| In recent years,the problems of energy shortage and environmental pollution are becoming more and more serious.There has been an increased emphasis on the development of advanced high strength steels(AHSS)in order to reduce vehicle weight and lower consumption of oil as well as to maintain or even improve safety in the automotive industries.Studies have shown that every 10%reduction in vehicle body mass will reduce fuel consumption by 8%-10%.Transformation induced plasticity(TRIP)steel is one of the AHSS,which was widely used in the automotive industries because of the comprehensive mechanical properties combined with high strength and excellent ductility.In this study,the Cu-TRIP steels after different rolled processes were heat treated by quenching and partitioning(Q&P)process.Subsequently,the tensile tests were carried out at different temperatures and strain rates to study the mechanical behavior of the Cu-TRIP steel.The volume fraction and carton content of retained austenite were analyzed by means of X-ray diffraction(XRD)and vibrating sample magnetometer(VSM).Microstructures were observed by optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electron back scattering diffraction(EBSD)techniques.The mainly results are shown as follows:.The microstructures of hot-rolled steel after the Q&P processing consist of the lamellar martensite,acicular ferrite,film-like retained austenite and bainite,while the microstructures of warm-rolled steel after the Q&P processing consist of M-A island,polygonal ferrite,Isolated island retained austenite and bainite.The former has a good combination of ultimate tensile strength(1117 MPa)and total elongation(44%).However,the latter exhibits a higher tensile ultimate strength(1330 MPa)and a lower total elongation(43%),with about 33%retained austenite,and the carton content of retained austenite is about 2.0%The differences of mechanical properties under different rolled processes are mainly attributed to the different microstructures,particularly the content and carbon concentration of retained austenite.For example,the former steel has a volume fraction of retained austenite about 33%,companied with carton concentration of?2.0%,whilst the latter has a volume fraction of retained austenite about 40.8%,and the carton concentration in retained austenite is about 1.81%.The tensile tests were carried out under different temperatures(25?-400?)and strain rates to study the influences of temperatures and strain rates on the mechanical properties.The results show that at temperatures in the range of 25? to 200?,the tensile strength decreased with increase of the deformation temperature at a constant strain rate;at a constant strain rate of 0.001 s-1,the tensile strength increased from 1154 MPa to 1227 MPa with the increasing temperature from 200? to 300?.At a constant temperature,the tensile strength of Cu-TRIP steel decreased with the increasing strain rate.The total elongation decreased with the increasing strain rate at a range of 100? to 400?.Keeping the strain rate at 0.001 s-1,the product of strength and elongation showed the highest value of 102706 MPa·%(tensile strength of 1154 MPa,elongation of 89%,yield strength of 744 MPa)under 200?.The softening effect of temperature,TRIP effect,and nanoscale precipitations are the main reason for the above phenomena.The effects of temperature and strain rates on the microstructures of Cu-TRIP steel were studied at various temperatures(25?-400?)and strain rates(0.001 s-1-l s-1).The results show that the volume fraction of retained austenite increased with the increase of the strain rate at a certain temperature.The micrographs of the fractural section in Cu-TRIP steel are characterized as obvious dimple at various temperatures,typically ductile fracture.At 400?,the number of elliptical voids increased and the dimple size increased with the increase of strain rate. |
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