| Advanced high-strength steels have a wide range of applications in key engineering areas of the national economy such as transportation,shipping,construction and even aerospace.In particular,one of these steels,medium manganese steel,has become a hot spot of interest and research as it has ultra-high strength and toughness while balancing cost and safety.The complexity of the microstructure of medium manganese steels and its potential mechanisms with respect to mechanical response have not been adequately discussed.In this paper,the causes of the band structure,the influence of the initial hot-rolled matrix,the role of nano-scale microalloy carbides and the intrinsic link between yield strength and microstructure are systematically investigated and discussed.The design concepts of "pre-deformationtempering","deep cooling-tempering",vanadium micro-alloying and low-temperature annealing processes are proposed to provide a theoretical basis and technical support for ultra-high toughness z-medium manganese steels.The specific work and results are as follows:(1)The "intercritical annealing-pre-strain-tempering" process is designed for medium to high manganese alloyed Fe-0.165C-10Mn-0.5V(wt.%)martensiticaustenitic mixed base Mn steels,eliminating the initial large size banded sub-stable austenite with low stability by introducing micro-deformation induced martensitic phase transformation,while the annealing process avoids stress concentration in the new martensite and improves ductility.Pre-strain increases the martensite fraction and decreases the austenite fraction.At a pre-strain of 0.02,the steel has optimum mechanical properties with a tensile strength of 1506 MPa,a yield strength of 1264 MPa and an overall elongation of 19%.The increase in yield strength of the steel is mainly due to the dislocation strengthening produced by strain-induced martensite and the precipitation strengthening of microalloyed carbides during the tempering stage.Excessive pre-strain reduces residual austenite,preventing it from providing sufficient slip space for dislocations during loading,resulting in a reduction in the ductility of the steel.(2)The "deep cooling+tempering" process is designed to increase the driving force of the martensitic phase transformation through the temperature difference generated by deep cooling and to promote the partial transformation of the banded austenite into thermally transformed martensite.Thermally transformed martensite is similar to strain-induced martensite in that it cuts through the initial banding and refines and improves austenite stability.At the same time,during the tempering or annealing process,the thermal phase transformation martensite undergoes an austenite reverse transformation(ART),which refines the steel whilst reducing its composition segregation.In addition,the deep cooling process promotes the precipitation of fine carbides and increases the strength of the steel.A combination of excellent tensile strength of 1400 MPa,yield strength of 1080 MPa and elongation of 26.8%was achieved by deep cooling.(3)A hot-rolled medium manganese steel "0.12C-10Mn-0.05V/0.5V" with martensite as the main matrix has been designed based on the concept of low carbon and vanadium microalloying.The organisation genetic effect of the hot-rolled martensitic matrix was clarified by performing an austenitic reverse transformation annealing(ART)study,which favours the formation of uniformly alternating lath martensite(intercritical ferrite)and sub-stable austenite.The vanadium carbide particles in the steel are mainly distributed in spherical,ellipsoidal and elongated fibrous shapes within the ferrite grains or at grain boundaries.The effect of "vanadium microalloying" has been optimised and,together with the "low temperature annealing"process,excellent comprehensive mechanical properties have been achieved by regulating the type of carbide,the amount of sub-stable austenite and the dislocation density.The mechanism of the effect of vanadium microalloying on the yield strength,ductility and work hardening rate of the steel was clarified.In high vanadium manganese steels,vanadium carbides are preferentially produced with carbon,preventing the formation of brittle cementite.At the same time,the high density of vanadium carbide slows down the growth of austenite and the recovery of dislocations.However,the carbides are easily solidified at high temperatures,so the strengthening effect of vanadium is favoured at low and medium temperatures during annealing.It is worth noting that 0.5 wt.%V tends to produce a segregation at the interface,which hinders the crossing-interface slip of dislocations and delays the occurrence of austenite phase transformation induced plasticity(TRIP),which is not conducive to strain coordination between austenite and ferrite and reduces the ductility of the steel.(4)A one-step hot dip aluminium process has been designed for the martensiticbased medium manganese steel "0.3C-4Mn" with low to medium manganese to achieve properties modulation and corrosion resistance enhancement of medium manganese steel.It was found that the immersion layer mainly consists of an aluminium layer and an FeAl3/Fe2Al5 intermetallic compound layer with a low ductility.The steel surface layer flakes off when the strain exceeds 5.2%,but the residual Fe2Al5 layer is still resistant to corrosion.The one-step hot-dip aluminium process simplifies the production process and improves the corrosion resistance of the steel,with the corrosion current density jcorr decreasing from 26.4 μA/cm2 to 4.5μA/cm2 while maintaining good mechanical properties with a tensile strength of 1217 MPa,yield strength of 953 MPa and elongation of 13.1%,at the same time. |
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