Microstructure And Mechanical Properties Of Ultrafine-Grained F-M High Strength Low Carbon Dual Phase Steel Plate

Ferrite-martensite dual-phase(DP)steels have been widely used in automotive industry due to their excellent mechanical properties,such as high work-hardening rate and a good compromise between strength and ductility,which allows for a high energy absorbing performance.In order to fully explore the potential of DP steels and to expand the application,the dual-phase microstructure has to be optimized for better mechanical performance.Tremendous efforts have been devoted to improve the mechanical properties of DP steels.Grain refinement is the most effective way to strengthen the metaillic materials.And the ultrafinegrained dual-phase steel(UFG-DP)has been proved as a promising way of microstructure engineering.Many processing routes for achieving UFG-DP steels have been proposed,involving severe plastic deformation(SPD),advanced thermomechanical processing(ATMP)routes,but these methods are difficult and uneconomical for large-scale production.The industrial implementation of UFG-DP steels remains a challenge.In this thesis,low-carbon low-alloy steel was used to produce ultrafine-grained ferrite-martensite DP steel by novel processing route combining rolling and heat treatment.The effects of rolling temperature and annealing on microstructure and mechanical properties of UFG-DP steel were studied.The main findings of this work are listed as follows:(1)A new processing route to produce UFG-DP steel has been proposed,involving coldrolling and subsequent intercritical annealing of a fibrous ferrite-martensite starting structure.UFG-DP steel with an average ferrite grain size of about ?2.7?m and an average martensite island size of ?2.9?m was achieved.Tensile testing revealed superior mechanical properties(the ultimate tensile strength of 1267 MPa and uniform elongation of 8.2%)in comparison with the fibrous DP steels.The superior mechanical properties are attributed to the influence of microstructure refinement on the work-hardening and fracture behavior.(2)The ultra-strong nano-heterostructured steel presents a strength up to 2.1 GPa and the lamellae thickness of 22 nm,which was processed by warm rolling the fibrous dual-phase structure at 300 °C.(3)The effects of different rolling temperatures on the microstructure and mechanical properties of the ultra-strong nano-heterostructured DP steel were studied.The results show that when the rolling temperature is below 300 °C,the strengthening efficiency is increased with increasing temperature;while if rolling is conducted beyond 300 °C,the strengthening efficiency of rolling is decreased with increasing temperature.(4)The comparative study shows that the grain refining by warm rolling is more effective than cold rolling.