Reliability of steel made from recycled scrap in Uganda

Abstract

While steel can be made from its ores, the industrial world has recognized the advantages of its manufacture through the recycling route especially in view of its economic and environmental friendliness. Steel in Uganda is mainly made through the recycling route. Given the reducing availability of scrap and the growing demand of steel, the quality of the scrap is quickly dropping. In this research, since the usefulness of a material is depicted by its properties, the quality of steel has been studied through the analysis of its mechanical properties namely; ductility, strength, weldability and hardenability. A quantitative experimental research approach has been used along with extensive review of literature. Samples have been selected for examination from the different manufacturers in the country. Attention has been paid to the popular semi-finished product forms like twisted and thermo-mechanically treated (TMT) bars to guarantee the correlation between raw materials input and end use. Samples have therefore been grouped according to product type but have otherwise been randomly selected to eliminate manufacturer identity. A high level of tramp element content has been found consistent in all sample groups. While the bars have been found generally of acceptable ductility, resilience, strength and metallographic properties in spite of relatively high and irregular carbon content, the incidence of fragile samples in each group related to residual element and inclusions content, rolling faults and others has been pointed out. The varying composition in individual steel bars has also been shown to be a factor in the quality of especially the TMT and twisted bars. The wide scatter in yield strength has been found to engender unpredictable concrete reinforcement value. The prevalence of Boron in the range 0.0003% and 0.003% has been shown to play an outstanding role in raising the yield strength and creating development lengths that often exceed the pre-calculated value. Examination of weldments of the bars showed post weld cracks in up to 13% of the samples both in cold and hot cracking. These were also shown to be due to elevated tramp element content rather than carbon content. The incidence uniaxiality of properties also associated to the unpredictable tramp element content was highlighted. On the overall, the use of better sorting methods, more elaborate refining especially at the teeming ladle stage and the exploitation of virgin iron resources to exploit the sponge iron alternative have been recommended. Reducing dependence on Boric acid binding for furnace and ladle lining and chemically reducing Boron content from liquid steel have also been suggested as possible solutions to the unpredictably fluctuating high yield of the steel bars.