Tensile Deformation Behavior Of High Strength Seismic Bars Under Different Temperatures

High strength seismic rebar was one of the most important materials to guarantee the structural safety of buildings, and it also played an important role in a variety of large and high-rise buildings with complex structures. Rebar with high strength, high toughness and high ratio of tensile strength and yield strength could absorb earthquake energy to a large extent. In this paper, the changes of tested rebars’ microstructure evolution and seismic performance were discussed according to tensile deformation tests at room temperature and high temperature by adopting the universal tensile testing machine, metallographic microscopy, microhardness tester, high temperature testing machine, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) et al.According to the tensile test of 5 kinds of tested rebars with different bainite contents of 3%,8%,15%,20% and 50% at room temperature, study shown that, each seismic index of tested rebars with different bainite contents met the requirements of 500MPa seismic rebar. With the increase of bainite contents, the ratio of tensile strength and yield strength was increasing. By analyzing the influence of bainite contents on the yield platform, results indicated that, the yield platform of tested rebars disappeared as the content of bainite reached up to 20%. There were two main reasons for the occurring of yield phenomenon:(1)the solute atoms clustered in dislocation areas and pinned dislocations, which blocked dislocation movement to result to the increase of the Peierls stress. Further movement of dislocations needed increased stress, which performed the upward trend in the curve; (2) the lath ferrite in bainite contained high density of dislocations, which made it easier for the tangling of dislocations with high density during deformation processing, and the dislocation movement needed increasing stress to keep the deformation processing.By respectively comparing the microstructure changes of tested rebars with the bainite contents of 8% and 50% in different deformation stages, conclusion could be drawn that, bainite bore the main deformation in the rebar of 50% bainite content, and the dislocation density of sub-lath in bainite decreased after the first increase with the increasing amount of deformation; while, in the rebar of 50% bainite content, ferrite and pearlite bore the main deformation, and the dislocation density of ferrite in tested rebar increased dramatically with the increasing amount of deformation, and pearlite bore the largest deformation, even fractured.High temperature tensile tests for five kinds of rebars with different bainite contents had been carried out at the temperature range of 200～600℃. The results shown that, at the temperature range of 200～400℃, obvious hardening phenomenon appeared in all tested rebars, while it appeared less obviously with the increase of temperature. When the tested temperature rose to 500～600℃, work hardening phenomenon largely disappeared, and the stress-strain curve even tended to be a horizontal straight line. The results of change evolution of yield strength and tensile strength of the test rebar in high temperature tensile tests at different temperatures shown that, both the yield strength and tensile strength of tested rebars decreased as the temperature increased at the temperature range of 200～600℃; However, the decline of yield strength and tensile strength was much more dramatical at the range of 400～600℃ than that of 200～400℃. Besides, at the range of 200～600℃, the tensile strength decreased larger than yield strength with the increase of temperature.Analysis of the ratio of tensile strength and yield strength of high temperature tested rebar shown:when the temperature was less than 500℃, the ratio of tensile strength and yield strength of tested rebar was higher than 1.25 and met the seismic requirements; the ratio of tensile strength and yield strength of different bainite contents tested rebars decreased after the first increase with the increase of temperature. High temperature seismic performance of tested rebars was confirmed by the contrast with the room temperature properties, after comparison, the ratio of tensile strength and yield strength of high temperature tested rebars was higher; the yield strength under high temperature was higher than two thirds of yield strength at room temperature unless the temperature was lower than 500℃, and it was suggested that tested rebars had excellent high temperature seismic performance.