Microstructure-Properties Regulation And Strengthening-Toughening Mechanism Of Ultra-low Carbon Medium Manganese Steel Plate

Medium manganese steel plate possesses excellent strength,ductility and toughness,which has great potential in the field of engineering machinery,offshore platform and petrochemical industry.However,the research and development on high strength and ultra-high strength medium manganese steel plate is still immature.More importantly,the microstructure-mechanical properties relationship and strengthening-toughening mechanism during the deformation and heat treatment processes are not clear yet.Taking ultra-low carbon medium manganese steel as research object,this paper analyzed the micro-mechanism of microstructure and properties control of medium manganese steel plate,from aspects of austenite stability,laminated microstructure,evolution and strengthening of precipitates and so on.Moreover,the strengthening-toughening essence under different alloy compositions and deformation and heat treatment processes was revealed.The main contents and results are as follows:(1)Thermal stability of reverted austenite and mechanical stability of retained austenite were studied,and toughening mechanism of metastable retained austenite was analyzed.The results showed that refinement of grain size significantly improved the thermal stability of reverted austenite in ultra-low carbon medium manganese steel,mainly by decreasing Ms and increasing elastic strain energy of martensitic transformation nucleation.However,its effect on mechanical stability of retained austenite was insignificant.Two-step intercritical annealing process endowed austenite with excellent thermal stability and relatively low mechanical stability.Excellent thermal stability guaranteed the stable existence of retained austenite with volume fraction of 24.3%at-80?.Low mechanical stability provided positive transformation-induced plasticity(TRIP)effect during impact deformation.TRIP effect of retained austenite alleviated local stress concentration,suppressed crack propagation,and hence improved the crack propagation energy significantly.(2)Medium manganese steel with laminated microstructure was prepared by non-recrystallization controlled rolling-warm rolling technology.Mechanism of delamination fracture and its toughening essence were studied.It was shown that the delamination fracture during impact deformation consisted of interface-decohesion delamination and quasi-cleavage delamination.The laminated micro structure and interfaces with weak bonding force could promote formation of interface-decohesion delamination,while numerous {100} cleavage planes that parallel to rolling plane were helpful in formation of quasi-cleavage delamination.The delamination fracture promoted the main crack to branch along brittle {100} crystal planes and interfaces of laminated microstructure that parallel to rolling direction.Thus,the propagation of main crack along V-notch direction was inhibited,and the resistance to impact fracture was significantly enhanced.Delamination fracture endowed the medium manganese steel plates with excellent impact toughness of room temperature impact energy higher than 450 J and-196? impact energy of 105 J.(3)Evolution of morphology,crystal structure and chemical composition of Cu-rich precipitates during nucleation,growth and coarsening was studied,and the corresponding strengthening mechanism as well as strengthening effect was also analyzed.The results revealed that Cu-rich precipitates tempered at 500? and 550? for 1 h possessed body-centered cubic structure and were coherent with the matrix,containing substantial Cu,Mn and Ni in the core region.The average equivalent radius was 1.35 nm and 2.59 nm,and the number density was 4.11×1023 m-3 and 1.43×1023 m-3,respectively.Precipitation strengthening mechanism for these two samples was shearing mechanism,with strengthening contribution of 266 MPa and 312 MPa,respectively.As tempering temperature increased to 600?,the concentration of Cu atoms in the core of Cu-rich precipitates was increased,and the concentration of Fe,Mn and Ni atoms was decreased.At the same time,these precipitates coarsened to ellipsoid or rod-like and possessed a stable face-centered cubic structure.The average equivalent radius of precipitates increased to 3.24 nm and number density decreased to 0.86×1023 m-3.Orowan strengthening with a contribution of 232 MPa became the only strengthening mechanism.(4)Relationship between microstructure and mechanical properties of Cu-precipitation strengthened medium manganese steel was studied.Strengthening-toughening theory involving the coupling effect of strengthening from Cu-rich precipitates,and plasticizing and toughening from austenite was explored.It was shown that introduction of Cu to medium manganese steel could not only facilitate formation of Cu-rich precipitates,but also promote dissolution of cementite particles and formation of reverted austenite.Cu-rich precipitates strengthened the matrix and increased the yield strength significantly.Formation of reverted austenite reduced brittle cementite particles at grain boundaries,inhibited intergranular fracture,and hence improved the impact toughness effectively.Retained austenite with volume fraction of 23%and appropriate stability improved the ductility.The coupling effect of strengthening from Cu-rich precipitates and plasticizing-toughening from austenite significantly improved the mechanical properties of medium manganese steel plates.(5)An "Intercritical annealing-tempering" heat treatment process was designed.Evolution of austenite and precipitates during annealing and tempering was studied,and the correlation of microstructure and mechanical properties was clarified.The results showed that intercritical annealing promoted dissolution of cementite and formation of retained austenite,and then improved ductility and toughness.While tempering treatment was helpful in formation of secondary precipitation strengthening,and then improved the yield and tensile strength.Secondary precipitation mainly came from two aspects:one was the precipitation of a small amount of Cu-rich precipitates and NiAl phase in ferrite matrix;the other one was the precipitation of high density nanoscale Cu-rich precipitates in secondary martensite.The precipitation strengthening effect in secondary martensite was significant,and increased yield strength of tempered medium manganese steel by 75 MPa.The "intercritical annealing-tempering" process optimized austenite reverted transformation and Cu precipitation simultaneously,which endowed the medium manganese steel plate with excellent combination of strength,ductility and toughness.The yield strength,tensile strength and total elongation of the medium manganese steel plate reached to 960 MPa,1010 MPa and 29.2%,respectively,with outstanding impact energy higher than 100 J at-40?.