Investigation On The Dynamic Mechanical Behavior And Formability Of High Strength-ductility Low Carbon Quenching-Partitioning-Tempering (Q-P-T) Steel

The research and development of advanced high strength steels?AHSSs?always follow the basic principles:energy conservation and environmental protection,high quality and low price.In 2007,Hsu proposed the quenching-partitioning-tempering?Q-P-T?process based on the quenching-partitioning?Q&P?process proposed by Speer et al.so that the Q-P-T steel became a new member of AHSSs.In recent ten years,the quasi-static(10^-510^-1 s^-1)mechanical behavior and the strength-ductility mechanism of the low,medium and high carbon Q-P-T steels were systematically studied by our group.In order to expand the application of the Q-P-T steels in collision components and complex forming components of car,it is necessary for us to further evaluate the dynamic mechanical behavior and formability of Q-P-T steels.We designed a low carbon and low alloyed steel?Fe-0.20C-1.49Mn-1.52Si-0.58Cr-0.051Nb?wt.%??in this work,and treated this steel by a novel Q-P-T process and traditional quenching and tempering?Q&T?one,respectively.The dynamic mechanical behaviors?tension&compression?were conducted with Hopkinson bar machine and their formability were performed with sheet forming test equipment,and the microstructure was characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?and transmission electron microscopy?TEM?for investigating microstructural evolutions.By comparing the microstructures of the Q-P-T steel with Q&T steel,the mechanism of high strength-ductility and excellent forming ability of the Q-P-T steel were revealed.The main research contents and achievements in this thesis are summarized as follows:?1?In the dynamic tensile test,the yield strength?YS?and ultimate tensile strength?UTS?of the Q-P-T martensitic steel with a considerable amount of the retained austenite are higher and the total elongation is slightly lower than those in the quasi-static tensile test.This differs from the enhancements in both the strength and the elongation of the Q&T martensitic steel with little retained austenite in the dynamic tensile test.However,the product of strength and elongation?PSE?of the Q-P-T steel is still higher than that of the Q&T steel in both the dynamic and quasi-static tensile tests.Meanwhile,the PSE of both the Q-P-T steel and Q&T steel in the dynamic tensile test is still higher than that in the quasi-static test.?2?In the dynamic tensile test,the flow stress of Q-P-T steel was almost invariant with increasing strain,indicating that the balance between dynamic hardening and adiabatic softening.The Q&T steel shows the different dynamic mechanical behavior,namely,the flow stress decreases with the increase of the strain,which means that dynamic hardening is less than adiabatic softening.The volume fraction of the retained austenite(V_RA)in the low carbon Q-P-T fractured sample was determined to be 3.3%at strain rate of 500 s^-1 in the dynamic tension and is higher than that at the quasi-static test,indicating that the adiabatic heating effect caused by high strain rate enhances the stability of retained austenite.Especially,it was found that high strain rate in dynamic tension almost suppresses the dislocation absorption by retained austenite?DARA?effect existing in quasi-static tension and weakens the tansformation induced plasticity?TRIP?effect in the Q-P-T steel.The suppression of the DARA effect and the weakening of TRIP effect favorable for ductility cannot be effectively compensated by the adiabatic softening of the martensite matrix,which leads to the slight decrease in the elongation as compared with that of the Q-P-T sample in the quasi-static tension.For Q&T steel,since there are not DARA effect and TRIP effect,high strain rate in dynamic tension produces higher strain rate hardening and the adiabatic softening comparing with quasi-static tension,which gives rise to higher strength and ductiliy in dynamic tension than in quasi-static tension.?3?During dynamic compression,the flow stress of Q-P-T steel increases observably with increasing strain rate,but the flow stress of Q&T steel exhibits a smaller change,that is,Q-P-T steel more obviously demonstrates the positive effect of dynamic strain rate hardening.Both Q-P-T steel and Q&T one show the adiabatic heating effect,the calculated result based on experiments indicates that with the increase of compressive strain rate from 700 s^-1 to 5000 s^-1,the temperature rises from about 20?to about 150?.?4?At a high compressive strain rate of 5000 s^-1,the morphology of martensite dramatically changes,including the destruction of the most long straight martensite laths accompanying with the formation of martensite chunks,which is attributed to the formation of micro-shear bands,the disappearance of grain boundaries between laths and the formation of dislocation cells.In the condition of same compressive strain rate,the adiabatic heating effects of Q-P-T steel and Q&T one are almost the same,and high strain rate in dynamic compression almost suppress the DARA effect existing in quasi-static compression,and such two factors make martensite in Q-P-T and Q&T steels almost undergo the same mechanical behavior.However,the retained austenite in Q-P-T steel makes its dynamic compressive strain be slightly higher than that of Q&T steel and the flow stress be slightly lower than that of Q&T steel.?5?The low carbon Q-P-T steel consists of a martensitic matrix with retained austenite,while the Q&T steel consists essentially of single-phase martensite.The forming limit diagram?FLD?of Q-P-T steel demonstrates a larger working safe-zone than Q&T steel,which reflects that the former has larger plane strain value?FLD₀?,major strain value??₁?and minor strain value??₂?in the tension-tension region.This is attributed to the difference in microstructure,leading to a higher strain hardening exponent?n?and a higher true uniform elongation??_u?in Q-P-T steel.?6?The high strain hardening exponent?n?of Q-P-T steel stems from a low dislocation density in the martensitic matrix prior to deformation and a high strain hardening rate of the retained austenite.The high true uniform elongation??_u?,on the other hand,is attributed to a softening of the martensitic matrix during deformation due to the DARA effect and the TRIP effect caused by retained austenite.There are no DARA and TRIP effects in t_ah_ne_d Q_?&_u.T steel with little retained austenite,so it has the lower values of n?7?Compared with Q&T steel,there are fewer shear bands,deeper dimples and a greater number of tearing ridges in the tension-tension region of the Q-P-T steel,implying better formability in this region.