The carbonization process is classified as follows: i) low-temperature carbonization, which occurs at temperatures between 300 and 1000°C, and ii) high-temperature carbonization, which occurs at temperatures between 1100 and 1600°C . Carbonization aims to cause the stabilized fibers to undergo crosslinking, cracking, condensation, and other chemical reactions by heating under higher temperatures and inert conditions, which induces a disordered graphite structure in the resulting carbon fibers . For lignin-based carbon fibers, the carbonization temperature ranges from 300 to 1000°C, and the selection of carbonization temperature depends on the type of lignin, stabilization conditions, carbonization conditions, and structural development . However, a higher carbonization temperature yields a carbon fiber with low tensile strength and high modulus of elasticity. As the carbonization temperature increases, the noncarbon heteroatoms (mainly Nitrogen) become excited, which improves the amount of elemental carbon and finally results in the formation of a disordered layer of graphite structure. During this process, the tensile strength also increases, and the maximum tensile strength is achieved when fiber orientation is equal to the bulk density of the disordered graphite structure. Beyond this temperature, the tensile strength decreases, and the modulus of elasticity increases .