Causes of banding formation and methods to eliminate it

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PART 1: Causes of banded structure. The banded structure refers to the phenomenon in hypoeutectoid steel where pearlite and ferrite are arranged in bands; it is a defective structure that forms during the steel-making process.      During the solidification process of the molten steel, selective crystallization occurs, resulting in a dendritic structure with uneven distribution of chemical composition. The coarse dendrites in the ingot are elongated in the direction of deformation during rolling, and gradually align with this direction, thereby forming zones that are depleted in carbon and alloying elements and zones that are enriched in these elements, which stack alternately with one another. Under slow cooling conditions, pro-eutectoid ferrite precipitates first in the depleted zones of carbon and alloying elements (where the stability of supercooled austenite is lower), releasing the excess carbon into the enriched zones on either side, ultimately resulting in zones dominated by ferrite ； Thereafter, a zone enriched in carbon and alloying elements (with higher stability of supercooled austenite) is formed, which gives rise to a zone dominated by pearlite; ultimately, a band structure with alternating layers of ferrite and pearlite is established. The more severe the compositional segregation, the more pronounced the banding structure that forms.  Figure 1: Band structure of 40 steel. Figure 1 shows the microstructure of 40 steel in its as-supplied state; after etching with a 4% nitric acid-alcohol solution, white ferrite and dark pearlite are distributed in bands.      Banded structure forms when phosphorus segregates in steel. When steel is cooled slowly in the A3–A1 range, the A3 temperature is higher in the high-phosphorus region, leading to the formation of ferrite first; carbon is concentrated in the low-phosphorus region, resulting in a carbon-rich area there. During subsequent cooling, an eutectoid transformation occurs, forming pearlite and causing the microstructure to arrange in layers.       Manganese is also an element that promotes the formation of banded segregation. In hot-rolled steel, the manganese content is generally higher in areas where pearlite is formed, while it is lower in areas where ferrite precipitates. After hot rolling, the steel is cooled slowly; first, eutectoid ferrite precipitates preferentially in the low-manganese areas along the direction of the deformed fibers. Then carbon moves to the high-manganese areas to form pearlite, resulting in a banded pattern with pearlite and ferrite distributed alternately.      If non-metallic inclusions in the steel are elongated in the rolling direction to form a band-like distribution, during cooling these inclusions can serve as nuclei for preferential precipitation of ferrite, resulting in the formation of ferrite bands; such defects are generally difficult to eliminate through normalizing. This strip-like structure must first be improved through high-temperature homogenization annealing followed by normalizing treatment.      If the distribution of alloying elements in austenite is uneven, it will result in varying tendencies for grain growth. Unreacted carbides tend to remain in the regions enriched with carbide-forming elements, which reduces the diffusion rate of carbon atoms and thereby inhibits grain growth ； In the depleted region, grains tend to grow larger, thus leading to the formation of a mixed-phase structure. During quenching, the areas with reduced concentrations of alloying elements have low hardenability, making it easy for non-martensitic microstructures to form. During carburizing and quenching, the large grains in the mixed crystal form coarse needle-like martensite, which increases the amount of residual austenite. Therefore, banded tissues all exhibit low mechanical properties after conventional heat treatment. Furthermore, compositional segregation leads to an increase in the differences in expansion coefficients and specific volumes before and after phase transformation, thereby increasing the quenching deformation of the parts.       Due to the layered microstructure of banded structures, their mechanical properties are directional: they exhibit high tensile strength and good toughness along the longitudinal direction of the bands, but their properties in the transverse direction are poor, with low strength and low toughness as well. This also reduces the machinability of the steel, and increases variations in deformation and hardness during subsequent heat treatment. If banded structure is present before quenching, it cannot be completely eliminated during the quenching heating process. The remaining banded structure after quenching will cause significant structural stress in the workpiece, and may even lead to cracking.      Generally, the homogenization temperature for carbon in austenite is above 950°C, while that for alloying elements is above 1100°C. The homogenization time is determined by the width of the banded structure, the concentration difference between the bands, and the desired degree of homogenization. Therefore, it is quite difficult to homogenize the carbon (especially alloying elements) in banded structures, and conventional heat treatment processes such as annealing, normalizing, quenching, and carburizing generally make it hard to eliminate banded structures.      Summarizing the reasons for the formation of banded structure, the external factor is rolling, while the internal factors are the segregation of elements such as phosphorus and sulfur within the steel ingot, as well as inclusions. The severity of banded structure can be assessed in accordance with GB/T 13299 \"Methods for evaluating the microstructure of steel\". PART 2 Methods for Eliminating Banded Structure Conventional heat treatment methods (such as annealing, normalizing, quenching, carburizing, etc.) cannot eliminate the segregation of alloying elements in the banded structure. Although rapid cooling can suppress the uneven distribution of carbon and prevent or reduce the formation of banded structure, banded structure reappears when the material is reheated and cooled slowly.      Therefore, banded structures need to be eliminated through methods such as electroslag remelting, increasing the crystallization rate, raising the final rolling temperature, increasing the forging ratio, and performing high-temperature homogenization annealing. PART 3 Conclusion: Banded structure is a phenomenon in steel characterized by uneven distribution of carbon and alloying elements. Depending on the severity of these bands, it can have varying effects on the mechanical properties of the workpiece.