Optimierung der Stahlbandgeometrie im Hinblick auf den Bandsäbel in Warmbreitbandstrassen

Lukasson-Herzig, Anna Margaretha; Kopp, Reiner (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2007, 2008)
Dissertation / PhD Thesis

Abstract

The final quality of steel strip is significantly affected by the first forming operations. Strip defects caused at this early stage are difficult or even impossible to remove in downstream lines, and may lead to a reduction of output figures as a result of their interference with forming and other subsequent processes. This work examined the formation of the so-called camber - the curvature of the strip in the strip plane - during the roughing process, which represents the first forming process in flat steel production. Inextricably linked with the camber phenomenon is the formation of an asymmetrical thickness profile (thickness wedge). The purpose of this work was to gain knowledge about the optimization potential of the roughing process in order to obtain a strip geometry with a maximum freedom from camber and thickness wedge formation. A great number of effects on camber formation is already known and has been described in the relevant literature. What has so far not been examined in detail is the edging effect frequently observed in the industrial practice. For that reason, initial laboratory tests were conducted to determine possible camber-affecting criteria in connection with the edging process. Three particular factors could be identified: different diameters of edging rolls, off-centre position of specimens and oblique head and tail ends of the strip resulted in a camber formation as early as during the edging process or after the subsequent horizontal rolling pass. Varying roughness values of edging rolls did not produce any camber. The main purpose of further tests was to examine the regeneration of a straight form without any camber and thickness wedge. To begin with, wedge-free specimens were rolled to a camber shape with tilted rolls. A restoration of the specimen's original straight shape was attempted a) by tilting the rolls in the opposite direction, b) by an only minor tilting of rolls and c) by their adjustment to a straight position. Test results proved that the straight, non-camber and wedge-free shape could be restored after the third rolling pass with only slightly tilted rolls. In addition to straight initial specimens rolled with tilted rolls, specimens with thickness wedges were rolled with straight rolls. The significant camber formation as a result of the asymmetrical thickness profile may be prevented by tilting the rolls according to the initial thickness wedge. This roll adjustment, however, does not cause a reduction of the thickness wedge. The rolled product exhibits either a camber or a thickness wedge or both defects. To obtain a straight specimen, an additional control element besides the tilting of rolls is required. For this reason, tests were conducted by using side guides. The positive effect of side guides on the strip form is frequently observed in the industrial practice. A comparison of specimens rolled with and without side guides proved that a reduction of both camber and thickness wedge can be achieved by using side guides during the rolling process. The thickness wedge was in part equalized as a result of an asymmetrical spread at both sides of the stand. In addition to laboratory tests, industrial process data were evaluated and analyzed by using data recorded by a strip form measuring system installed at the roughing train of the EKO-Stahl hot-rolling mill. This system measures the position of strip edges at both sides of the stand and at two points each in the entry and exit area of the stand. These data were used to determine the curvature of the strip, which provides a precise information about the size and direction of the strip camber. This calculation allows an online conclusion as to the strip camber at any point along the strip length. The curvature and all other relevant process data of EKO-Stahl were collected in a data base and were analyzed with regard to the strip camber, with various data-based methods being used to determine and evaluate the influential factors of the strip camber. This evaluation and the relevant test results were used to select the input variables of a data-based modelling for a pre-calculation of the strip curvature by means of the neural network technique. The obtained correlation factors between the curvature data determined on the basis of measured edge position data and those calculated by means of the model amount to 0.7-0.8 in the validation. In conclusion, a procedure for preventing camber and thickness wedge formation during hot roughing was elaborated and submitted to the German Patent Office. This procedure could allow the production of a straight form free from camber and thickness wedge formation for straight slabs as well as for slabs with a thickness wedge. A proposal for the control of the roughing process on the basis of this procedure is included.

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