Research On The Microstructure And Mechanical Properties For Cold-rolled Q&P980 Steel With Intercritical Annealing

As a significant way to reduce the emission from vehicle,the advanced high strength steel(AHSS)has run to the third generation so far.Q&P steel which is the typical third generation AHSS,has developed into industrial production and application for vehicle.Howerer,most of the researchers focu on the the full austenized type Q&P steel and have no more works on Q&P process which involved with intercritical annealing.This work based on the traditional TRIP composition has added some microalloy element,such as Nb,V,Ti to enhance the mechanical properties of tested steel by the means of precipitation strengthing and fine-grain strengthing.In order to research the basic thermal mechanic parameters,the priciples of microstructure evolution and the relationship between the microstructure and mechanical properties in the intercritical annealing Q&P process,the author used the methods of theoretical model for simulation,thermal and dynamic model for simulation,processing simulation by dilatometer,adjustment and control of annealing parameters in laboratory.Based on the research in this work,the results can be obtained as follows:(1)The alloyed carbide can consumed 0.0728%(wt.%)carbon(most of which is VC)in equilibrium state;As calculated by CCE model,the optical quenching temperature of annealed in 850? is 226.5? and the most volume fraction of RA is 18%with carbon concentration of 1.1%(wt.%);In Dictra simulation,the volume fraction of RA increases gradually and reach the theoretical level from CCE model with the prolong of partitioning time when the phase interface is immobile and the quenching temperature is smaller than the optical quenching temperature.While the volume fraction of RA increases firstly and then decreases when the quenching temperature is above the optical quenching temperature,and the largest volume fraction of retained austenite is larger than that of simulation from CCE model.When the phase interface is mobile,the phase interface moves to austenite and then moves reversed to martensite with the decreased volume fraction of RA(3.159%)compared with that of immobile phase interface in the same annealing condition(9.108%).(2)The results of dilatometer is as follows:Acl is 675.5?,Ac3 is 938.6?,Arl is 615.7?,Ar3 is 891.3 ?.The intercritical cooling rate of martensite transformation is 20 ?/s from static CCT curve.As partition for long time by dilatometer,the expansions is ascribed to bainite transformation and the volume decrease attributed to the matensite temper with carbide precipitation,recovery of dislocation and sub-structure.(3)As the stress-strain curve shows,the dynamic recrystallization launch easier when the deformation temperature is higher and the strain rate is lower;when the strain rate is constant and improved the deformation temperature,the start time of dynamic recrystallization decreases from the RTT curve.The activation energy of dynamic recrystallization is 424.820 kJ/mol in this steel.When the interval time is 10s,the static recrystallization has be closed to the finished state.(4)The tensile strength increase with the improvement of coiling temperature,while yield strength and elongation increase firstly followed by decrease.And the largest PSE is 20694MPa·%which obtained in 570? coiled and the tensile strength is 1085MPa,the yield strength is 648MPa,the elongation is 19.1%.(5)With annealing temperature increasing,the tensile strength and yield strength increase while the elongation decreases followed by increaseing.The optical mechanical properties has obtained with annealed in 850?.And the tensile strength is 1151MPa,the yield strength is 594MPa,the elongation is 15.3%and the PSE is 17557MPa·%.The volume fraction of RA is also the largest which is 9.58%with the carbon concentration of 1.16%.The volule fraction of RA decreased between the tensile which typically means the TRIP effect is also the largest(7.26%)with different annealing temperatue(6)With the improvement of partitioning time,the tensile strength decreases and yield strength decreases followed by increaseing while the elongation increases followed by slightly decrease of partitioning for 3600s.The optical mechanical properties have obtained with partitioned for 600s.In this condition,the tensile strength is 1004MPa,the yield strength is 591MPa,the elongation is 19.56%and the PSE is 19638MPa·%.The volume fraction of RA is also the largest which is 6.5%with the carbon concentration of 1.03%.