Mechanisms of Pure Fe Layer Formation on Hot Rolled C–Mn–Si Steel under Vacuum Sealing Conditions

Advanced High-Strength Steels (AHSS) are integral to the automotive industry’s ongoing quest for materials that satisfy stringent safety and weight reduction requirements. The addition of alloy elements such as Mn, Si, Cr, Ni, and Al, while beneficial for achieving desired mechanical properties, complicates production processes and elevates costs, posing challenges for widespread application in automotive manufacturing.  A new study published in the Vaccum Journal led by Professor Guangxin Wu, Dr. Yihao Liu, and Dr. Jieyu Zhang from the Shanghai University alongside Dr. Xinyan Jin and Dr. Weichen Mao from the Baosteel Research Institute, the research team investigated the formation mechanisms of a surface pure Fe layer on hot rolled C–Mn–Si steel sheets under vacuum sealing conditions. Their experimental framework was aimed at replicating the conditions during the coiling process of steel manufacturing, particularly focusing on understanding the role of internal oxidation of Mn and Si, alongside decarburization reactions, in creating a distinct surface morphology.  The researchers selected two types of steel for their experiments: C–Mn–Si steel and ultra-low carbon steel, representing typical compositions used in the industry. The chosen specimens retained the tertiary oxide scale formed during the finishing rolling stage, a critical aspect for studying scale transformations. They prepared three categories of samples: industrial samples cut from strip centers at varying coiling temperatures, laboratory samples for high-temperature coiling simulations, and samples for vacuum sealing heat treatment to mimic oxygen pressure conditions during coiling. The core of the experiment involved placing cleaned steel samples in a quartz glass tube under vacuum, followed by heat treatment at targeted temperatures to simulate internal oxidation and scale transformation processes. This approach allowed for a controlled study of the oxygen pressure effects on the sample’s surface and subsurface structures.

The authors analyzed the morphological, structural, and compositional changes using a range of advanced characterization techniques light optical microscopy and scanning electron microscopy for detailed morphological observations of the sample surfaces and cross-sections. They used energy dispersive spectroscopy for elemental composition analysis.  The experiments led to several critical insights into the formation of the pure Fe layer and the overall transformation of oxide scales under vacuum-sealed heat treatment. The formation of a surface pure Fe layer on hot rolled C–Mn–Si steel is attributed to the internal oxidation of Mn and Si within the substrate and a decarburization reaction under vacuum conditions. This process reduces the oxygen pressure, causing the decomposition of oxide scales and leading to the emergence of a pure Fe surface layer. The researchers demonstrated that the gradual reduction of oxygen pressure in a sealed environment is a key factor in the transformation process. Mn and Si, due to their high affinity for oxygen compared to Fe, undergo selective oxidation, which significantly contributes to the reduction in oxygen pressure and promotes the formation of a pure Fe layer. Detailed microstructural analysis revealed a complex layering on the steel surface, comprising an outermost pure Fe layer, followed by an intermediate oxide scale layer and an internal oxidation zone enriched with Mn and Si oxides. This unique structure suggests a layered mechanism of oxidation and reduction processes. XRD and SEM/EDS analyses confirmed the presence of distinct phases and compositions across the surface and subsurface layers, highlighting the role of thermal treatment and elemental diffusion in dictating the phase distribution and the formation of the pure Fe layer.

The new study by Professor Guangxin Wu and Xinyan Jin concluded that the vacuum sealing heat treatment effectively simulates the coiling process’s conditions, allowing for the first-time reproduction of the continuous pure Fe layer formation on the hot rolled steel surface in a laboratory setting. The findings emphasize the critical role of internal oxidation of Mn and Si, alongside decarburization, under reduced oxygen pressure conditions, in achieving the desired surface morphology. These insights not only deepen the understanding of surface transformations in steel manufacturing but also open pathways for optimizing steel processing techniques for enhanced material properties. The implications for the automotive industry and beyond, in terms of material innovation and application, are profound, potentially leading to safer, lighter, and more environmentally friendly vehicles.