Quantifying the Total Amounts of Tramp Elements Associated with Carbon Steel Production in Japan

Steel is one of the most recyclable materials, and thus it is recycled repeatedly. When steel is recovered from end-of-life products, some materials containing tramp elements besides carbon steel are mixed into the scrap. Therefore, we must prevent contamination by tramp elements during repeated recycling. Meanwhile, there is little knowledge on tramp element contents in carbon steel. This study aims to quantify tramp element contents in carbon steel produced in Japan. The tramp element contents of more than 500 samples of carbon steel were analyzed. The specimens were classified by steel product forms because the tolerance for tramp element contents differs by form. The average contents and confidence intervals for populations were calculated and then multiplied by the annual carbon steel production in Japan. The amounts of Ni and Mo impurities associated with carbon steel produced annually accounted for more than 15% of the annual consumption of these metals in Japan. In addition, the consumptions for plating were considered. The results were confirmed by a comparison with the contents in carbon steel produced from steel scrap generated in Japan after non-ferrous metals were separated from the scrap, which was performed in Vietnam. It is hypothesized that the tramp element contents in the Vietnamese steel are equal to the weighted average contents of carbon steels consumed in Japan. The comparison for Cu and Sn showed good agreement. We conclude that tramp element contents in carbon steel obtained in this study are representative values, even though the number of samples was limited.