Research On The Fabrication Technology Of Nano/Ultrafine Grained High-Strength Steel And Microstructure-Mechanical Properties Control

Grain refinement is one of the most effective ways to increase the strength of materials by concurrently maintaining toughness with less alloy addition.Multiple processing treatments have been successively developed to explore the ultimate performance of（ultrafine grain）UFG steel.Nonetheless,the most dramatic drawbacks of UFG steels are,without doubt,poor ductility or work hardening ability and poor thermal stabilization.The further refined UFG steels with a super high strength could not meet the requirement of engineering application materials due to their terrible ductility.In the present study,a novel method of cryorolling combined with annealing for fabricating UFG steels with attractive strength was proposed,providing a new idea for the preparation of new high-strength UFG steels.Microalloy element Nb,showing excellent grain refinement and precipitation strengthening effects in hot-rolled steels,was moderately added in a low carbon steel.The purpose is to explore an effective way to further improve the strength of UFG steel with better ductility or work hardening ability.Conclusions are as follows:（1）At 50%reduction,compared with ambient temperature deformation,more distortion energy and deformation defects were accumulated during cryorolling process at liquid nitrogen temperature,leading to the decreased recrystallization temperature of ferrite.In addition,the dynamic recovery of dislocations was inhibited during cryorolling process,and more nucleation points can be formed,resulting in the refinement of ferrite grain（330 nm-132 nm）.However,the strength and elongation of the UFG steel prepared by cryorolling decreased due to the inferior work hardening ability caused by finer ferrite grains and larger Fe₃C particles.（2）The ferrite grain first refined and then coarsened with the increase of cryorolling reduction from 50%to 80%,and there was valley value in the grain size of ferrite under the reduction of 70%.With the reduction increasing to 80%,the further growth of ferrite grain is attributed to the secondary recrystallization of ferrite.The optimal tensile strength of 978.1 MPa and adequate elongation of 12.30%were obtained under the reduction of 65%and annealing at 500°C for 30 min.However,the mechanical properties were still worse than that of UFG steel（860.7 MPa and 16.70%）at ambient temperature deformation.（3）The work hardening of UFG steels is affected byαandθphase in theα+θphase low-carbon UFG steels.The relative coarseαphase and large number of fine intragranularθparticles contribute to the better work hardening.The intragranularθparticles play a significant role in the improvement of work hardening,because the geometrically necessary dislocations are apt to form and store around the intragranularθparticles.The intergranularθparticles are more sensitive to the processing route and are easy to be coarsened compared to the intragranularθparticles.Therefore,the intergranularθparticles result in the decreased dislocation accumulation ability ofα-ferrite,and impair the strength of grain boundaries and work hardening ofα+θultrafine grain steels.（4）The optimal tensile strength of 990.8 MPa and adequate elongation of 16.71%were obtained by adding 0.028 wt.%Nb in a 0.165 wt.%carbon UFG steel under the reduction of 65%and annealing at 500°C for 45 min.The strength was substantially enhanced without any sacrifice of elongation comparing to the normal UFG steel（without Nb）.The main increased strengthening mechanisms in Nb-UFG steel were the precipitation and dislocation strengthening.The improved work hardening and adequate elongation of Nb-UFG steel were guaranteed by the geometrically necessary dislocations（GNDs）and heterogeneous ferrite grains.（5）Two main reasons accounting for the better work hardening in the Nb-UFG steel are schematized in two types.The first contributor is the initiation and accumulation of GNDs induced by the Nb C particles.Another is the heterogeneous ferrite grains which generate the local strain gradient（the coarse grains tend to be deformed due to the low deforming resistance）in the deformed grains during tensile,making the uniaxial stress state become much complex and finally promoting the dislocation accumulation and interaction.（6）The strength and elongation of Nb-UFG steel remained unchanged with the increased annealing temperature and prolonged annealing time,showing better mechanical properties stability.The grain refinement mechanism was guaranteed,and to avoid the nonequilibrium transition.In addition,the heterogeneous ferrite grains in Nb-UFG steels result from the discontinuous static recrystallization transformed to uniform distribution with the annealing time.（7）Compared to the Nb-free UFG steel,the completely dissolved Nb in the Nb-UFG steel before annealing was beneficial to improve the thermal stability and strength but harmful to plasticity.However,the thermal stability and strength of the Nb-UFG steel were also enhanced without any elongation sacrifice when Nb was partially dissolved in matrix before annealing.In addition,although the thermal stability of the steel with completely solute Nb before annealing was better than that with partially solute Nb,the strength and elongation of the former decreased due to the inferior work hardening ability caused by finer ferrite grains and larger Fe₃C particles.