| Ultrafine bainitic steel,which was derived from the nanostructured carbide-free bainitic steel developed by university of Cambridge,has a remarkable combination of ultra high strength and excellent toughness.However,the tediously long time heat treatment limited its application in the industrial production.In this paper,combined with the composition optimization,supercooled austenite ausforming and prior martensitic transformation,a new ausforming-quenching and bainitic transformation(A-QBT)process was conducted to shorten the manufacturing period,and meet the demand of industrial production.The phase transformation mechanism and plastic deformation behavior of ultrafine bainitic steel have been systematically studied.The main work and results are as follow.Using the traditional nanobainitic steel as reference,three compositions of medium carbon ultrafine bainitic steels have been designed with different C,Mn,Cr,B or A1 contents.The transformation behavior of investigated steels was determined from MUCG 83 software,as well as CCT and TTT methods.The results showed that increasing Mn and Cr content decreased the chemical driving force for the transformation of austenite into ferrite,and dramatically extended the incubation period of bainitic transformation.B could effectively reduce the critical cooling rate,but the incubation period of isothermal bainitic transformation was increased and the transformation rate was decreased.The effect of different ausforming temperatures and strains on bainitic transformation behavior and plastic deformation mechanism were conducted using thermal simulation method and warm rolling.The results showed that the ausforming of supercooled austenite could accelerate the bainitic transformation when austempering at 300 ?.However,the ausforming process increased the stability of the austenite,that leading to the decrease of the final volume fraction of bainite.With the decrease of ausforming temperature,the incubation period and phase transformation completion time were significantly decreased,and the bainitic transformation rate was greatly accelerated.Meanwhile,the thickness of bainitic ferrite plates and retained austenite films were refined.When ausforming at low temperature with a small deformation(ausforming strain less than 30%at 300 ?),the bainitic nucleation rate was improved and the completion time was decreased,while the acceleration effect could be diminished with large ausforming strain.The crystal orientation of bainitic ferrite after ausforming was studied with EBSD.The results indicated that ausforming process reduced the number of variants in a single austenite grain,and with the decreasing ausforming temperature,the variant selection became more obvious.Compared to the traditional isothermal transformation process,the ausforming process could effectively refine the microstructure,as well as improve the hardness and strength of investigated steels.The size and the proportion of blocky retained austenite were decreased and the formation of deformation-induced martensite at the early stage of deformation was prevented.Therefore,the ductility was improved.However,the variant selection after ausforming deteriorated the toughness of the investigated steels.The effect of prior martensitic transformation on the bainitc transformation behavior was conducted using a dilatometer,and different volume fractions of martensite were introduced by quenching to certain temperatures followed by reheating to the isothermal bainitic transformation temperature.The results showed that with increasing prior martensite fraction,the nucleation sites and nucleation ratio of bainitic ferrite near the martensite plates increased,while the incubation period and completion time of bainitic transformation were significantly shortened.The effect of austempering time at the quenching temperature below the Ms temperature on the bainitic transformation and precipitation behavior of carbides in prior martensite was also investigated.Theoretical calculation and experimental results showed that less austempering time could lead to shorter diffusion distance of carbon atom,resulting in the increase of carbide precipitation time and more time for the formation of carbon depletion region.This phenomenon could increase the incubation time of bainitic transformation.The crystallographic relationship between prior martensite plates and adjacent bainitic plates was observed by EBSD.The results showed that prior martensite plates could affect the crystal growth of adjacent bainite,and bainitic lathes formed adjacent to prior martensite plate shared an almost identical orientation.The prior martensitic transformation could effectively reduce the scale of blocky retained austenite,refine the bainitic microstructure and improve the hardness,strength and ductility of investigated steels,but the impact was slightly decreased.The mixed effect of ausforming and prior martensitic transformation on subsequent bainitic transformation behavior has been elucidated,and the microstructural evolution models of different processes were established.Under the condition of high temperature deformation and slight deformation at low temperature,the nucleation of bainitic ferrite occurred at the prior martensite boundary and the slip plain of austenite.The ausforming and prior martensitic transformation process could both accelerate the bainitic transformation.However,the acceleration effect of prior martensite could be inhibited to some extent when ausformed at a low temperature.After comprehensive evalution of the combined effect of ausforming and prior martensite transformation,a novel process called A-QBT was conducted to fabricate the bainitic steel with nano carbide-free bainite.retained austenite and a small quality of martensite.The steels with yield strength of 1600 MPa,ultimate tensile strength of 1900 MPa and total elongation of more than 20%could be produced by isothermal heat treatment at 300 ? after ausforming at 300 ? with one-step compression strain of 30%.A multi-scale analysis method has been employed to analyse the essential relation between microstructure features and properties of the ultrafine bainitic steel.At the macroscopic level,the effect of various processing parameters on the strength and ductility of the investigated steel was studied.The fracture morphology indicated that the decreased dimension of retained austenite and refined microstructure could avoid the generation and propagation of cracks during tensile deformation.The TRIP effect of ultrafine bainitic steel has been studied by quantitative tensile tests.The results indicated that retained austenite could hardly transforme into martensite in the first stage of deformation.Both the ausforming and A-QBT process could improve the stability of retained austenite and refine bainitic microstructure.When the tensile strain was ranging from 0.02 to 0.1,the variation of retained austenite with strain was remarkable in ultrafine bainitic steel produced through ausforming process,while the A-QBT bainitic steel exhibited pronounced work hardening behavior in the later deformation stage.At the microscopic level,the constitutive model parameters of different phases were obtained by the combination of nano-indentation and finite element analysis.Two-dimensional representative volume element models were used to simulate the plastic deformation process of ultrafine bainitic steel.The results showed that the prior martensitic transformation and the ausforming process could inhibit martensitic transformation during the initial deformation stage.Moreover,the prior martensite undertook most of the stress,exerting a significant improvement of the yield strength. |
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