| Today's society is facing the dual challenges of energy crisis and environmental degradation.It is urgent to save energy and reduce emissions.As a typical representative of the third-generation advanced high-strength steel,medium Mn steel has the advantages of low density and low cost.It has a combination of ultra-fine metastable austenite and ferrite.Through the transformation induced plasticity(TRIP)effect,an excellent combination of high strength and high elongation can be obtained,which is of great significance for improving fuel economy and collision safety.This paper takes 4Mn-3Al-0.38 C wt.% forged and hot-rolled medium Mn steel as the research object,and adopts the heat treatment process of sub-temperature quenching + low temperature tempering.Exploring the microstructure transformation law,mechanical properties and strain hardening behavior of medium Mn steel in forged and hot-rolled under different sub-temperature quenching temperatures.Finally,analyzing the relationship between its organizational structure and mechanical properties,explore its strong plastic mechanism and organizational control strategies.Providing theoretical guidance for the industrial production of high product of strength and elongation(PSE)medium Mn steel materials.The main conclusions are summarized as follows:(1)The forged experimental steel was water quenched at 730 °C for 60 minutes and then tempered at 200 °C for 20 minutes to obtain a dual-phase homogeneous medium Mn steel material of ultrafine ferrite and retained austenite.Its retained austenite content reaches55 vol.%.The process of tensile produces obvious TRIP effect.The dislocation density increases after tensile fracture,and the Portevine-Le Chatelier(PLC)effect caused by strain localization is not found.The deformation coordination is good,the tensile strength reaches900 MPa,and the elongation reaches 76.1 %,with an ultra-high PSE of 68.5 GPa·%.The samples were water-quenched at 780 ? for 60 minutes and tempered at 200 ? for 20 minutes to obtain a three-phase structure of ferrite,martensite and retained austenite.The strain hardening rate is high,the tensile strength is up to 1373 MPa,and the elongation rate is up to(2)After the hot-rolled experimental steel was water quenched at 720 °C for 60 minutes and tempered at 200 °C,the cross section of the steel obtained heterogeneous lamella structure(HLS)composed of ultrafine austenite layer and coarse ferrite layer.It is called heterogeneous medium manganese steel material.Its retained austenite content reaches 63.8 vol.%.The grain size has a wide range of distribution,and a continuous TRIP effect is produced in a wide strain range.The heterogeneous material has a hetero-deformation induced(HDI)hardening effect.After the tensile fracture,the whole and local structure of the sample deformed evenly.Its tensile strength reaches 957.4 MPa.Compared with the homogeneous medium manganese steel material under the same heat treatment temperature,the heterogeneous medium manganese steel has higher strength and the elongation rate reaches 77.0%.Obtained an ultrahigh PSE of 73.7 GPa·%.(3)During the heat treatment process,the C,Mn,and Al elements have obvious partition behavior.Among them,the C and Mn elements tend to be enriched in austenite,and the Al elements tend to be enriched in ferrite.The holding time is fixed,the quenching temperature rises,and the austenite grows up.The content of C and Mn in the austenite decreases,and the thermal and mechanical stability of the austenite decreases.In austenite with low mechanical stability,the TRIP effect is discontinuous in the tensile deformation,and a zigzag band appears on the stress-strain curve,that is,the PLC effect.As the quenching temperature increases,the tensile strength of the experimental steel continues to increase,and the PSE tend to increase first and then decrease.After tensile fracture,the microhardness increases,and <111> oriented austenite has higher mechanical stability. |
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