THICKNESS IMPROVEMENTS ASSOCIATED WITH ST RETCHING AND STAND MECHANICS FOR THE HOT-ROLLING MILL

In the contact of overall improvement of the thickness success for hot finishing mills, the work mentioned was realized by a three-sided wor k group: Irsid, Sollac, Clecim. The main points of current performance improvement in thickness, specified in this article, are broken down into the following themes: Improvement of the theoretical knowledge of stretching of a rolling mill stand at the sheet center For this, an a ccurate phenomenological model was realized, explicitly integrating th e influence of all explanatory parameters such as stand and roll geome try (radii, chamfers, table lengths), their metallurgic characteristic s, rolling and inter-roll pressures deduced from conventional formulat ions, sheet width, the contact are at the roll gap, off-centering of t he work rolls, the counter-bending and the nature of the roll gap, sym metry or asymmetry depending on exploitation conditions. The model, ad justed and validated with a strict original methodology was installed successfully on-line on all the finishing stands of the Sollac Dunkerq ue and Sollac Seremange strip mills, making it possible, after substit ution of the former models, to improve performances of the gagemeter m odel by about 30 %.Locating and quantification of friction in the stan d From a specific study of skidding in an industrial stand and a first theoretical estimate of friction levels, development of a skidding mo del integrating friction made it possible to interpret all experimenta l hysteresis readings done on the industrial site. This information ha s turned out to be very precious, making it possible to meet effective ly the following major points: choice of the priority technological ev olutions for decreasing stand hysteresis phenomena, definition of a ma intenance and stand mechanical follow-up log available on the industri al site, making it possible to make the stand behavior reliable in tim e, integration of hysteresis phenomena in the behavior of the AGC type thickness regulation loop. Study of the stability of the AGC loop wit h hysteresis From the above information, an original approach for stab ility resolution of the AGC loop with non-linearity was undertaken. Th is theoritical analysis thus made it possible to explain some unstable thickness behaviors noted on the industrial site during << mechanical crises >>, showing the preponderant role of friction distribution in the stand, of the rotation direction compared with hysteresis cycles i n stretch and in pressure, of the locating the pressure sensors used b y AGC regulation. These results have made it possible to realize techn ological evolutions and accurate regulations on the Sollac Dunkerque a nd Sollac Fos sites, marking it possible to increase the AGC gains in the last finishing stands by more than 30 % on the average between the middle of 1992 and the middle of 1993, while eliminating all thicknes s unstability to date.