| With the rapid increase of steel production and the arrival of its lifespan, the scrap steel quantity in China will increase so remarkably in the near future that the way to use the scrap steel resources is urgly needed. However, the efficient recycling of scrap steel is a world problem for many years mainly involving the cycle enrichment of the harmful residual elements Cu, Sn, As, etc. Since the oxidation tendency of the residual elements is weaker than iron, they are difficult to remove economically and effectively in the current steelmaking process. Due to microsegregation, grain boundary segregation and oxidation enrichment, the residual elements tend to result in the deterioration of hot ductility, hot workability, temper brittleness and mechanical properties of steels. The problem of arsenic enrichment in the process of recycling of scrap steel was focused on in this paper. To eliminate the harmful effect of arsenic on properties of steel and enhance the utilization efficiency of recycling scrap steel, the systematic work has been conducted on the precipitation behavior of asenic, metallurgical conditions of arsenic-rich phase formation after adding rare earth element into the steel. Moreover, the influence of arsenic on hot ductility, high temperature oxidation behavior and mechanical properties of C-Mn steel and their improvements by Ce addition have been systematically investigated.The study on the distribution behavior of arsenic in steel showed that in the case of low-arsenic content, solidification microsegregation of arsenic were not observed by EPMA in Fe-0.5wt%As alloys quenched from 1600℃ and 1200℃. However, the average arsenic concentration at grain boundary in the two alloys detected by Transmission Electron Microscope (TEM) was higher than that in the matrix, meaning that arsenic tends to segregate to grain boundary. In the case of high-arsenic content, the eutectic Fe2As phase distributed discontinuously in the interdendritic region of a-Fe phase for Fe-4wt%As and Fe-10wt%As alloys quenched from 1600℃ and 1420℃, while the eutectic Fe2As phase distributed continuously in the interdendritic region of a-Fe phase for Fe-10wt%As alloy quenched from 1200℃. Moreover, the amount of eutectic Fe2As phase increased with increasing arsenic content and decreasing quenching temperature.The inclusion composition analysis after modification treatment by adding Ce in the steel containing arsenic showed that different types of arsenious rare earth inclusions were formed with different Ce content. As Ce content increased from 0.037wt% to 0.095wt%, the dominant inclusion was changed from the Ce-S-O inclusion fully coated by Ce-S-As inclusion to the Ce-S-As inclusion completely covered by Ce-As inclusion. Simultaneously, the single Ce-S-As and Ce-As inclusions were also detected when Ce content was more than 0.055wt%. The rapid quenching experimental results combined with the SEM-mappings of inclusions indicated that the Ce-As inclusion was formed due to the reaction of the microsegregation element during solidification process, which not only formed on the pre-formed inclusion by heterogeneous nucleation but also nucleated independently during solidification process. The results of electron diffraction patterns by TEM indicated that the Ce-As inclusion was the face-centered cubic CeAs phase. Furthermore, the concentration of arsenic on grain boundaries was decreased to the level of the matrix due to the formation of arsenious rare earth inclusions, detected by TEM. Therefore, the formation of arsenious rare earth inclusions is beneficial to control grain boundary segregation of arsenic and relieve embrittlement behavior.The effect of arsenic on the hot ductility, high temperature oxidation behavior and mechanical properties of C-Mn steel has been systematically investigated. For hot ductility, the results showed that whether arsenic exists alone or copper and arsenic exist together, the hot ductility of C-Mn steel gradually become worse with increasing arsenic content. In austenite single phase zone, 0.16wt%As in the case of arsenic alone and 0.075wt%As in the case of copper and arsenic together significantly reduced the hot ductility in the temperature range of 850℃ to 900℃. Auger Electron Spectroscopy (AES) analysis indicated that arsenic segeration to grain boundary deteriorated the hot ductility in the above temperature range. For high temperature oxidation behavior, the results showed that arsenic alone contributed to the grain boundary oxidation, and thus led to the formation of obvious oxide particles band at grain boundary. The penetration depth of oxide particles band increased with increasing the oxidation temperature from 1000℃ to 1050℃. Electron Probe Microanalyzer (EPMA) analysis demonstrated that the enrichment concentration of arsenic at the scale/steel matrix interface first increased and then decrease with increasing oxidation temperature, and the arsenic enrichment degree was most serious at 1050℃. The enrichment regularity with increasing temperature when copper and arsenic exist together is the same as that when arsenic exists alone. Similarly, at 1050℃, the penetration into grain boundary by copper-enriched liquid was most severe, while the penetration phenomenon disappeared when the temperature was higher than 1100℃. Energy Dispersive Spectrometer (EDS) combined with the phase diagram analysis indicated that arsenic reduced the melting point of copper phase, which made the molten copper-rich phase containing arsenic penetrate into grain boundary at 1050℃ below pure copper phase melting point. The hot compression experiment by Gleeble confirmed that arsenic/copper and arsenic exacerbated the hot crack sensitivity of the C-Mn steel. The cracking degree at 1050℃ was the worst, while the hot crack eliminated when the oxidation temperature exceeded 1100℃. This is consistent with oxidation enrichment regularity obtained by the thermogravimetric experiments. For the mechanical properties, arsenic mainly deteriorated the impact property of C-Mn steel, especially the impact property at low temperature.The improvement of hot ductility, high temperature oxidation behavior and mechanical properties of C-Mn Steel containing arsenic by rare earth modification treatment were systematically investigated. For hot ductility, the results indicated that adding 0.016wt%～0.035wt%Ce enhanced the hot ductility of C-Mn steel containing arsenic. As Ce content increased from 0% to 0.035wt%, the hot ductility at the temperature range of 750℃～950℃ increased significantly. When Ce content was more than 0.027wt%, the improvement degree of hot ductility increased smoothly. For high temperature oxidation behavior, adding 0.016wt%-0.035wt%Ce reduced the enrichment degree of arsenic at the scale/steel matrix interface. The reduction of arsenic enrichment was most remarkable when Ce content was 0.027wt%. Furthermore, the hot compression experiments confirmed that when Ce content was 0.016wt%～0.035wt%, the cracking at 1050℃ caused by arsenic enrichment completely eliminated. For the mechanical properties, the impact toughness at -60℃～0℃ were improved by adding 0.016wt%Ce, while adding 0.027wt%Ce deteriorated the impact toughness due to the formation of large size carbides and a large number of inclusions. Therefore, considering the improvement of the hot ductility, suppressing surface cracking formation and enhancing impact toughness, adding 0.016wt%Ce in the C-Mn steel is more appropriate. |
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