Surface cracking in hot-rolled plain round rebars is closely causally related to the quality of the continuously cast billet, its source of production. As the raw material for the hot rolling process, the internal defects and surface condition of the continuously cast billet are directly transmitted to the finished product, playing a crucial role, especially in the formation of surface cracks. This correlation is mainly reflected in the metallurgical quality, solidification characteristics, and stress distribution of the continuously cast billet during subsequent processing.
Non-metallic inclusions in the continuously cast billet are one of the core factors initiating surface cracks. During continuous casting, if non-metallic inclusions such as oxides and sulfides in the molten steel fail to float and separate sufficiently, they will remain inside or on the surface of the billet. These inclusions become stress concentration points during subsequent rolling. When metal flow is obstructed, cracks initiate and propagate from the interface between the inclusion and the matrix. For example, when inclusions are distributed in chains or aggregated on the surface of the billet, continuous surface cracks are easily formed during rolling, resulting in visible defects on the surface of the hot-rolled plain round rebar. Surface quality defects in continuously cast billets directly translate into surface cracks in hot-rolled plain round rebars. Common surface cracks in continuously cast billet production, such as longitudinal cracks, transverse cracks, and star-shaped cracks, if not thoroughly removed before heating or rolling through grinding or other methods, will be exacerbated during high-temperature rolling due to the indentation of iron oxide scale or uneven metal deformation. Especially when microcracks exist on the billet surface, the tensile stress during rolling will cause them to propagate, forming straight or serrated cracks along the rolling direction, severely affecting the surface integrity of the hot-rolled plain round rebar.
Internal cracks in continuously cast billets also become apparent during the rolling process. Internal cracks typically originate from the solidification characteristics during continuous casting, such as weak columnar crystal regions, central segregation, or bulging deformation. When internal cracks exist in the billet, the high temperature and pressure during rolling will gradually close them. However, if the crack depth is large or extends to the surface, the closed interface may reopen under subsequent cooling or stress due to insufficient bonding strength, ultimately forming cracks on the hot-rolled plain round rebar surface.
The influence of compositional segregation in continuously cast billets on surface cracks in hot-rolled plain round rebars cannot be ignored. Segregation of elements such as carbon, sulfur, and phosphorus in steel reduces the toughness of local materials, especially in the corners or edges of the billet. The increased brittleness caused by segregation makes it more prone to cracking during rolling. For example, excessive sulfur content can form low-melting-point sulfide inclusions, reducing the hot ductility of the steel and thus inducing cracks during high-temperature rolling.
The cooling process of continuously cast billets has an indirect impact on crack formation. Uneven cooling intensity in the secondary cooling zone can lead to excessive temperature gradients on the billet surface, thereby inducing thermal stress or phase transformation stress. If the tensile stress experienced by the billet during straightening or bending exceeds its tensile strength, cracks will occur. These types of cracks may propagate during subsequent rolling due to metal deformation, eventually manifesting as macroscopic cracks on the surface of the hot-rolled plain round rebar.
Shape defects in continuously cast billets can also exacerbate the formation of surface cracks on hot-rolled plain round rebars. For example, bulging or rhomboid deformation of the billet can lead to uneven metal flow during rolling, creating localized stress concentrations. When the stress exceeds the material's yield strength, cracks will initiate in the stress concentration area and propagate along the rolling direction, forming continuous surface defects.
There is a direct causal relationship between surface cracks in hot-rolled plain round rebars and the quality of continuously cast billets. From the residue of non-metallic inclusions, the transmission of surface and internal defects, to the effects of compositional segregation and cooling processes, every quality defect in the continuously cast billet can potentially contribute to surface cracks. Therefore, controlling the purity of continuously cast billets, optimizing solidification processes, improving cooling uniformity, and strengthening surface quality inspection are key aspects of reducing surface cracks in hot-rolled plain round rebars and improving product quality.
