Rolling flat products : heavy reductions on first stands of a tandem cold mill : final report

Increasing the reduction rate in the tandem cold mill is a means of improving hot strip mill and pickling line productivity by increasing hot strip thickness. The most promising ways to increase cold mill reduction seemed to be ferritic hot rolling route and friction decrease. Using ferritically hot rolled coils on stand 1 resulted in 5-12 % lower rolling force. Rolling force sensitivity to friction is very high (ΔF/Δμ 3 000-11 000 t/m). It is thus very interesting to optimise friction by improved lubrication and/or roughness. For instance, stand reduction could be increased after having worn rolls for 10-20 km. It has been shown that 1.5-3 % extra reduction can be achieved on stand 1 by decreasing work-roll diameters by ~~20 mm. Unfortunately, this has negative effects on roll bending, roll crown and strip shape. When possible, increasing strip tension may help. Calculations showed the interest of using pre-heated strips (about - 10 % rolling force at 130°C). However, industrial application would require a thorough examination of lubricant behaviour in the stand and thermal consequences on stands 2, 3 and 4 of the tandem mill. Asymmetric rolling allowed an increase in reduction on the stand 1 by about 5 % for 30 % speed difference. However, a strong power mismatch between upper and lower motors and the onset of stand vibrations when asymmetry becomes too high are associated drawbacks. The best solution to increase cold rolling reduction may be a personalised combination of all these different ways, depending on the specificities of each cold rolling mill.