Effect Of Annealing Process On Microstructure And Properties Of 0.2C-1.03Si-2.04Mn-1.90Al Steel

With the increasing requirements for energy conservation and environmental protection,steel used in industries such as automobiles is moving towards lighter weight.This article is based on the traditional Fe-C-Mn-Si series steel adding lightweight element Al to achieve the purpose of lightening,but still retain a small amount of Si to improve surface quality.A kinetic model of carbon atom diffusion controlled continuous heating austenitization was established to provide a theoretical basis for studying the microstructure and mechanical properties of tested steel.Through salt bath annealing experiments,two heat treatment processes are designed-Bainitic Isothermal(BI)and Quenching & Partitioning(Q&P).Explore the influence of different processes on the microstructure and mechanical properties.At the same time,the changes of microstructure and mechanical properties during the stretching process are studied through the work hardening index analysis equation,which provides a theoretical basis for changes in mechanical properties.The main conclusions are as follows:(1)The influence of carbon atom diffusion on the austenite transformation fraction was studied.An isothermal solute precipitation analysis model based on Fan position saturation nucleation,isotropic growth,and linear approximation of the concentration gradient,introduced a carbon diffusion coefficient equation considering carbon atoms,temperature,and alloying elements,and austenite and ferrite containing carbon at different temperatures Based on the quantity variables,a carbon diffusion controlled continuous heating austenitization model was established.Matlab software was used to calculate the simulated austenite transformation fraction when the annealing temperature was 850 ?-annealing time of 120 s,and the simulated austenite transformation fraction was 40.85%when the annealing temperature was 880 ? and annealing time of 120 s.(2)The influence of the bainite isothermal process on the microstructure and mechanical properties of the tested steel was studied.The tested results show that for the BI process,as the annealing temperature and bainite isothermal temperature increase,and the bainite isothermal time increases,the mechanical properties show a trend of increasing plasticity and decreasing strength.Annealing temperature 850 ?-bainite isothermal temperature 400 ?-bainite isothermal time 300 s,tensile strength reaches the maximum value of 968 MPa;annealing temperature 880 ?-bainite isothermal temperature 400 ?-bainite isothermal time 600 s,The elongation reaches the maximum value of 21.78%.For the Q&P process,as the holding temperature and the quenching temperature distribution temperature increase,the mechanical properties show a trend of increasing plasticity and decreasing strength.When the holding temperature is 880 ?-quenching temperature260 ?-distribution temperature 360 ?,the tensile strength reaches the maximum value of 958 MPa;when the holding temperature 880 ?-quenching temperature 280 ?-distribution temperature 400 ?,the elongation after fracture reaches the maximum value of 21.2%.(3)According to the analysis equations of Hollomon and C-J,the influence of the microstructure and mechanical properties of the uniaxial tension stage is studied.For the BI process,the three stages of change in the stretching process are all related to the composition,quantity and type of the microstructure.The first stage is mainly caused by the elastic deformation of ?-ferrite,the second stage is affected by the constraint of martensite's coordinated deformation of ?-ferrite,and the third stage is affected by both?-ferrite and martensite.The impact of complex plastic deformation occurs.For the Q&P process,there are also three stages of change in the stretching process.The first stage is affected by the plastic deformation of ?-ferrite and reticulated bainite and the elastoplastic deformation of martensite,and the second stage is affected by the plasticity of ?-ferrite and granular bainite by martensite.Affected by the difficulty of deformation,the third stage is affected by the complicated deformation of ?-ferrite and martensite at the same time.