Investigation On The Control Of Fine Microstructure In Ultra-Low Carbon Bainitic Steel

Compared with "ferrite+pearlite "steel and tempered martensite steel,ultra-low carbon bainitic steel is a kind of steel with excellent strength and toughness,low process cost and wide application.The appearance of ultra-low carbon bainitic steel is the inevitable result of the development of social economic and metallurgical technology.The inhomogeneity of deformation and cooling rate between the surface and core during hot rolling of thick surcture steel is a problem hard to solve.Recently,the research and production have focused on the improvement of rolling,capacity of cooling equipment and hardenability of steel plate in order to obtain the microstrcture of refined bainite.This work focus on the phase transformation during the cooling process in order to slove the issue that the thick structure steel is difficult to prepare.we expect to obtain the refined bainite micro structure through the several regulation of transformation behaviors.The main research are as follows:(1)Static continuous cooling experiments were carried out under the composition design of Fe-0.047C-2.4Mn-0.08Nb alloy,and its micro structure was analyzed.The results show that the ferrite structure appears at the cooling rate of 0.1?/s and uniform lath bainite structure is obtained at the cooling rate of 0.5?20?/s and martensite transformation occurs at the cooling rate of 20?30?/s under the static continuous cooling experiment.The uniform bainit structures are obtained at 440?and 480? under the static isothermal experiments.The combination of bainite variants is CP group and more types of bainite variants are present at lower temperatures,thus,the bainite structure is refined.(2)The phase transformation behavior of acicular ferrite was studied based on addition the elements of V and N to low-carbon medium-.manganese steel.The results show that bainite occupies the nucleation position of acicular ferrite and inhibits its nucleation and growth in initial austenite grains under the static continuous cooling experiment.The proeutectoid ferrite can inhibit bainite transformation and providing favorable conditions for the formation of acicular ferrite in initial austenite grains under static isothermal experiments.However,the decreased of transformation temperature of retained austenite and the formation of acicular ferrite is restrained due to the carbon enrichment when the proportion of proeutectoid ferrite is relatively high.,(3)The amount of precipitates in the second phase will affect the formation of acicular ferrite in the initial austenite grains.The introduction of Ti will affect the precipitation kinetics of V(CN),thus reducing the driving force of the second phase nucleation and increasing the critical nucleation power by comparing the experimental steels with different alloy compositions and the second-phase precipitates are difficult to form.(4)The effect of deformation on acicular ferrite was studied under the composition design of ultra-low carbon Nb and Mo microalloys.The uniform bainite structure was obtained in the cooling rate range of 5?/s-20?/s under the static continuous cooling experiment.With the decrease of cooling rate,the lath characteristics of bainite structure gradually weakened,and some granular bainite structure was obtained at 5?/s.However,the confused transformation microstructure and fine acicular ferrite are obtained under the dynamic continuous cooling and isothermal experiments.With the decrease of transformation temperature and the increase of deformation,the proportion of acicular ferrite increases and becomes finer.mWith the increase of deformation and the decrease of phase transition temperature,the proportion of acicular ferrite increases and becomes finer.