Abstract. High strength requirement of Thermo-mechanically treated (TMT) rebars is crucial in the construction of flyovers, bridges and high rise buildings because of the good combination of the mechanical properties. The yield strength is expected to be between 450 MPa and 550 MPa after the hot rolling process depending on prescribed standards. A series of experimental trials during a hot rolling process were carried out in a steel plant in which parameters such as the water flow rate and the processing time were varied to study their effect on the evolving mechanical properties of the rebars. Four "heats"(A "heat" is a batch of molten steel, referred to as tap to tap cycle and involves furnace charging with scrap, melting, deslagging, tapping molten steel and furnace turn-around. Furnace turn-around is the period following completion of tapping until the furnace is recharged for the next "heat") were done to produce Y 12 mm reinforced bars (rebars). For every "heat" done, tensile tests were carried out on the samples every after 15 minutes to establish the yield strength of the rebar. At least eight samples were tested in every "heat". It was observed that some samples showed low values of yield strength (< 450 MPa) which falls short of the minimum guaranteed yield strength. It was further noted that the water flow rate in the water cooling chamber was far below 600 m h ⁄ for this size of rebar in some cases. A series of these tests were conducted and the water flow rate adjusted in order to arrive at the optimum flow rate corresponding to the expected yield strength and microstructure. The quenching time in the water cooling chamber was in the range 0.1 to 0.5 seconds and the results obtained both for the tensile tests and microstructure after several adjustments to the flow rate in particular yielded optimum results consistent with prescribed standards.
Reinforced bars (rebars) are thermomechanically treated (TMT) bars that are hot-rolled from steel billets produced from scrap melted in an electric arc furnace at a temperature of about 1,600°C (usually 1,580°C ). The weight per meter of low-carbon steel rebars is one aspect that has been neglected by some steel producers during the tensile testing in rod mills. Determination of the weight per meter is explicitly required for a TMT rebar. Any reduction in mass will mean a lowering in the capacity of the steel reinforcing bar. A series of "heat" numbers or batches of molten steel from an electric arc furnace for the production of Steel Sample A (Y10 and Y12 rebars) were observed at a steel plant to investigate the effect of the weight per meter of a reinforced bar on the mechanical properties and microstructure. The rolling speed range was 3-14 m/s for different "heat values." Two other steel samples, B and C, were sourced from the local market to compare with Sample A. Samples collected from different sources from the local market, however, showed lower values of weight per meter, different from the prescribed standards. This affected not only the ultimate tensile strengths (UTSs) that were higher than normal, but also the microstructure that deviated from the standard for this material. Sample A showed not only a good combination of tensile strength and yield stress (YS) of 450 MPa and a maximum tensile strength of 650 MPa, but also a standard pearlite-ferrite microstructure, whereas Samples B and C exhibited excessively high strengths and brittle behavior and were prone to failure.
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