Study On Microstructures And Properties Of Heat-treated Dual-phase Seismic Resistant Rebars

Due to frequent earthquakes and the catastrophic consequences they brought, researchers both from home and aboard paied highly attention to development of seismic resistant rebars for buildings. Traditional seismic resistant rebars, containing microstructures of ferrite and pearlite, were obtained by both adding microalloying elements such as Nb, V and Ti in low carbon steel and adopting proper heat treatment or TMCP process. Based on the chemical compositions of low carbon steel, the present study melted four kinds of steels with different contents, using various heat treatment paths to get dual-phase seismic resistant rebars with microstructures of ferrite and martensite. The effects of different heat treatment processes and parameters on microstructures and martensite morphology of dual-phase seismic resistant rebars were investigated, using optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Strength, plasticity and impact toughness of test steels were examined by conducting tensile and Chary impact test. The main works of this paper are as follows:(1) Microstructures were varied by three different heat treatments (or quenching paths), namely step quenching (SQ), intermediate quenching (ImQ), and intercritical quenching (IcQ). For the SQ specimen, the microstructure showed two phase aggregates of large, blocky shaped martensite islands surrounded by the coarse ferrite matrix. On the other hand, the ImQ specimen revealed fiber dual phase structure, consisting of lath martensite, lath-like ferrite, and small amount of polygonal ferrite. The IcQ specimen had the microstructure of martensite islands dispersed in polygonal ferrite grain boundaries. The ferrite grain size of IcQ specimen was fine compared to that of SQ specimen.(2) Intercritical quenching time did not much affect the microstructure. Intercritical quenching temperature showed significant influence on test steels’microstructures. A small amount of martensite which distributed in grain boundaries could be obtained when quenching temperature was low. And a growing number of martensite content was observed with the increasing temperature. The martensite morphology and dislocation density around martensite were different at various intercritical quenching temperatures, with scattering precipitates being observed in ferrite. When the intercritical temperature was low, twin martensite could be obtained and the dislocation density in ferrite was small, however with further increase of quenching temperature, lath martensite and high density dislocation could be measured.(3) The yield strength of test steel could reach400-490MPa by adopting proper intercritical quenching.08Mn2Nb and08SiMn2Nb’s yield strength first increased and then decreased with the increasing temperature and the maximum value could be found at780℃. The yield strength of08CrMn2Nb and11Mn2Nb increased with the increasing temperature but the increasing rate was low after800℃. Test steel’s tensile strength could reach750-950MPa. All four test steels’curves of tensile strength changing with intercritical quenching temperature were characterized by existing a peak point, which was located at780℃for08Mn2Nb and08SiMn2Nb,800℃for08CrMn2Nb and11Mn2Nb. The impact toughness of08CrMn2Nb was the best, followed by08SiMn2Nb, whose was a little better compared to that of08Mn2Nb and11Mn2Nb. Through optimizing of processes and microstructures,08CrMn2Nb had excellent mechanical properties in terms of yield strength420MPa, tensile strength850MPa, percentage elongation at maximum force13%,percentage elongation after fracture20%, and impact toughness at room temperature50J.(4) Phase transformation strengthening is main strengthening method of dual-phase steel. On the basic of phase transformation strengthening, micro-alloying of Nb is important to promote strength grade of DP steel by strengthening of grain refining and precipitation. In the process of tensile test, void nucleation occurs predominantly along the ferrite-martensite interface or fractured martensite. The morphology and distribution of martensite strongly influenced the crack production and propagation during shrunken phase.