Dynamic Tensile Behavior Of Nano-structured Bainitic And Martensitic Steels

In order to reduce the weight of a car,the carbon emissions and environmental protection,the third generation of AHSS(advanced high strength steels,AHSS)is paid more attention to investigate and develop.Therefore,medium carbon steel has been investigated in the present work,mainly focusing on the influence of different heat treatments on conventional and dynamic tensile mechanical properties under different strain rates.Initially,one-step,two-step,three-step low temperature bainite transformation,quenching and tempering(QT)and quenching and partitioning(Q&P)processes were carried out to obtain microstructures including nano-structured bainite/martensite,retained austenite and carbide.Then,the microstructure scale and volume fraction of retained austenite were analyzed by OM(optical microscopy),SEM(scanning electron microscopy),TEM(transmission electron microscopy)and XRD(X-ray diffraction).The traditional tensile test was carried out and the micro-hardness was measured by Vickers hardness tester.Finally,one-step,three-step low temperature bainite transformation,QT and Q&P processes were chosen for the dynamic tensile tests under different strain rates to investigate the high speed tensile properties of nano-structured bainitic and martensitic steels.The main results are summarized as follows:(1)Compared with the traditional one-step bainitic transformation,multi-step low temperature bainitic transformation effectively reduced the average thickness of bainitic ferrite laths or plates as well as the size and volume fraction of retained austenite.Mechanical properties of two-step and three-step bainite transformation were greatly improved,i.e.,the strength was enhanced by 230～300 MPa and the elongation was improved by 5～10 % comparing with one-step bainitic transformation.(2)For QT and Q&P processes,nano-structured martensite and block retained austenite were obtained.Compared to the steel treated by three-step low temperature bainite transformation,nano-structured martensitic steel had much worse mechanical properties.(3)In the high speed tensile tests,steels with four different heat treatments showed good strain rate sensitivity,i.e,with the increasing strain rate,the strength and elongation were also improved.After dynamic tensile tests,the volume fraction of retained austenite reduced owing to the occurrence of phase transformation induced plasticity(TRIP)effect.(4)The product of strength and elongation for nano-structured bainitic steel treated by three-step low temperature bainite transformation was highest and increased from 13872 MPa% to 20184 MPa% with the strain rates increasing due to the finer microstructures and the TRIP effect,especially the existence of film-like retained austenite..(5)Scanning electronic microscopy observations showed that many dimples with different sizes were formed in all fractured surfaces of high speed tensile tested steels.The fracture mode of all tested steels with different heat treatments was ductile fracture.