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Bending and Fracture of Cold Bending Ribbed Reinforcing Steel in Hot Rolling Strip
Cold bending reinforcement steel is a type of steel strip and bar used for structural reinforcement in construction. It is divided into cold-rolled ribbed reinforcing steel and hot-rolled ribbed reinforcing steel according to production technology. As the name suggests, cold-rolled ribbed reinforcing steel is produced by cold rolling the material, while hot-rolled reinforcing steel is produced by hot rolling. Hot-rolled ribbed reinforcing steel has high strength, good surface finish, uniform rib height and wide rib pitch, and is widely used in construction projects. In addition, the properties of hot-rolled ribbed reinforcing steel make it suitable for cold-bending operation. This paper studies the cold-bending mechanics and fracture of hot-rolled ribbed reinforcing steel.
As a branch of metal forming technology, cold bending is the mechanical processing in which a steel strip or bar is bent in a cold state by external forces to form a permanent bending angle. A common application is bending reinforcement steel at a right angle to form a straight section. In the cold bending process of hot-rolled ribbed reinforcing steel, compressive residual stress (CRS) was formed on the inner side of the bent section surface due to plastic deformation, while tensile residual stress (TRS) was formed on the outer side of the bent section surface under the action of tension.
Due to the combination of these two residual stresses, a compressive elastic recovery force was generated on the bending section. The magnitude of the compressive elastic recovery force depends on the magnitude of CRS and TRS and the curvature of the bending section. The compressive elastic recovery force will counter the pulling force applied to the bent section, and the moment balance of the entire bending section may reach a stable point.
The behavior of hot-rolled ribbed reinforcing steel under bending is strongly influenced by the helical rib pattern. During the cold bending process, the TRS generated at the outer surface of the bent section makes it more difficult for the ribs of the steel to slide. As a result, the TRS causes the rib pattern to become more serious, and the widening of ribs imparts a large compression stress to the bent section.
On the other hand, the CRS formed at the inner surface of the bent section increases the materials resistance to further deformation, which improves the yield strength of the bent section. This CRS also helps the rib pattern to remain smooth, thereby improving the ductility of the material.
Despite these positive effects, excessive compressive stress can ultimately lead to fracture of the bent section. In the case of cold-bent hot-rolled ribbed steel, the fracture usually occurs at the rib apex due to premature failure of the steel. The fracture surface of failed specimens usually remains smooth, showing typical features of ductile fracture. The fracture surface may sometimes display a ribbed cusp caused by the rib pattern.
The results of laboratory tests conducted on hot-rolled ribbed reinforcing steel in cold-bending operation showed that the ribs of the steel increased in width and depth during the bending process. In addition, an increase in the curvature of the bent section resulted in a decrease in the thickness of the steel. These observations indicate that high bending curvatures combined with the ribbed nature of the steel can lead to premature failure of the steel and fracture of the bent section.
In conclusion, hot-rolled ribbed reinforcing steel is well suited for cold-bending operations, but under certain conditions, the ribbed nature of the steel can lead to premature failure and fracture of the bent section. It is important for engineers to consider the effects of ribbed nature and bending curvature of hot-rolled ribbed reinforcing steel when designing cold-bent structures.
