Rotary forging die Hebei Yuanyue Trading Co.,Ltd , https://www.hbyuanyue.com
In the process of smelting and rolling carbon steel, various issues can arise due to equipment, process, or operational errors. These include phenomena like scabs, cracks, residual pores, delamination, white spots, segregation, non-metallic inclusions, looseness, and banding structures.
Scabs refer to metallic or non-metallic chunks on the steel surface that fail to bond with the substrate. Some scabs are partially attached and resemble tongues, while others are detached and appear flaky. These unattached scabs may fall off during processing, leaving pits behind. Scabs caused by steelmaking (casting) often contain visible non-metallic inclusions. Those formed during rolling are typically referred to as "rolls," and usually consist of just iron oxide beneath the surface.
The primary causes of scabs during steelmaking and casting include:
- Failure to implement splash prevention measures during the casting of the top ingot or excessive force when casting the bottom ingot, leading to splashing and scabbing.
- Poor performance of the lower ingot slag or an unclean and wet mold, which results in surface or subsurface inclusions, bubbles, and heavy skins on the steel ingot (or continuous casting slab).
- Severely damaged mold walls or excessively high casting temperatures causing crown and mold sticking, which then forms scabs during rolling or forging.
For scabs occurring during rolling, the reasons could be:
- Defective rolls (rails) or guide devices before the final product causing protrusions, ears, or strokes in the rolled piece, which turn into scabs upon re-rolling.
- Flame cleaning of slabs being too aggressive or remnants not being fully removed, allowing foreign objects to fall onto the slab and get rolled into scabs.
Scab defects significantly impact the appearance and mechanical properties of the steel. Finished steel must not contain any scabs; problematic areas can be ground and repaired. After grinding, the steel dimensions should comply with standards. To minimize and eliminate scabs, both steelmaking and rolling processes need improvement, along with focusing on addressing billet surface defects or thoroughly cleaning the skin.
Cracks present various names depending on their shape and formation cause, including cracking, transverse cracking, splitting, hairline fractures, brittle cracking (fracture), rolling, and shearing. Cracks can occur at nearly every stage from steelmaking through to deep processing.
During steelmaking:
- High sulfur and phosphorus content in steel reduces its strength and ductility; excessively high casting temperatures, overly rapid casting speeds, incorrect casting flows, poorly designed molds or crystallizers, insufficient or uneven cooling can lead to thin or localized stresses in the chill layer; serious defects in the ingot mold or improper installation of the thermal insulation cap during the solidification process of the steel ingot; poor performance of protective slags, mold tides, and various casting operations can all contribute to poor surface quality of the steel ingot and result in cracks.
Almost every step in steel production carries the potential for cracking, whether it's during casting, rolling, or subsequent processing stages. Addressing these requires careful attention to material composition, casting conditions, and processing techniques.
To summarize, ensuring proper handling and management across all stages of steel production is crucial for reducing defects like scabs and cracks. This involves continuous optimization of processes and vigilance against potential pitfalls at each step of manufacturing.
Rotary forging die