In steel rolling production, cast billets quality has a significant impact on whether the rolling process can proceed smoothly. Among them, the occurrence of steel piling accidents is often closely related to billet quality defects. So which billet defects will have such serious consequences on the rolling process?
Which cast billets quality leads to rolling steel piling
Head steel piling caused by internal cracks
Internal defects such as middle cracks and center cracks generated in the billet during continuous casting will cause stress concentration at the crack during rolling, which will gradually expand during the rolling process and easily cause splitting and steel piling. For example, when rolling bars, the middle cracks of the billet are oxidized in the heating furnace, and oxide layers are formed on both sides of the cracks. The head splits during rolling, causing the rolling mill to fail to bite normally. Studies have shown that Nb-V micro-alloyed steels are prone to form microcracks under stress due to the precipitation of Nb (C, N) along the grain boundaries during solidification, which reduces the grain boundary binding energy. The crack expansion during rolling causes steel piling. Among the steel piling accidents during rough rolling of HRB400 bars at Jiugang, 65% of the cases were related to the center crack of the ingot. When the crack depth exceeded 2mm, the probability of steel piling increased significantly.
Brittle fracture caused by inclusions
Inclusions such as Al₂O₃ and calcium aluminate in the ingot will reduce the continuity of the steel matrix. Analysis of the steel pile sample of ML08Al-S cold heading wire rod of Handan Iron and Steel shows that Al₂O₃ accounts for 64.46% of the inclusions and MgO accounts for 19.34%. These hard and brittle inclusions become crack sources under rolling stress. During the continuous casting process, the fall of nodules at the water inlet of the tundish is an important way for inclusions to enter the ingot. The Al₂O₃ content in the nodules is as high as 63.02%. After falling into the molten steel, it causes the rolled piece to break and pile up during rolling. Statistics show that steel piles caused by inclusions account for 41.3% of cold heading steel rolling accidents.
Superposition of surface defects and shape defects
Defects such as warping, scarring (such as entrapment of protective slag, large fluctuation of the liquid level of the crystallizer, etc.) and iron oxide scale on the surface of the ingot will peel off during rolling to form foreign matter, blocking the guide or rolling groove. Long-term accumulation will cause the guide to block the steel pile.
In addition, during rolling, the ingots of uneven sizes entering the rolling mill are difficult to bite into, and the deformation is uneven, which is easy to cause steel piles, such as square off and bulging. Square off of the ingot (cross-sectional dimension deviation exceeds 7mm) will cause uneven force on the rolled piece during rough rolling, resulting in bending or warping. For example, among the steel piles at the entrance of the 6-frame rough rolling of Jiugang No. 2 High-speed Line, 28% of the cases are related to square off of the ingot. For every 1% increase in square off rate, the probability of steel piles increases by 12%.
Preventive measures for steel piles based on ingot quality control
Optimization of secondary cooling system: Accurately adjust the secondary cooling intensity and water distribution of each section according to the steel type and cross-sectional dimension of the ingot. Adopt dynamic secondary cooling control, adjust the cooling water volume in real time according to the billet drawing speed, molten steel temperature, etc., so that the billet is cooled evenly and the thermal stress is reduced. For example, for Nb-containing micro-alloyed billets, reasonably reduce the secondary cooling water volume, adjust the water volume of each cooling section, control the billet temperature recovery, and reduce intermediate cracks.
Control the superheat of molten steel: strictly control the superheat of continuous casting molten steel, generally control the superheat within a reasonable range (such as 20-30℃), and reduce the problems of developed columnar crystals, central looseness and cracks in the billet caused by high superheat of molten steel.
Improve the solidification structure of the billet: refine the solidification structure of the billet through electromagnetic stirring, light pressure and other technologies, reduce component segregation, increase the proportion of equiaxed crystals, and reduce central looseness or cracks.
Dimension control during continuous casting: determine the appropriate crystallizer taper according to the cross-sectional size of the billet and the type of steel to reduce billet bulging and rhombus deformation. Stabilize the billet drawing speed, adopt a suitable billet drawing speed control system, combined with the crystallizer liquid level control and secondary cooling control to ensure the uniform cross-sectional dimensions of the billet. Accurately debug the roller table, pinch roller and other equipment of the continuous casting machine to ensure that the billet does not deform during transportation and maintain dimensional accuracy.
Steel pile accidents are caused by multiple factors, including the impact of billet quality and the impact of the rolling process. When a steel pile accident occurs, the quality traceability system can be used to quickly find the cause of the accident, whether it is a billet quality problem or a rolling process problem, so as to make targeted improvements, and at the same time provide data support for subsequent production quality improvements.
