Study On Preparation And Properties Of Ferrite/martensite Ultrafine Low Carbon Steel

Low-carbon steel is the important engineering materials.It has been widely used in bridge construction,shipbuilding,and other engineering fields.Obtaining ultra-fine grain structures with excellent ductility and toughness through severe plastic deformation technology is the hot research of current iron and steel materials.In this thesis,low-carbon steel Q345 b is selected as the research object,and a layered nanocrystalline low-carbon steel plate is obtained through the quench-heavy cold rolling process,and the carbon supersaturated deformed ferrite is deformed under different deformation temperatures and stresses.using OM,SEM and TEM methods to observe and analyze the dynamic recrystallization of the ferrite grain and cementite grain morphology in the deformation,to explore the effect of deformation temperature and stress on the microstructure.The results show that the dynamic recrystallization process occurs at 60MPa-650°C hot deformation-stress aging,and the ultrafine structure with finely dispersed fine cementite grains on the ferrite matrix is obtained.As the deformation stress increased,the cementite particles on the matrix gradually dissolve and coarsen,following the Ostwald ripening mechanism,the ferrite grain size is refined to 2?m at 280MPa-650°C,and the grain refinement effect is significant.However,when the deformation stress increases to a certain extent,the effect of grain refinement is limited because the recrystallization nucleation sites on the substrate gradually approach saturation.The dynamic recrystallization occurs at 550°C-210 MPa deformation-stress aging,and ultrafine structure composed of ferrite grains and cementite grains is obtained,as the deformation temperature increased,and the ferrite grains begin to grow.The cementite particles aggregated and spheroidized at the intersections of the grain boundaries.When the deformation temperature increased above 700°C,the cementite particles on the matrix dissolved and disappeared,and twinning martensite with a ferrite grain size appeared.The homogeneous mixed structure composed of body and retained austenite forms a new mixed phase.This homogeneous microstructure of the new mixed phase increases in strength and toughness simultaneously,ie,the ultra-fine grain steel with excellent overall performance is obtained.The mechanism of forming a new mixed phase in hot deformation is that when the sample reaches a high critical deformation amount,the cementite on the deformation-induced ferrite grain gradually dissolves,and then the carbon atom passes through the bite under the action of heat activation.The wrong channel rapidly spreads into the retained austenite that can hold more carbon atoms,forming an austenite supersaturation region,and is induced by stress to form twin martensite.