Microstructure And Mechanical Properties Of The 0.1C5Mn3Al Dual-phase Steel With Laminated Structure

Rencently heterogeneous lamella structure material are characterized by an excellent combination of high strength and good ductility.It is expected to enhance the strength and improve the ductility of dual phase steels by obtain heterogeneous structure.A laminated structure ferrite/martensite dual-phase steel 0.1C5Mn3Al which fabricated by hot-rolled has been cold-forged up to 60%,followed by annealing at temperatures up to750 oC.The the as-received,deformed and annealled specimens were measured by microhardness,tensile tests and nanoindentation.Optical microscope,scanning electron microscope and electron back scattered diffraction were used to characterize the microstructure after deformation and subsquent annealing.The main findings are summarized as follows:(1)After cold-forged to 60%thickness reduction a laminated structure were preserved,there is no change in the volume fraction of the two compotent phases i.e.ferrite and martensite phase.The average phase boundary spacings of ferrite and martensite phases were refined from 6.8?m and 6.2?m before cold forge to 2.2?m and2.6?m after cold forge respectively.The strength of the material after cold forging is 1.3GPa,but the plasticity is significantly reduced,the total elongation is less than 5%.(2)The lamella structure of dual phase steels has good thermal stability.After high temperature annealing for a long time,the microstructure of the dual phase steel can still maintain obvious lamellar structure.(3)With the increase of annealing temperature and the prolongation of time,the ferrite is obviously recrystallized,and recrystallized grain grows mainly along the original hot rolling direction.When annealed at 650 ~oC for 5 min,the recrystallized ferrite grains are mainly equiaxed grains.However,continue to increase the temperature or prolong the annealing time,the recrystallized grain grows into a bamboo shape.(4)The martensite phase decomposes at low annealing temperature and form carbide precipitates and new ferrite phases.At high annealing temperatures,the original martensite region evolves into equiaxed fine ferrite grains and martensite/austenite islands.With the increase of annealing temperature and annealing time,the size of martensite/austenite island increases obviously.(5)The results of nanoindentation test suggest that the hardness of ferrite and martensite is improved by cold forge,and the hardness of the interface between two phases is conformed to the rule of mixture.The interaction between dislocation and interface varies with annealing condition.