Microstructure Regulation And Strengthening Mechanism Of Aluminum-Containing Hot Rolled Medium Manganese Steel With High Product Of Strength And Elongation

With the development of the automobile industry,ultra-high strength steel is used in the automobile industry to improve safety,fuel economy and protect the environment.Medium manganese(3 ～ 15 wt.% Mn)Steel,which belongs to the third generation of Advanced High Strength Steel(AHSS),is becoming more and more attractive due to its advantages in mechanical properties,material cost and material processing.The Transformation-induced Plasticity(TRIP)effect and Twinning-induced Plasticity effect may occur in medium manganese steel during deformation.Many studies have shown that the deformation behavior of Fe-Mn-Al-C series manganese steel is closely related to the content and stability of Retained Austenite(RA)in steel,and the above factors depend on composition design,processing and heat treatment.Aluminine-contained medium manganese steel with medium carbon content(0.3～0.35 wt.%)demonstrates excellent mechanical properties and good weldability,which has a wide application prospect.However,its microstructure control strategy and deformation mechanism still need to be further investigated.In this study,the phase volume fraction evolution,lattice strain and phase strain load distribution of critical quenching and cyclic quenching 5Mn hot rolled steel during uniaxial tensile process have been studied by means of High Energy X-ray Diffraction(HE-XRD).The effects of critical quenching state and cyclic quenching on microstructure,deformation behavior,strengthening mechanism,mechanical properties and strain hardening behavior of9 Mn hot-rolled quenched steel were analyzed by using Electron Backscatter Diffraction(EBSD)technique.The results show that:(1)The 9Mn experimental steel has complex structure of Mn-rich segregation band and packet structure,and the austenite orientation in the same packet structure is consistent;Appropriate critical quenching process(650 ℃×1 h)promotes austenite reversing and avoids excessive grain growth and coarsening.The yield strength(YE),ultimate tensile strength(UTS),total of elongation(TE)and product of stenth and(PSE)reached up to 788 MPa,1180 MPa,61.4% and 72.5 GPa ·% respectively.After cyclic quenching,the YE,UTE,TE and PSE of 9Mn hot-rolled quenched steel are further improved to 750 MPa,1152 MPa,67.0%and 77.2 GPa·%,respectively.Compared with the critical annealed experimental steel,it is reduced by 11.8%,2.3%,9.7% and increased by 9.1%,respectively.The strain hardening behavior analysis shows that the cyclic quenching further controls the stability distribution of RA,promotes the full transformation of the RA,guarantees the discontinuous TRIP effect,and finally obtains more excellent mechanical properties.(2)During the uniaxial tensile process of critical quenched 9Mn hot-rolled experimental steel,the favorable orientation packet structure absorbs strain through rotation,while the adjacent unfavorable orientation packet structure undergoes martensitic transformation to withstand strain.The two substructures coordinate to adapt to deformation.A phase distribution parallel to the Loading direction(LD)is formed.With the increase of deformation degree,martensitic transformation occurs in the favorable orientation of the packet austenite,while in the unfavorable orientation of the packet martensitic/ferrite grains,the formation of low angle grain boundaries(LABs)and further evolution into high angle grain boundaries(HABs),leading to the grain fragmentation of the ferrite/martensitic phase.Appropriate critical quenching process(650 ℃×1 h)can obtain the packet structure with wide distribution of austenite grain size.The wide distribution of austenite stability provides a guarantee condition for the continuous strain-induced martensite transformation in a larger strain range,which makes the austenite in the segregation region with high stability also undergo a large number of transformations,and obtains excellent mechanical properties.(3)For 5Mn hot rolled quenched steel,cyclic quenching significantly increases the C/Mn content in austenite,but has no significant effect on the C/Mn content in ferrite;There is no martensitic transformation of critical quenched experimental steel during uniaxial tensile process.After the elastic stage,the austenite changes into martensite gradually,and the volume fraction of each phase shows a discontinuous transformation characteristic of step-change.When the critical corresponding change point is reached,the austenite changes to martensite,the load is distributed to martensite,and the corresponding change of austenite decreases.