Microstructural Evolution And Mechanical Behavior Of Ferrite/Bainite Steels With High Stretch-Flangeability

Dual-phase steels are one of the most widely used AHSS(Advanced High Strength Steel) sheets,because of their characteristic mechanical properties,such as high strength,low yield ratio,high initial work hardening rate as well as good combination of strength and toughness. As compared to FMDP(Ferrite/Martensite Dual-Phase) steels,FBDP(Ferrite/Bainite Dual-Phase) steels exhibit a superior combination of strength and cold formability,especially excellent welding and fatigue properties as well as high stretch-flangeability.Therefore, FBDP steel may be an ideal candidate for the complex press-formed parts,such as automotive chassis and wheels.The work of this dissertation was carried out integrating with the project of 'Phase Transformation and Strengthening Mechanism of Ferrite/Bainite Dual-Phase Steels with High Hole-Expansion Ratio',which is supported by the Baoshan Iron and Steel Group.The continuous cooling and isothermal transformation behavior of low carbon Si-Mn steels was studied.Economical hot-rolled FBDP steels were developed by TMCP(Thermo-Mechanical Control Process) technology and the stretch flangeability of these steels with different strength grades was evaluated by hole-expansion tests.Deformation behavior and fracture characteristics in the uniaxial tension,and crack formation and propagation behavior during the hole-expansion were also investigated.The main work involved as follows:(1) The continuous cooling transformation behavior of low carbon steels with different Si and Mn contents was investigated by thermal simulation experiment.Effects of alloying elements,hot-deformation parameters and cooling rates on ferrite transformation start temperature(Ar₃),microstructure and properties were studied.The results showed:1) Ar₃ rises with the increase in Si,while it drops with the increase in Mn.The effect of Mn on Ar₃ is more obvious than that of Si;2)Influence of Si on Ar₃ is dependent on deformation,and such effect is more significant without the prior deformation of austenite;3) Si promotes the formation of polygonal ferrite,but inhibited the transformation of bainite.Mn not only refines the microstructure of ferrite and bainite,but also promotes the formation of bainite;4)In case of the deformation with true strain of 0.4,the length of bainitic ferrite laths is significantly decreased in the low Si steel,whereas,the M/A constituents become more uniform in the high Si steel.(2) The influence of Si and prior deformation on the isothermal transformation of ferrite and bainite in low carbon steels was studied.The numerical simulation of phase transformation kinetics and thermodynamics was carried out by the super-element method and modified Zener's parameters.The results were showed as follows:1) With Si content increasing,the incubation time of ferritic transformation decreases,while that of bainitic transformation increases;2) The ferrite grains are refined and the bainitic ferrite laths become shorter with increasing Si content;3) After the prior deformation of austenite,the ferrite grains become equiaxed and more refined,and the volume fraction of bainite decreases;4) Deformation or increasing Si content increases the carbon equilibrium concentration onγ/(γ+α) orα/(α+γ) phase boundary and driving force for ferrite transformation.(3) The economical hot-rolled FBDP steels were developed by TMCP technology.Some factors affecting the microstructure and properties of FBDP steels were investigated.1) FBDP steels with high stretch-flangeability were exploited by designing Si and Mn contents as well as TMCP parameters.The tensile strength,total elongation and hole-expansion ratio may reach 472.5～721.5MPa,27.4～43.8%and 95～117.8%,respectively,which meets the standards of high stretch-flangeability steels(λ≥80%,A≥20%);2) The increase in Mn content increases both the strength and toughness,but decreases the ductility and stretch-flangeability; 3) When Si content increases from 0.06%to 0.57%,the values of tensile strength,impact toughness and stretch-flangeability all increase.When Si content increases to 0.92%,the strength increases,whereas,the values of toughness and stretch-fiangeability decrease.The steel with 0.57%Si exhibits the best combination of all the three property parameters;4) Compared with ferrite/pearlite and FMDP steels,FBDP steels have a better balance between strength and stretch-flangeability;5) The decrease in finish rolling temperature promotes the formation of fine-grained ferrite and improves the ductility and toughness of experimental steels,but results in the lower tensile strength and higher yield ratio of steels due to the decrease in the volume fraction ofbainite.(4) The crack formation and propagation behavior during the hole-expansion were investigated.Since the edge of punched hole is the dangous zone of deformation,the necking or cracking will occur when the tangential elongation is more than the forming limit here.The crack of FBDP steels is formed by the mode of microvoid coalescence.When a microcrack meets the bainite,it mostly propagates along the phase interface between ferrite and bainite by cutting off ferrite grains.In the crack propagation,the majority of equiaxed ferrite grains are elongated along the direction perpendicular to the crack,which shows that ferrite grains with lower strength will have a greater plastic deformation.This helps to reduce the stress concentration near the crack and thus restrain the crack propagation.(5) The deformation behavior and fracture characteristics in the uniaxial tension were studied.The curves of the instantaneous work-hardening factor n^* value versus true strainεare made up with three stages during the uniform plastic deformation:n^* value is relatively higher at stageâ… ,decreases slowly withεin'stageâ…¡,and then decreases quickly withεat stageâ…¢.Compared to the ferrite/pearlite and FMDP steels,FBDP steels show higher n^* value in the low strain region.The uniaxial compression-tension testing was performed,and the Bauschinger effect parameter and work-hardening behavior were examined to understand the deformation mechanism of FBDP steels under complex loading conditions.The values of dσ/dεand n firstly increase and then decrease with the prior compressive deformation increasing from O to 1.0mm.