The Strength Evolution Mechanism of Cold-Pressed and Heat-Treated Carbon-Bearing Lumps During Reduction Process

Under the demand for green and efficient development of the steel industry, cold-pressed and heat-treated carbon-containing lumps (CCLs) have emerged as key materials for optimizing the blast furnace burden structures. However, the strength evolution mechanism of CCLs during high-temperature reduction process has not well understood. In this study, biomass-based CCLs were prepared by cold-pressed and medium–low-temperature heat treatment. The evolution law and micro-mechanism of lump strength during the reduction process at 600 °C~1200 °C were explored through compressive strength testing, X-ray diffraction, scanning electron microscopy, and van der Waals force theoretical calculations. The results show that the compressive strength of CCLs first decreases and then increases with the rise of temperature. The lowest strength occurs at 900 °C~1000 °C due to the consumption of binders and the formation of FeO. At 1100 °C~1200 °C, the interconnection of metallic iron crystals and the filling of pores by slag phase significantly improve the strength, with the maximum compressive strength exceeding 3000 N. During reduction, Fe₃O₄ is gradually reduced to FeO and metallic Fe. Metallic Fe is rapidly generated above 1000 °C and forms an interconnected crystal structure above 1100 °C. The micro-mechanism indicates that van der Waals force dominates the inter-particle bonding force, and the formation of metallic interconnected crystals at high temperatures enhances the mechanical interlocking and metallurgical bonding between particles. This study provides a theoretical basis for the optimization of carbon-containing lump preparation technology and the regulation of high-temperature properties.