Phase Transformation Kinetics And Microstructure Control In Ultra-fine Carbide-free Bainitic Steel

Ultra-fine carbide-free bainitic(UCFB)steel,also known as nano-bainite(NB)steel,is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths.The UCFB(or NB)steel has an excellent combination of strength and toughness and is subject to wide attention in academia and industry.In the past two decades,many experimental and theoretical studies have been conducted on the composition design,process preparation,isothermal transformation mechanism,and strengthening and toughening mechanism of UCFB steel or NB steel.But there are still unresolved technical problems and unclear mechanisms,such as accelerated technology,phase transformation kinetics,and the evolution mechanism of retained austenite during plastic deformation.Therefore,the UCFB steels with medium and high carbon composition are designed in this present work to systematically study transformation kinetics of bainite and the evolution of microstructure and properties,reveal the influence of heat treatment processes on the microstructure and properties and strengthening and toughening mechanism of the UCFB steels.The work and results are as follows:The CCT(Continuous Cooling Transformation)curve and the TTT(Time,Temperature,Transformation)curve of two experimental steels(0.53C and 0.73C)are calculated using JMatPro software to obtain the critical solid phase transformation temperature parameters of the experimental steels.Three isothermal heat treatment processes are designed,and their influence on the bainite transformation behavior is studied.The three heat treatment processes are conventional austempering process(Conventional Austempering,abbreviation:CA,the same below),interrupted by martensite transformation austempering(IMA),and intercritical annealing-austempering(IAA).Compared with the C A process,the results show that a certain level of introduced martensite by the IMA process can increase the maximum bainite transformation rate and shorten the bainite transformation’s completion time to some extent.The increasing trend in overall bainite transformation rate can be observed with the increase in martensite content.The corresponding bainite transformation time becomes shortened;however,the maximum bainite transformation rate does not change significantly.The introduction of martensite can notably reduce the dimension of blocky retained austenite(BRA).Regarding the IAA process,as the content of intercitical ferrite increases,the incubation period and completion time of bainite transformation are both reduced,and the maximum bainite transformation rate is increased.Meanwhile,the introduction of intercitical ferrite can also lessen the dimension of BRA.The dint of EBSD observation determines the orientation relationship between different constituents.The corresponding resuts indicate that the crystallographic orientation of bainite ferrite(BF)and parent austenite belongs to N-W relationship,and the martensite plates and bainite adjacent to the interface belong to the same variant,Bain group or CP group.The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in the CA,IMA,and IAA processes based on the shear mechanism combined with the dilatometer test.Simultaneously,the energy of BF is obtained,including the initial nucleation activation energy and initial autocatalytic nucleation activation energy.The research results are as follows:during the CA process,as the isothermal transformation temperature decreases,the activation energy for BF nucleation decreases.For example,when the isothermal temperature decreases from 330℃ to 270℃,the initial energy of BF at the grain boundary is reduced from 172 kJ/mol to 164 kJ/mol,and the initial autocatalytic energy of BF is reduced from 141 kJ/mol to 132 kJ/mol.The kinetics model of bainite transformation during the CA process is optimized,and the formula of nucleation rate of BF at the martensite/austenite interface under the IMA process is deduced as(?).Similarly,the equation of nucleation rate of the BF at the intercitical ferrite/austenite interface in the IAA process is deduced as(?).The regulation procedures of mechanical properties of UCFB steel and the transformation toughening mechanisms of RA during plastic deformation have been studied.The results show that the proper heat treatment cycles can effectively tailor the dimension and distribution of RA and improve RA’s mechanical stability,which in turn avoids the formation of high work hardening behavior at the early stage of deformation,improving the mechanical properties.By controlling RA in the 0.53C steel,the tensile strength is increased from 1498MPa to 1831 MPa;the elongation is enhanced from 3.0%to 9.8%.The tensile strength of the 0.53C steel after microstructure optimization is equivalent to that of the 0.73C steel.When the content of film-like retained austenite(FRA)is equal,RA’s stability is mainly affected by the dimension and content of BRA.The transformation characteristics of RA of different shapes observed by the TEM technology indicate that the BRA and FRA both transform into martensite after tensile deformation.The twinning can be detected inside the transformed martensite.The transformation sequence of RA into martensite during tensile deformation is:BRA with a diameter greater than 1.26μm,FRA connected to BRA,and unconnected to BRA.Meanwhile,the stability of FRA is negatively correlated with thickness.