Modelling of structure-property relationship of TMT high strength structural steel bars

Abstract

The 500W locally produced thermomechanically treated (TMT) high strength reinforcing steel bars have been collected from four different companies. The chemical compositions of the TMT steel rebars were determined by optical emission spectroscopy (OES) and carbon equivalents (CE) were calculated from the results of the OES. The macro and micro structure of the rebars were studied using metallographic techniques. To determine hardness, Rockwell hardness test and Vickers hardness tests were conducted. Tensile tests were carried out using a Universal Testing Machine (UTM) to evaluate the tensile properties (tensile strength, yield strength, % of elongation) of the rebars in three different conditions viz. with and without rib and without case. The mathematical modelling were developed between the tensile properties (tensile and yield strength) and hardness in the light of previous studies on it. Failure modes of the high strength steel rebars in tensile loading were studied by observing fracture surfaces after tensile tests. It was found that all TMT steel rebars were low carbon steels having 0.18 percentage of carbon in average. Optical microscopy revealed low carbon martensite in the case and ferrites and pearlites in the core. A transition zone of refined, mixed constituents was also observed in the microstructures. Difference in size, shape and appearance of grains and constituents has been observed. Variations in area of hardened case, core and transition zone of the rebars were revealed in the macrostructures. Differentiation in hardness among the TMT rebars was observed due to the variation in microstructures and chemical compositions. It was also found that the hardness of the cases was higher than that of the cores. The dissimilarity in tensile properties of the TMT steel rebars was found too. The evaluation of tensile property from the mathematical models was consistent with the tensile properties obtained from the mechanical tensile tests. The yield strength calculated from the strength– hardness models also showed consistency with the test values. The effects of microstructures and carbon equivalent on the hardness and tensile test results were evaluated. Description of the evaluated tensile properties from the mathematical models was also given in terms of the microstructures of the TMT steel rebars. The calculated and test values of tensile properties showed good agreement with the microstructures. The fracture behaviours of the TMT steel rebars also showed good dependency on microstructures. Thus, overall structure-property relationship was established with existing mathematical models.