| In recent years,the total amount of vehicles in China has been increasing,leading to serious problems such as exhaust and energy consumption caused by automobile emissions.How to improve the product of strength and elongation(PSE)of sheet steel for automobile and realize the light weight of automobile as much as possible while taking into account the driving safety,so as to achieve the values such as energy saving and emission reduction,safety and low consumption has become the focus of attention and research.At present,as the third generation of advanced high-strength steel,medium manganese steel(3?11 wt.%of Mn)has a great development prospect in automobile application due to its excellent strength,elongation,PSE,crashworthiness and safetyIn this paper,medium manganese steel with three different composition systems,such as 5Mn,5Mn-Nb-Mo and 4Mn-Nb-Mo are designed.The microstructure evolution,mechanical propertie,work hardening behavior,strengthening and plasticizing mechanism,austenite stability and TRIP effect of the experimental steel after varietys of microstructure control heat treatment processes are mainly studied,which provides experimental and theoretical basis for the microstructure regulation,mechanical properties optimization and industrial application of the medium manganese steel.The main experimental results obtained in this paper are summarized as follows(1)The optimum parameters of austenite reverted transformation(ART)process for 5Mn experimental steel are intercritical annealing at 625? for 4h and water cooling to room temperature.The hot-rolled+ART,warm-rolled+ART,and cold-rolled+ART experimental steels all show excellent PSE,among which the 500? warm-rolled+ART experimental steel has the best performance,i.e.the content of retained austenite reaches 56.8%,the tensile strength of 1001 MPa,the elongation of 57.5%,and the PSE of 57.6 GPa·%.(2)The mechanical properties of 5Mn-Nb-Mo cold-rolled experimental steel after quenching and tempering(Q&T)are better than those of hot-rolled experimental steel The cold-rolled experimental steel,which was quenched at 625?675? for 30 min and then tempered at 200? for 15 min,obtained excellent comprehensive properties,i.e.the retained austenite content of 39%,the tensile strength of 1059?1190 MPa,elongation of 33?40%,and PSE of?41.0 GPa·%;comparing with the best hot-rolled HR-650 sample,the best cold-rolled CR-650 sample has a larger and deeper dimple size,which in turn shows a better elongation.(3)The optimum parameters of intercritical annealing+quenching and partitioning(IA&QP)process for 4Mn-Nb-Mo hot rolled experimental steel are intercritical annealing at 760? for 30min,isothermal quenching at 180? for 10s and then isothermal partitioning at 350? for 180s;the hot-rolled experimental steel showed the optimum mechanical properties under this process,i.e.,tensile strength of 1231 MPa,elongation of 24.8%,and the PSE of 30.5 GPa·%.After the IA&QP process,the tensile strength of 4Mn-Nb and Mo hot-rolled experimental steels exceeded 1024 MPa,but the elongation and residual austenite content were not high.(4)The 4Mn-Nb-Mo cold-rolled experimental steel treated with a novel cyclic quenching and austenite reverted transformation(CQ-ART)process has been significantly refined in grain size and the retained austenite content has been significantly increased.The cold rolled CQ2-ART sample after twice cyclic quenching has the highest retained austenite content(62.0%),the best grain size(0.40 ?m)and stability;they provide a strong guarantee for the generation of TRIP effect during the deformation of retained austenite,and finally obtain the optimal comprehensive properties,that is,the tensile strength of 838 MPa,the elongation of 90.8%,and thePSE reaches 76.1 GPa·%.(5)The stability factors of austenite in 4Mn-Nb-Mo and 5Mn-Nb-Mo experimental steels were studied.It was found that the content of Mn element was the main factor affecting the stability.Retained austenite with different grain size and Mn content has different levels of austenite stability.There is obvious Mn partitioning behavior in the retained austenite of the experimental steel,which in turn results in the retained austenite having different levels of stability and therefore shows different work hardening behavior.Both 4Mn-Nb-Mo and 5Mn-Nb-Mo low-alloy medium-Mn experimental steels designed in this paper have the obvious comprehensive performance advantages while achieving the purpose of minimizing the total alloy element content.(6)Large fluctuations in the work hardening rate curves of the three experimental steels at the S3 stage are due to the discontinuous TRIP effect.The reason for the discontinuous TRIP effect is that the retained austenite transformed into martensite during the tensile process and volume expansion occurs,which in turn offsets part of the stress concentration and causes the stress to be transferred to the surrounding phase to produce cooperative deformation,accompanied by stress relaxation and transfer;secondly,the retained austenite in the experimental steel needs to have the stability of different grade batches,and the TRIP effect can only occur when the stress reaches the critical value of the phase transformation of the retained austenite in the batch(7)The MS? point is affected by the chemical composition,grain size,yield strength,and stress state of the retained austenite.More and more stable retained austenite can be obtained by controlling the MS? temperature of the experimental steel below the service temperature,thus producing more extensive trip effect,and finally improving the comprehensive properties of the experimental steel. |
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