¦£ ¡ú ¦Á Phase Transformation In Microalloyed Steels, The Experimental Study And Numerical Simulation

Microalloyed steel is a modern metallurgical production under the condition of the high-tech steel production.Addition of a small quantity of microalloying elements (mainly Nb,Ti and V,etc.)can substantially improve strength and organizational performance.Niobium can have grain refining effect and precipitation strength effect in steel,and it can improve steel's toughness and strength efficiently.This widens the investigation and production region of microalloyed steel containing niobium. Therefore the control of transformation of austenitic to ferrite plays a decisive role in the organization of microalloy steel eventually.It can provide reference for the actual production of new steel and the development of controlled rolling and controlled cooling technology.Three low carbon steels of different mass fraction 0.021%,0.081%and 0.17%were used to investigate the effect of deformation on transformation of austenitic to ferrite and the grain size of ferrite.The deformation of rolled sample was calculated by finite element software Marc,and it analysed the effect of deformation on the transition of austenitic and ferrite.The model was developed on the thermodynamic basis of super-element and KRC models and the kinetic basis of Cahn's transformation theory from the nucleation and growth mechanisms of ferrite.The time - temperature -transformation curve of three test steels was calculated about the transformation of austenite to ferrite.Finally,the time - temperature - transformation curve of a steel of business was calculated by the model of isothermal transformation.It further verified the accuracy of model compared with the measured values.The main results are as follow:Through the thermal simulation experiments,the rules of effect of different elements, different deformation and different holding temperature on austenitic to ferrite and the grain size of ferrite of the three test steels were acceivd.The grain size of ferrite is small while the niobium content is high,the grain size of ferrite is small while the temperature of isothermal transition is low,the grain size of ferrite is large while the holding time is long,the grain size is larger when 35%deformed than undeformed of isothermal transition in the same holding temperature. The stress value of deformation of 20%and 50%for the three test steels was simulated.Compared with the value of experiment,the error is about 6%.The result is accord with requirements of finite element simulation.It shows that the result is reliable and the model is rational.The stress value of deformation of 35%for No.3 steel(C:0.036%,Mn:1.10%,Si:0.20%,Nb:0.170%)was simulated.It made a base of deformation energy for calculation of isothermal transition of austenite and ferrite.It simulated the isothermal transition curve with the calculated model.It verified the final content of product through quantitative microstructure analysis and found that calculated and experimental results are in good agreement.It shows that the result is reliable and the model is rational.