Controlled Rolling And Controlled Cooling Process And Microstructure Control Of Rare Earth Contained Microalloy Steel

Low carbon microalloyed steel has been widely used because of its characteristics such as high strength,high toughness,excellent welding performance and low production cost.Adding microalloying elements such as niobium,titanium to the steel and using reasonable controlled rolling and controlled cooling process,making the carbides precipitate in the hot-working process of the steel.Comprehensive utilization of second phase in steel to improve the microstructure and properties of steel by precipitation strengthening and fine-grain strengthening,which can make the steel obtain a good combination of strength and toughness.Rare earth elements in steel have the functions of purification,modification,microalloying and so on,by influencing the dissolution and precipitation of microalloying elements niobium and titanium,thus the effect of grain refinement and precipitation strengthening was affected.The steel that contains trace solid solution rare earth in itself produced by Bayan Obo iron ore raw material,which can play an obvious role in microalloying in steel.However,due to the limitations of current experimental methods,in the actual research process,laws of interaction between rare earth and microalloying elements in steel are still lacking in profound study.Based on the advantage resources of rare earth in Baotou Bayan Obo area,the effect of rare earth element lanthanum on second phase solidification and precipitation behavior in microalloy steel is systematically studied in this paper.Combined with the determination results of critical point and CCT curve,the rolling process and the cooling process after rolling were developed to study the effects of lanthanum on the after rolling microstructure,precipitation phases and mechanical properties of the experimental steel.Four components of experimental steel with and without lanthanum were divided into groups for comparative experiments.According to the calculation results of Thermo-Calc software,different heat treatment processes of high temperature solidification experiments was developed respectively,and the influence of lanthanum on the second phase solidification behavior in steel is investigated by the growth law of austenitic grains.Two kinds of thermal deformation processes were designed,the effect of lanthanum on the second phase precipitation behavior was studied by stress relaxation method,and the action law of lanthanum on the growth and precipitation phase of austenitic grains under different insulation times were studied.The results show that the dissolution of the second phase and the growth of austenitic grains were accelerated by adding lanthanum.In the process of strain-induced precipitation,the incubation period of the second phase was prolonged and the precipitation of the second phase in austenite was inhibited by adding lanthanum.In addition,the size of precipitation phase was refined and the number of precipitation phases was decreased of lanthanum contained experimental steel,which is of great significance for the control of the precipitated phase in the actual rolling process.The critical point and the thermal expansion curve at different cooling rates during continuous cooling process of the experimental steel were measured by quenching dilatometer.Combined with metallographic method and hardness measurement,the CCT curve of lanthanum contained experimental steel was drawn,which provided basic data and reference basis for formulating reasonable rolling process.According to the determination results of the critical point and the CCT curve of the experimental steel,a hot rolling process of two-stage control rolling was developed.After pilot rolling,the tissue after hot rolling is fine and uniform,mainly for isometric ferrite,as well as a certain proportion of bainite and a small amount of pearlite.After adding lanthanum,the microstructure and precipitation phase dimensions were refined,the number of precipitated phase was increased,and the distribution was more uniform of hot rolled experimental steel.The results of mechanical properties test show that the experimental steels after hot rolling have achieved good strong toughness.Based on the above experimental and theoretical research,the action law and mechanism of rare earth element lanthanum in niobium contained microalloy steel are systematically explored,which provides better theoretical and experimental support for its practical application.