In the early stage of production, a steel plant had a large number of short scratches on the billets. The main reason was that the clamps were not selected properly, resulting in a large number of short scratches (clamp marks). The following pictures are some of the scratches:
It is understood that the billets in the plant are divided into multiple different sections, but there is no distinction in the selection of clamps and hangers. Only a general clamp is selected. When the clamp is used to lift the billet with a small end face, the billet itself can be fully wrapped, but when lifting the large section, the front end of the clamp will be in the lower half of the billet, and the billet cannot be wrapped, which is easy to cause clamp marks.
There are also many aspects of the damage to the billet caused by the clamp:
Pinch and scratch: If the jaw design of the clamp hanger does not match the billet cross-section (such as too large, too small or inconsistent in shape), uneven pressure will be applied to the billet surface during the clamping process, resulting in deep scratches, pits or pinch marks on the surface. For example, if the arc of the jaws does not fit the corners of the billet when clamping the billet, stress concentration is likely to occur at the corners, causing surface damage. The photo shown above is one of the typical examples.
Increased oxide scale shedding: Improper clamping force may cause the oxide scale formed on the surface of the billet to fall off over a large area, which not only affects the surface finish, but also may expose the internal metal and increase the cost of rust removal during subsequent processing.
Stress concentration and cracks: If the clamping points of the clamp hoist are not distributed reasonably (such as too large or too small spacing), the billet will be subjected to excessive shear force or bending stress locally. For billets that have not yet fully solidified or have low strength (especially high-temperature billets that are directly lifted after continuous casting), internal microcracks or even through-cracks may be caused. For example, when lifting large-sized slabs, if the clamp spacing exceeds the reasonable support span of the slab, the middle of the slab will sag due to its own weight, resulting in bending cracks.
The impact of inappropriate clamp hoists on billets covers multiple aspects such as surface quality, internal structure, safety and cost. To avoid these problems, it is necessary to select matching lifting tools according to the size, shape, temperature and material of the billet (such as using special corner clamps for square billets and multi-point lifting tools for slabs), and regularly check the wear and clamping performance of the lifting tools to ensure uniform force and firm clamping during the lifting process. In addition, operating specifications (such as lifting speed and balance control) are also key factors in reducing billet damage.
