Microstructure And Mechanical Properties Of A Low Carbon Microalloyed Steel Through Symmetric And Asymmetric Hot Rolling

In order to reduce the vehicle weight and improve its safety,the high-strength steel has been widely used in the automobile industry.However,the tensile ductility and toughness of steels generally decrease due to the absence of work hardening capability,accompanied by a dramatic increase in tensile strength.The inverse relationship between strength and ductility/toughness has become a major issue in the applications of high strength or ultra-high strength steels.In order to address the plastic instability of ultrafine grained/nano-ferrous materials,this work therefore,will focus on developing an ultrafine-grained gradient structure throughout the sheet thickness by asymmetric hot rolling,and thus to improve the overall performance of the material.Using optical microscopy(OM),scanning electron microscopy(SEM),tensile testing,Vickers hardness,nanoindentation etc.,the effects of austenitizing temperature,final rolling temperature,and rolling reduction on the evolutions of microstructure and hardness along the sheet thickness and mechanical properties of a low carbon micro-alloyed steel through symmetric/asymmetric hot rolling.The main contents and conclusions are summarized as follows:(1)The effects of symmetric/asymmetric rolling(SR/AsR)on the evolutions of microstructure and texture show:when comparing the SR-processed samples,the AsR-processed samples exhibit ultrafine ferrite grains and its size is generally distributed gradient through the sheet thickness.The martensite of AsR samples is distributed at the grain boundaries of ferrite and an obvious shearing texture ?(<110>//TD)was observed due to the existence of intensive shearing deformation.(2)The effects of rolling reduction on the evolutions of microstructure and hardness along the thickness under AsR showed that as the rolling reduction increased,the ferrite grains were refined and the volume fraction of martensite decreased;meanwhile,the difference between ferrite and martensite decreased as well to some extent.(3)The effects of austenitizing temperature on the evolutions of microstructure and hardness along the thickness under AsR showed that when the austenitizing temperature dropped from 1200 to 950?,the ferrite grains were refined,resulting in an increase in hardness and a decrease in the fraction of martensite which is distributed dispersedly at ferrite grain boundaries.(4)The effects of final rolling temperature on the evolutions of microstructure and hardness along the thickness under AsR rolling showed that as the final rolling temperature decreased,some amount of band-shaped ferrite grains appeared at the surface layer with a relatively large faction of pro-eutectoid ferrite;the low-temperature rolling promoteed grain refinement of ferrite.(5)The effects of thermo-mechanical processing parameters on the mechanical properties of the AsR/SR-processed samples showed that as the tensile curves of that the AsR samples exhibited continuous yielding behavior,and had relatively high tensile strength of 930-1005 MPa and good tensile ductility of 16.8-21.6%.The relationship between yield strength and ferrite grain size met the Hall-Petch equation as follows:?s=316.23+572.35 d-1/2.