Surface Nanocrystallization And Properties Of Ferrite/Martensite Dual Phase Steel

Gradient nanostructure means that the structural unit(such as grain size or thickness)of a material continuously increases from the nanoscale to the macroscale in space.Gradient nanostructure can coordinate the microstructures with different feature sizes to improve the material properties(strength,hardness,wear resistance,fatigue properties,etc.).Ferritie/martensite dual-phase steel processes higher mechanical properties and great formability comparatively.So,it is helpful to prepare the gradient nanostructures on the surface of dual-phase steel by exploring the micro-deformation behavior and studying the effect of the two-phase deformation coordination during the preparation of gradient nanostructures.The low-carbon steels were heat-treated by using three heat treatment processes,intercritical annealing(IA),intermediate quenching(IQ)and step quenching(SQ),to obtain the dual-phase steel with different martensitic phase morphologies and volume fraction.Firstly,supersonic fine particles bombardment(SFPB)was used to prepare gradient nanostructures on the surfaces of dual-phase steels.Then,the mechanical properties,corrosion resistance were tested.Lastly,and finite element simulation was used to study the mechanical distribution.The results showed that:(1)Under the same process,three kinds of martensite microstructure morphologies produced gradient nanostructure layer,in which the island-shaped martensite(IA process)formed smaller particles and dispersed in the deformation layer,the fibers martensite(IQ process)became fine particles and uniformly disperses the subcrystal layer.The bulk martensite(SQ process)morphology undergoes surface plastic deformation and formed a sheet-like shape.Under the IA process,the deformation layer morphology of different martensite volume fraction samples was basically similar.With the increase of martensite volume fraction,the deformation layer thickness also increases,but the surface grain refinement amplitude was generally reduced trend.(2)After the preparation of the gradient nanostructure layer,the strength of the samples was significantly increased,while the ductility was reduced.For samples,the fibrous martensitic dual-phase steel had the largest increase in strength and the lowest decrease in plasticity,while the bulk martensite dual-phase steel had the largest plastic loss.For the island-like martensitic dual phase steel,with the volume fraction of martensite increased,the strength of samples generally showed an increasing trend.However,the tendency of plasticity decreased firstly and then decreased.After gradient nanocrystallization,the hardness gradient distribution existed on the surface of the specimen,the hardness of the surface layer of the massive martensitic dual phase steel was the largest.For the island martensitic morphology,with the volume of the martensite increased,the maximum hardness of the surface gradually decreased and the gradient of hardness changed slowly.(3)The results of electrochemical corrosion showed that the self-corrosion rate of the samples with island-like and fibrous martensitic morphology increased and the corrosion resistance decreased,the island-like martensitic morphology steel had the largest increase after nano-gradient.However,the corrosion rate of the bulk martensite dual phase steel decreased.For island-like martensitic dual-phase steels,the increase rate of self-etching rate decreased with the increase of martensite volume fraction.(4)The micro flow stress model precisely represented the micro-mechanical properties of martensite and ferrite.The simulation results accurately closed to the true stress-strain curves.When the quenching temperatures were 740° C and 800° C,the strain of surface of samples was the smallest.When the volume fraction was around 40%,the micro strain difference was larger and the grain refinement was better.