| This work aims to carry out intercritical annealing(IA)plus low temperature tempering(IT)heat treatment for a novel medium Mn steel with a small amount of Zr element based on“Multiphase,Metastable,and Multiscale”M~3theory.The effects of different heat treatment on the mechanical properties and microstructural of medium Mn steels were studied in this work.In addition,a new type of medium Mn TRIP steel with a heterogeneous structure was successfully fabricated through microstructural regulation,and the strengthening and toughening mechanism of the new type of medium Mn steel was explored,providing theoretical guidance and experimental support for the development and research of the third generation of advanced high strength steel.The main conclusions are as follows:(1)The IT process of cold rolled medium Mn steel is studied.The result shows that the comprehensive mechanical property of the sample is the best,when it is annealed at680℃for 10 min,and then tempered at 200℃for 20 min.Its UTS,YS,TE,and PSE are 1280 MPa,1022 MPa,53%,and 69 GPa·%,respectively.(2)The effect of low temperature tempering on the properties of cold rolled medium Mn steel annealed at different temperatures is studied.The results show that tempering treatment reduces the mechanical properties when the sample annealed at660°C.When the annealing temperature increases to 680℃,the UTS and TE increase from 1220 MPa and 48%for the un-tempered sample to 1280 MPa and 53%for the tempered sample,respectively,indicating an increase of the PSE from 59 GPa·%to 68GPa·%.When the annealing temperature is further increased to 700℃and 720℃,the tempering treatment has a little effect on the properties of the medium Mn steel.(3)The effect of low temperature tempering on the properties of cold rolled medium Mn steel annealed at different times is studied.The results show that the UTS,TE,and PSE of the samples have no obvious change when the annealing times are 5,30,and 120 min.When the annealing time is 10 min,the UTS,TE,and PSE are increased by 60 MPa,5%,and 9 GPa·%,respectively.(4)The micro-mechanism of the temperature tempering effect on the medium Mn steel is studied.The results show that temperature tempering treatment increases austenite stability by partitioning C from ferrite to austenite.When the annealing temperature is relatively low(660℃),the sample possesses a low volume fraction of austenite with high stability.It is found that low temperature tempering further increases the stability of austenite,and the TRIP effect is seriously suppressed during the tensile process,reducing the mechanical properties of the sample.The concentration of austenite in the cold rolled medium Mn steel samples is high when annealed at high temperatures(700℃and 720℃)or for extended periods(30 and 120 min).Thus the content of C in ferrite is relatively low,which has a weak effect on the stability of austenite;hence,the effect on its properties can be ignored.The austenite stability of medium Mn steel can be adequately increased by tempering treatment only when the annealing temperature and time are suitable(holding at 680℃for 10 min),and the TRIP effect can persistently occur.Accordingly,the comprehensive properties of medium Mn steel can be greatly improved.(5)Further studies found that the strengthening–toughening mechanism of the heterostructured medium Mn steel can be summarized as follows.Austenite of different morphologies has varying mechanical stability,allowing the TRIP effect to persist occur over across a wide strain range and resulting in excellent strength and high plasticity.The results reveal that the HDI strengthening of the heterogeneous structure is observed during the deformation stage and its value increase with the strain increase,which enhances not only the yield strength but also the work hardening ability of the medium Mn steel,which improves its plasticity.Therefore,the results show that dislocation strengthening,HDI stress hardening,TWIP effect,and TRIP effect contribute to the exceptional strength–ductility synergy of the heterostructured medium Mn steel. |
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