Study On The Evolution Behavior And Strengthening And Toughening Mechanism Of Cold Rolled Low Carbon Martensitic Hot Stretching

As an important material of iron and steel,low carbon martensitic steel has high strength and good toughness,and its substructure is fine,which provides ideal original structure for grain refinement of recrystallization in large plastic deformation.The analysis of the effect of thermal deformation on the recrystallization of deformed martensite has important academic value and engineering application prospect.In this paper,lath martensite microstructure was obtained by quenching heat treatment in 15 steel.The lath martensite was cold-rolled by 80% reduction,and hot tensile deformation at 300?-650?for 1 hour was carried out,respectively.Nanocrystalline/submicron ultrafine grained steel plates were prepared.The effects of thermal tensile recrystallization mechanism at different temperatures and delamination fracture on the toughness of the materials were systematically analyzed by means of OM?SEM?TEM and so on.The results show that obtained grain sizes of 50 nm,300nm,1.5 ? m and 3 ? m nano/submicron steel plates at 400?,500?,600? and 650? heat drawing for 1h;Mechanical properties measured at 400?heat drawing tensile,tensile strength and yield strength,respectively 1724 MPa,1691MPa,elongation rate is 10.1%,and at each temperature of heat drawing,yield ratio of the sample is 1,which indicated that the yield strength has significantly improved.SEM observation and analysis show that there are a large n?mber of weak interfaces in the steel,and the mechanical properties are obviously improved.The weak interface hinders the dislocation movement,deflects the main crack propagation direction,produces discontinuous cracks and forms layered fractures.This fracture mechanism increases the surface area of the fracture surface and improves the toughness of the material.The results of TEM analysis show that the mechanism of recrystallization of cold deformed lath martensite is "continuous recrystallization",and the speed of the grain growth is slower than that of conventional recrystallization.The deformed lath martensite and dislocation cell substructure are gradually decomposed into ultrafine grains andnanocrystalline carbide is precipitated uniformly along the weak interface and arranged sequentially on the equiaxed ferrite matrix.Based on the analysis of hardness,yield strength and tensile strength curve,the heat tensile temperature of deformed lath martensite ranges from 550? to 600?,the hardness decreased from 351 HV to 149 HV,and the tensile strength decreased by 787 MPa,the yield strength decreased by 787 MPa.Therefore,the mechanical properties of the material decreased dramatically at this temperature stage,indicating that in the temperature range the material began to lose stability compared with the conventional annealing,the unstable temperature of heat tensile deformation is increased by 150 ?,which indicates that hot deformation can improve the thermal stability of deformed martensite of 15 steel.