1.What are the inherent differences between the head, tail, and middle sections?
Before hot rolling and coiling, the cooling conditions at the beginning and end of the strip and in the middle are drastically different. The beginning and end of the strip travel at higher speeds on the roller conveyor, having longer contact time with air and residual cooling water, resulting in a much faster cooling rate than the strip tightly wrapped in the middle of the coil. This difference in cooling rate directly leads to inhomogeneity in the microstructure:
Fine grains at the beginning and end: Due to rapid cooling, the grains do not have time to grow before "solidifying," resulting in higher strength and hardness at the beginning and end.
Coarse grains in the middle: Slower cooling inside the coil allows sufficient time for grain growth, leading to relatively lower strength in the middle.
Inhomogeneous properties: It is precisely this difference in grain size between the beginning/end and the middle that causes uneven mechanical properties along the length of the entire strip coil.
2.What are the direct "traumas" of the hot rolling process?
Head wrinkles: In the initial stage of hot rolling coiling, if tension control is improper or the roller speed is mismatched, "diagonal" wrinkles are easily generated at the head of the strip. These defects are concentrated within a range of about 20 meters at the head, directly disrupting the continuity of the substrate.
Coiling tension fluctuations: During the tension establishment process at the head of the strip on the coiler, if the tension setting is too low or fluctuates, the first few turns will not be tightly coiled, resulting in a slight "steel piling" phenomenon, which in turn forms folding defects inside the coil.
3.What is the "scale-up" effect of the cold rolling process?
Microstructural differences cause rolling force fluctuations: Because hot-rolled strips have high strength at the beginning and end and low strength in the middle, the rolling force applied to the beginning and end during cold rolling is much greater than that in the middle. For example, in the production of high-strength steel, the rolling force at the beginning and end can be 1500-3000 kN higher than that in the middle. This huge fluctuation in rolling force directly leads to process instability in cold rolling, which in turn causes thickness fluctuations at the beginning and end.
Tension and speed interference: Tension changes during cold rolling are one of the direct factors affecting the thickness at the beginning and end. In particular, the tension build-up and release process is unstable when the beginning and end pass through the mill and coiler, resulting in tension fluctuations. These fluctuations can cause abrupt changes in strip thickness at the beginning and end.
4.Does the shape of the beginning and end of the hot-rolled coil have an impact?
Problems with the shape of the strip at the beginning and end of the coil, such as camber (where one side of the strip is longer than the other, curving like a sickle), are also one of the causes of performance issues at the beginning and end of cold rolling. This curvature can cause deviation during cold rolling threading and may even lead to strip breakage accidents.
5.What are the general performance differences between the beginning and end of cold-rolled coils?
The performance differences between the beginning and end of cold-rolled coils are a typical multi-process problem. It begins with unavoidable cooling differences and process fluctuations in the hot rolling process, and is significantly amplified during the cold rolling process due to changes in rolling force and tension, ultimately manifesting as inconsistencies in thickness, shape, and mechanical properties between the beginning and end and the middle.