Research On Processes For Composite Microalloyed Low Carbon Steels By Simulation And Rolling Experiments

Microalloyed low carbon steels have been widely used in automobiles,ships,bridges,pressure vessels and other engineering structures because of its good strength,toughness,weldability,formability,good economy and other comprehensive performance.Therefore,microalloyed low carbon steels have broad prospects in application.In order to fully exploit its performance and comprehensively upgrade the properties of steel materials,it is an important research direction to design,develop and manufacture high-performance,low cost microalloyed high strength steels.In this paper,the composite microalloyed low carbon steels were selected by using continuous cooling process,simulation and rolling experiments,and the influence of process parameters on the microstructures and properties of steels were researched,and the results can be provided for the industrial production.The main studied contents and results of this paper are as follows:(1)The continuous cooling transformation behaviors of the steels were studied by thermal expansion method combined with metallographic,and the static and dynamic CCT curves were established.The results showed that the transformation temperatures of the steels decreased and the amount of bainite increased with the increasing of cooling rate.The phase transition temperature of the steels decreased and the micro-hardness decreased first and then increased with the increasing of cooling temperature.The critical temperature points Ac1,Ac3 increased by adding Mo and Nb elements,and the CCT curves of the steels were made to move to right.The transformation temperatures increased,and the ferrite transformation curve was made to move to left when the anstentie was deformed.The precipitate can be refined when adding alloying elements or applying deformation.(2)The effects of deformation temperature,holding temperature and holding time on the microstructures and precipitates of the steels were investigated by simulation experiments.The results showed that the original austenite grain size was refined and the size of precipitates were decreased with the decreasing of deformation temperature.The micro-hardness value was largerest for the steel when the isothermal temperature was 600? because of the lots of fine precipitates in the steel.The amount of precipitates and the hardness value were increased with the increasing of isothermal holding times.(3)The effect of hot-rolling parameters on the micro structure and properties of the steels was studied by hot rolled experiments The results showed that the yield strength and tensile strength of the steels increased first and then decreased with the increasing of the intermediate thickness,and the dispersion precipitation and interphase precipitation were observed in the steels.The size of precipitate increased,and nano hardness and Young 's modulus decreased with the increasing of holding temperature.The amount of large angle grain boundaries in the steels increased with the increasing of isothermal holding time.The yield strength and tensile strength of the steels can be increased,and the size of ferrite grain size can be refined by adding of Mo or Nb.