Novel Methods for Cold Rolling Process Modeling, Providing Cold Rolled Strip Quality Improvement, Energy Savings and Continuous Mill Rolling Speed Increase

摘要

There are several tendencies in the global development of flat steel rolling, which are characteristic of the late 20th and beginning of the 21st century: - Increased quality requirements imposed on cold rolled sheets, particularly for automotive applications: strength, plasticity, roughness and surface cleanliness, dimension precision and flatness; - Increased demand for ultra-thin cold rolled sheets (structural grades with thickness of 0.3 mm and under, food quality tin sheet with thickness of up to 0.1 mm); - Desire to decrease energy consumption at all stages of cold rolled sheet production, directly related to energy crisis. These tendencies have stimulated the development of various methods for cold rolling process modeling due to the fact that recognized mathematical models used in steel mill control systems did not permit for calculation of rational and cost-effective technological modes, which would make it possible to turn out high quality products meeting new, more stringent requirements. A number of theoretical issues, which were supposedly well explored and described in the works of rolling theory founders during 1950-80's had to undergo revision. This is caused by improvements in both equipment and technology of cold rolling mills, that have changed deformation zone structure in the working stands and contact stress distribution along the arc of roll contact. For example, application of new lubricant-coolant fluids at cold rolling mills producing structural and automotive products caused 2-3 time reduction of strip-on-roll friction coefficient (from 0.07–0.12 to 0.02–0.07), thus approaching the friction coefficient values achieved during tin plate rolling with palm oil. Combined with reduced strip thickness, this has influenced the entire set of rolling process energy and power parameters and caused an increase in the length of deformation zone's elastic sections where plasticity condition is not effective. In the case of rolling 0.2–0.5 ?? Thick strips, the length of elastic sections has achieved 50–70% of total deformation zone length [1]. The structure of plastic areas in deformation zones has changed as well: in the case of friction coefficient reduction to 0.02-0.03 the length of backward slip zone has increased up to 80–95% of total plastic area length. Conventional cold rolling models lack the body of mathematics allowing for the accounting of the above changes during contact stress, rolling force and rolling power calculation. Therefore, old model implementation in automatic process control systems of rolling mills under new operating conditions created energy and power parameters calculation error up to 20-50%, or higher. This inhibits the production of high quality cold rolled sheets, wastes energy and prevents process running at high rolling speeds. In order to overcome the above difficulties, some of the rolling equipment producers are basing their automatic process control software not on physical models but on statistical and/or regression models, which does not necessarily lead to positive results. In view of the above, the authors have conducted extensive research during 2000–2006 in order to create up-to-date physical models of the cold rolling process based on elasticity and plasticity laws, taking into consideration the abovementioned changes in technology. The models created were verified in practical production environment. They provided high calculation accuracy of power and energy parameters of the rolling mill and were used for raising production efficiency and upgrading cold rolled sheet quality. Below information is an abstract of the most important results from this research. Some of them have already been presented on International Conferences in Japan, USA and Russia, while a part of new results is stated in this paper for the first time.