Study On Annealing Process Of Rapid Heating And Its Microstructural Evolution Mechanism Of Cold Rolled Dual Phase Steel

Based on Baosteel rapid heated and Galvanized simulation equipment and cold rolled plate DP590,the microstructure evolution,mechanical properties and bake hardenability of the cold rolled dual phase steel during the annealing process of rapid heating was investigated by means of OM,SEM,TEM,EPMA and mechanical testing.The main contents of the dissertation are as follows:Firstly,the effect of heating rate on microstructure evolution of cold rolled DP590 were studied.It was found that the ferrite recrystallization and austenite formation process overlaps under faster heating rate,which affects the nucleation site and growth process of austenite.The austenite nucleated at recrystallised ferrite grain boundaries and grew along them under low heating rate.The carbon should first migrate from the carbide particles in grains to the grain boundaries and then towards the edges of the austenite by grain boundary diffusion.The increase in heating rate resulted in austenite nucleating at the deformed-ferrite/carbide interface.Growth of austenite into the deformed ferrite was controlled by diffusion of carbon in the austenite.In addition,the kinetics of austenitization is analyzed by the Johnson–Mehl–Avrami–Kolmogorov?JMAK?equation and a kinetic equation is proposed which catches the ferrite recrystallization and substance of austenitization.It was found that the deformed microstructure can greatly improve the favorable nucleation site of austenite.While the heating rate is increased from 5?/s to 500?/s,the Kⁿ rise 8 orders to 3.79×10⁹.Secondly,the influence of different annealing process parameters of rapid heating on the microstructure and mechanical properties of DP590steel was studied.Two typical microstructures of DP steel were produced by fast heating at different soaking temperatures.One is granular martensite particles linked in bands parallel to the rolling direction and located in grain boundaries of banded ferrite.The other is blocky martensite uniformly distributed in the fine proeutectoid ferrite matrix.The annealing of rapid heating can significantly refine ferrite grain and improve the austenitic hardenability.When the heating rate raise from 5?/s to300?/s,the grain size of ferrite was decreased from 9.7?m to 2.2?m,the volume of martensite was increased from 22.6%to 27%.As a result,the yield strength was increased more than 20%,the tensile strength and total elongation reached 687 MPa and 21.7%.Thirdly,the mechanical properties and fracture mechanism of DP steel with similar martensite volume fractions produced by annealing process of rapid heating?FH1,FH2?and slow heating?SH0?were compared.It was found that the tensile strength was not evidently changed but the yield strength was increased 27%percent.In addition,analysis of the work-hardening behaviors of the DP microstructures using the differential Crussard–Jaoul technique indicated that the initial strain-hardening exponent of the FH2 sample is 0.53,considerably higher than the 0.31 and0.37 for SH0 and FH1.Metallographic observations revealed that the microvoids appear as ferrite/martensite interface decohesion,separation of ferrite grain boundary immediate martensite and adjacent martensite within the SH0 and FH2 samples,whereas the microvoids are nucleated by fracture of banded martensite particles in FH1 specimen.Since the strain focused on the narrow regions of the banded martensite,which lead to total elongation of FH1 was decreased 3.1%compared to SH0.Then,the bake-hardening behavior?BH value?of dual-phase steel annealing under rapid heating was investigated.The results showed that the solute carbon atoms segregated to the mobile dislocations during baking,resulting in the formation of Cottrell atmosphere and precipitation of carbide.Consequently,bake-hardening could be attributed to the formation of Cottrell atmosphere and precipitation of carbide.And bake-hardening included 3 stages:Cottrell atmosphere formation,carbide precipitation and martensite tempering.The ferrite grain size decreased and the martensite volume fraction increased with the heating rate increasing,which resulted in the amount of mobile dislocation in ferrite was obviously enhanced.Thus,when the heating rate raise from 5?/s to 300?/s,the BH value of the DP steel increased from 41 MPa to 75 MPa,the growth rate reached almost 83%.When the baking temperature exceeded 220?the BH value of the DP steel reached a maximum value of 151 MPa due to the precipitation of carbide in both ferrite and matensite phases.As the pre-straining exceeded strain transition point(?_tr),there is a slow increase in the BH value,because the dislocations located at the ferrite/martensite interface actuated and slipped into the martensite phase.Increasing in pre-straining from 2%to 8%led to only 24 MPa increase in BH value.Finally,a micmechanical model was constructed to reflect the stress-strain relationship of rapid heating-DP steel,and the flow stress of rapid heating-DP steel has been calculated by using the stepwise approach of Tomota and adding an inner stress relief process,where the Eshelby inclusion theory and Mori-Tanaka mean field theory are combined.Calculation and analysis results show that the model considering the internal stress release caused by microviod formation,a more accurate description of the micro-deformation of dual phase steel,effectively reduced the calculation error.