Modeling And Optimization For Work Roll Shifting And Draft Scheduling In Schedule-Free Rolling

Since the industrial society, the steel has been the most important basic raw materials in the world. As one of the most important pillar industries in China, the steel sales volume has achieved about 8% of the GDP, and the annual output about 50% of the whole world. However, the profit of the steel production has been less and less, because of which reducing the steel production cost has become the crux of the survival of the iron and steel enterprises. Meanwhile, the marketing demands have become more and more various, and the requirement for the product specification & quality has become stricter and stricter. As the result, the flexible production mode has been the inevitable choice of the iron and steel enterprises, which can meet the low cost and diversification requirement at the same time. Then the technology of schedule-free rolling (SFR) has been developed.SFR is the effective way to achieve flexible production organization in hot strip mills. It is a rolling technology which realizes free and unrestricted switch of the steel grade, thickness and width. It can effectively improve the freedom of the production organization, and reduce the product cost of the iron and steel enterprises.However, limited by hardware equipment and optimal control technology and other factors, the technology of SFR currently has not been popularized in China. In recent years, with the improvement of equipment technology in hot rolling, the SFR equipment has been installed on some domestic hot rolling lines. The point is that the related optimal control technology has been the bottleneck of SFR.The difficulty of the optimal control technology in SFR lies in two aspects. Firstly, process modeling is complex, because the accuracy requirement of optimization objectives of SFR is on micron level, and the accuracy is affected by rolling specifications, process parameters, device status, and many other factors. Secondly, optimization algorithm is complicated. The optimization problems in SFR are multi-objective optimization ones, and the calculation process is very complex and takes a long time, so it is not easy to achieve the online control.In recent years, with the improvement of computer’s performance, comprehensive optimization for the SFR problems has been made possible. Therefore, this paper studied the modeling and optimization for work roll shifting and draft scheduling, centering on the problems during the implementation of SFR at Baosteel 1880HSM. The optimal control technology for SFR is developed to overcome the changing local uneven roll wear and thermal crown in the rolling process of a rolling campaign and keep the rolling stability of finishing process. The main work and fruits are listed as follows:1. On the basis of a detailed analysis of influencing factors on roll wear, a work roll wear model has been established, which considered work roll shifting and adopted the slice method. The evolution of roll wear in a rolling campaign was analyzed by the step-by-step accumulation method. It is found that work roll shifting can make roll wear uniform in the strip width direction, and eliminate severe local cat-ear wear in the roll zone corresponding to the strip ends. It is beneficial to the achievement of SFR.2. A roll thermal deformation model was established by the numerical simulation of roll temperature field. Considering the effect of work roll shifting on roll thermal crown, a finite difference method of PR format for roll temperature field was presented. It is found that the phenomenon of "shallow effect" exists in the effect of rolled strip on roll temperature field. The roll thermal deformation was simulated in a rolling campaign. It is pointed out that work roll shifting can disperse the thermal expansion of the roll body especially the edge to make roll thermal contour uniform.3. A multi-objective optimization approach for roll shifting strategy was presented. A new multi-objective differential evolution algorithm based on decomposition, called MODE/D, was proposed. MODE/D used differential evolution instead of genetic algorithm as the main search engine. The roll shifting strategy with varying stroke and its recursive algorithm were proposed, which can control roll shifting strategy of a rolling campaign by some decision variables such as shifting step and shifting stroke. The effect of work roll shifting on roll comprehensive contour is analyzed by numerical simulation. Two smoothness indexes were presented to evaluate the effect of work roll shifting on gap contour in the contact zone between work roll and strip rolled. Body smoothness average and edge smoothness average of all strips in a rolling campaign were selected as objective functions, and then a multi-objective optimization model of roll shifting strategy was built. On this basis, a two-stage optimization method for roll shifting strategy for alternate rolling campaigns was proposed.4. A multi-objective optimization approach for draft scheduling was presented. A high-precision integrated model of rolling force and torque was proposed, which can provide the force and energy parameters for draft scheduling calculation. An algorithm of change load ratio by adjusting draft (CLAD) was developed to achieve rolling force ratio distribution online in hot strip mills. The multi-objective optimization model was proposed to determine the target load distribution ratios, which takes into account the minimum energy consumption, load distribution balance and good strip shape. The contradictory relationship between the different objective functions was analyzed.The work of this paper provides the theoretical basis for the optimal design of work roll shifting strategy and draft scheduling for finishing stands. Meanwhile, the work targets the online control of SFR and thus has strong practical value. The proposed related fruits have been applied to Baosteel 1880HSM. It has significantly expanded the ability of SFR at 1880HSM. For example, the rolling length of the same width has been extended from 30km to 60km, and the total length of batch rolling from 70km to 100km. And SFR technology has been achieved at Baosteel 1880HSM for the mass production of mixed-grade steel products which is usually difficult to be produced.