| TRIP(Transformation-Induced Plasticity)steel has been widely used in automotive industry as the first generation of advanced high strength steel.The prominent characteristic of this kind steel is the good combination of high strength and plasticity.In this study,a novel quenching and partitioning(Q&P)process has been applied to heat-treat TRIP590 steel,in order to obtain excellent mechanical properties.The investigated TRIP590 steel has been heat treated by different processes using salt-bath furnaces.Microstructures and the contents of retained austenite have been analyzed by using the optical micrograph,scanning electron microscopy(SEM),X-ray diffraction(XRD)and Vibrating sample magnetometer(VSM).On the other hand,the mechanical properties of the obtained steel after heat treatment were characterized by tensile tests at various temperatures,strain rates and strains.The corresponding microstructures were characterized by SEM,EBSD and TEM.The obtained results are as follows:After Q&P heat treatment,the multiphase microstructures mainly consisted of ferrite,retained austenite,bainite and martensite.The majority of retained austenite distributed around the boundaries of ferrite.Keeping the time constant as 60 s,with an increase in the partitioning temperature from 380 ? to 450 ?,the VSM measurement revealed that the content of retained austenite firstly decreased from 26%to 18%,then increased to 20%while the carbon content of austenite increased from 1.61%to 1.77%.At room temperature,under a strain rate of 1.7×10-3/s,at partitioning temperature 380 ?.the tensile strength reached a maximum value of 1007 MPa.The elongation increased from 34.6%to 39.0%with increasing temperature.Heat-treated at a partitioning temperature of 450 ?,the production of strength and ductility exhibited the maximum value of 38649 MPa·%(tensile strength of 991 MPa,elongation of 39%).At a constant temperature of 450 ?,the content of retained austenite gradually decreased from 21%to 12%with an increase in the partitioning time from 10 s to 300 s.In addition,the carbon content of retained austenite reached the maximum value of 1.77%for the steel partitioning for 60 s at 450 ?.At room temperature,under a strain rate of 1.7×10-3/s,the tensile strength gradually decreased from 999 MPa to 871 MPa with an increase in the partitioning time from 10 s to 300 s.For the steel partitioning for 300 s,both the tensile strength and elongation revealed the minimum value of 871 MPa and 30.0%,respectively.At a constant strain rate,both the yield strength and tensile strength decreased with an increase in the deformation temperature.Elongation exhibited a highest value at room temperature.The product of strength and ductility showed the highest value of 49610 MPa\·%(tensile strength of 1210 MPa,elongation of 41.0%)at the lowest temperature of-70 ?.At 300 ?,the product of strength and ductility reached the lowest value of 36812 MPa·%(tensile strength of 918 MPa,elongation of 33.8%).The yield strength,tensile strength and elongation gradually decreased with increasing strain rates.At-70 ?,the tensile strength and elongation decreased from 1210 MPa and 41%to 1136 MPa and 34.9%,respectively,as strain rate increased from 2×10-3/s to 2×10-1/s.At room temperature and a constant strain rate of 5×10-3/s,the content of retained austenite determined by XRD gradually decreased from 12%to 2%with an increase in the strain from 0.1 to 0.5,The KAM analysis showed that the transformed amount of austenite was gradually and slowly.The transformation of austenite into martensite played a key role in the plastic deformation at lower strain levels.During the deformation,the coordinate deformation of ferrite and austenite maintained a uniform process of plastic deformation,resulting in a higher product of strength and ductility. |
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