The Study Of Hot Rolling Process And Stretch-Flangeability On FB Steels

Dual-phase steels are one of the most widely used AHSS(Advanced High Strength Steel)sheets because of their characteristic mechanicalproperties,such as high strength,low yield ratio,high initial work hardening rate as well as good combination of strength and toughness.Compared to FM(Ferrite Martensite Dual-Phase)steels,FB(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,FB steel may be an ideal candidate for the complex press-formed parts,such as automotive chassis and wheels.The targets of this dissertation are to develop the low cost hot rolled FB steels.The experimental steels with different chemistries(C-Mn-Cr and C-Mn-Nb-Ti)were used as the research objects.The transformation behaviors of the two experimental steels in the process of continuous cooling were studied.The influence of the process control parameters on the microstructural evolution in the TMCP process were discussed,and then low cost hot rolled FB steels with high mechanical properties were developed.The stretch-flangeability of these FB steels were evaluated through hole-expansion tests.Crack formation and propagation behavior during the hole-expansion were also investigated.The main work involved as follows:(1)Through thermal simulation experiment,the curves of static and dynamic continuous cooling transformation were established by the thermal dilation method and metallographic analysis.Comparing CCT curves and microstructures showed that it is easier to obtain ferrite-bainite dual phase structure for C-Mn-Ti steel by controlled rolling and cooling.The deformation increases the phase transition start temperature of supercooled austenite,and pushes CCT curves upwards and towards left which obviously enlarge the ferritic phase transformation zone.The combination of deformation and accelerated cooling decreases the grains of austenitic microstructure and homogenizes the grains.(2)Microstructure and mechanical properties of test steels with the UFC + air cooling + UFC cooling mode were researched through hot rolling experiment at laboratory,and the optimized process parameters were obtained.The results showed that the effect of process parameters on the microstructure and the properties of C-Mn-Cr steel are more obvious,compared to C-Mn-Ti steel.The finishing temperature,the cooling rate and final temperature of the first UFC and the air cooling time commonly affected the morphology,the amount and the grain size of ferrite,and then affected the subsequent transformation and microstructure.The coiling temperature and coiling way were the key process parameters to obtain the bainite.(3)The microstructure of FB steels,especially the morphology and microstructure of bainite,were observed through the scanning electron microscope(SEM)and transmission electron microscopy(TEM).The morphology of bainite in FB steels varies with the parameters of controlled rolling and controlled cooling,it is mainly composed of lath bainite and the width of lath bainite ferrite in 0.5?1 ?m.These ultra-micronization lathes can effectively improve the strength and toughness of the steel.There was a certain amount of TiC and TiN in experiments steel B.(4)Hot rolled FB steels with good mechanical properties and stretch-flangeability were developed through the reasonable parameters of controlled rolling and controlled cooling process.The yield strength,the tensile strength,the total elongation,the yield ratio,the work hardening index(n)and the hole-expansion ratio of C-Mn-Cr steel sheets reach 366.18MPa,606.68MPa,25%,0.6,0.18 and 77%,respectively.The yield strength,tensile strength,total elongation,yield ratio,work hardening index(n)and hole-expansion ratio of C-Mn-Ti steels sheets reach 473.49-490.41 MPa,567.19-581.19MPa,26%,0.82-0.86,0.10-0.12 and 77-107%.(5)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 FB steels is formed by the 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.