Effects of Ca Treatment and Mg Treatment on Inclusions, Microstructure, and Mechanical Properties of X80 Pipeline Steel

The Al ₂ O ₃ inclusions formed in Al‐deoxidized X80 pipeline steel tend to cause nozzle clogging and impair mechanical properties. Although Ca treatment can modify Al ₂ O ₃ inclusions, it may lead to the formation of large, brittle calcium aluminates or harmful CaS inclusions. This study systematically compared the effects of Ca treatment and Mg treatment on the inclusion characteristics, microstructure, and mechanical properties of ×80 pipeline steel. The results showed that after Mg treatment, the inclusion density decreased from 40.03 to 16.83 mm ⁻² , and inclusions larger than 10 μm were almost eliminated. In Ca‐treated steel, the inclusions were predominantly CaAl ₂ O ₄ and CaO‐Al ₂ O ₃ ‐CaS, while in Mg‐treated steel, they were mainly finely dispersed MgAl ₂ O ₄ and MgO. Mg treatment significantly refined ferrite grains and produced a uniform distribution of the bainitic secondary phase. The average microhardness of the Mg‐treated specimens increased from 204.28 HV to 229.74 HV, and the impact energy improved by approximately 17.5% at the same temperature. By generating fine, dispersed Mg‐containing inclusions, Mg treatment not only reduced inclusion density but also provided effective heterogeneous nucleation sites for grain refinement. These effects give Mg treatment clear advantages over Ca treatment.