ferrite transformation and made CCT curve shift to left.The adding of Ti into the steel decreased the start and finish temperature of austenite transformation,inhibited ferrite and pearlite transformation and promoted the bainite transformation.(2)The influence of Ti content on the microstructure and precipitation behavior was investigated through isothermal quenching experiments.The results indicated that with the adding of Ti,a large number of nano-precipitate particles formed during the process and this will significantly increase the hardness of ferrite.Simultaneously,with the increasing of Ti content,the row spacing of interphase precipitation decreased from 41.7 nm to 22.7 nm,enhanceing the precipitation strenthing.Interphase precipitation could be observed in all steels with different Ti content and the TiC precipitations had NaCl-type crystal structure and obeyed the Baker-Nutting(B-N)orientation relationship with respect to the ferrite matrix.(3)The influence of isothermal temperature on the microstructure and precipitation behavior as well as micro-hardness was investigated by means of thermal simulation experiments.The results indicated that with the increasing of isothermal temperature the precipitation mechanism changed from PIP to CIP and the spacing of interphase precipitation increased.At the same time the size of precipitates increased from 2.59 nm to 4.31 nm.(4)The effect of finish cooling temperature on the microstructure and mechanical properties were investigated by hot rolling experiments.The results indicated that with the decreasing of finish cooling temperature both the ultimate tensile strength and yield strength decreased.Charpy impact fracture surface analysis by SEM indicated that all the samples showed britle fracture mechanism and with the increasing of finish cooling temperature the cleavage surfaces decreased. |
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