Mechanisms Of Phase Transformation And Strengthening/plasticizating Of Lamellar-structured Low-alloy TRIP Steels

Lamellar-structured low-alloy TRIP steels with alternating bainite and ferrite layers exhibit high strength and plasticity but with low alloy cost and simple production processes,and are one of the solutions to the growing demand for lighter weight and higher safety in the automotive industry.However,the morphology and crystallographic orientation of intercritical ferrite and austenite on bainite transformation kinetics remain unclear.The crystallographic orientation of lamellar-structured low-alloy TRIP steels and their effect on tensile properties,and the effects of heat treatment parameters on the composition content proportion and strength-plasticity are still unclear.In this study,a systematic investigation of lamellar-structured low-alloy TRIP steel was carried out by thermal expansion,electron backscattered diffraction,quasi-in-situ X-ray,in-situ tension with micro digital image correlation techniques,and quantitative metallography.In addition,the regression analysis of heat treatment parameters on the mechanical properties of lamellar-structured TRIP steel was carried out using response surface analysis.These efforts will provide valuable guidance for renewing traditional low-alloy TRIP steels to achieve the goal of lighter weight,energy efficiency,and lower automobile production costs.The main findings are as follows.The effect of morphology and crystallographic orientation of intercritical ferrite and austenite on the kinetics of bainitic phase transformation was investigated.The results indicate that the interfacial length of the intercritical ferrite and austenite per unit area in lamellar-structured specimens is much higher than that in the polygonal-structured specimens.This provides a large number of nucleation sites for the subsequent bainite nucleation.In addition,the intercritical ferrite and austenite of lamellar-structured specimens exhibit a Kurdjumov–Sachs(K–S)orientation relationship.The activation energy of bainite nucleation at the K–S-oriented ferrite/austenite interface is much lower than that at the irrational-oriented ferrite/austenite interface.The mechanical properties,mechanical stability of RA,crystallographic orientation,and strain partitioning behaviour of lamellar structured low-alloy TRIP steels were systematically investigated and compared with polygonal structured specimens.RA grains in lamellar-structured specimens show high mechanical stability,allowing TRIP effects to persist at higher strains.In addition,the microstructure of lamellar-structured TRIP steels exhibits a hierarchical structure of the block,packet,and parent austenite grain.The misorientation angle between ferrite and bainite layers is small within a block,allowing plastic strain in ferrite layers to penetrate the adjacent bainite.The refined ferrite grains reduce the strength difference between the ferrite and bainite layers,improving the synergistic deformation ability of ferrite and bainite layers.The effect of the heat treatment process on the strength and plasticity of lamellar-structured low-alloy TRIP steel was systematically investigated.The results show that the intercritical annealing temperature affects the morphology and proportion of ferrite,bainite,RA,and pre-formed martensite.In the intercritical annealing temperature range of 780 ～ 860 ℃,the plasticity of tested steel increases with increasing RA fraction as well as the tensile strength.At a specific intercritical annealing temperature,the bainite austempering process affects the proportion of bainite,RA,and fresh martensite.The microstructure of specimens consists of ferrite,bainite,and RA in the austempering temperature range of 300 ～ 340 ℃.The tensile strength changes little,but the plasticity increases with increasing RA fraction and decreasing difference in ferrite/bainite lamellae strength.The appearance of pre-formed martensite and fresh martensite both reduce the plasticity of the test steel,but the pre-formed martensite effect the plasticity little.The effects of intercritical annealing temperature and time and the austempering temperature and time on the mechanical properties of lamellar structured TRIP steels were analyzed regressively.In present steel,the optimized heat treatment process for lamellar-structured TRIP steel is 850 ～890 ℃ for intercritical annealing temperature,8 ～ 14 min for intercritical annealing time,340 ～360 ℃ for austempering temperature,and 35 ～ 55 min for austempering time.The tensile strength of the test steel can reach more than 1100 MPa.The product of strength and elongation can reach more than 26 GPa·%.