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Nowadays the steel market is becoming ever more competitive for European steelworks, especially as far as flat steel products are concerned. As such competition determines the price products, profit can be increased only by lowering production and commercial costs. Production yield can be significantly increased through an appropriate scheduling of the semi-manufactured products among the available sub-processes, to ensure that customers’ orders are timely completed, resources are optimally exploited, and delays are minimized. Therefore, an ever-increasing attention is paid toward production optimization through efficient scheduling strategies in the scientific and industrial communities. This paper proposes a hybrid approach to improve the flexibility of production scheduling in steelworks producing flat steel products. Such approach combines three methods holding different scopes and modelling different aspects: an auction-based multi-agent system is applied to face production uncertainties, multi-objective mixed-integer linear programming is used for global optimal scheduling of resources under steady conditions, while a continuous flow model copes with long-term production scheduling. According to the obtained simulation results, the integration and combination of these three approaches allow scheduling production in a flexible way by providing the capability to adapt to different production conditions.

Part 14: Multi Agent Systems; International audience; In recent years, the European Steel Industry, in particular flat steel production, is facing an increasingly competitive market situation. The product price is determined by competition, and the only way to increase profit is to reduce production and commercial costs. One method to increase production yield is to create proper scheduling for the components on the available machines, so that an order is timely completed, optimizing resource exploitation and minimizing delays. The optimization of production using efficient scheduling strategies has received ever increasing attention over time and is one of the most investigated optimization problems. The paper presents three approaches for improving flexibility of production scheduling in flat steel facilities. Each method has different scopes and modelling aspects: an auction-based multi-agent system is used to deal with production uncertainties, a multi-objective mixed-integer linear programming-based approach is applied for global optimal scheduling of resources under steady conditions, and a continuous flow model approach provides long-term production scheduling. Simulation results show the goodness of each method and their suitability to different production conditions, by highlighting their advantages and limitations.

The paper presents a machine learning-based system aimed at improving the homogeneity of tensile properties of steel strips for automotive applications over their strip length in the annealing and hot dip galvanizing lines. A novel modular approach is proposed exploiting process and product data and combining smart data pre-processing and cleansing algorithms, an ensemble of neural networks targeted to specific product classes and an ad-hoc developed iterative procedure for identifying the variability ranges of the most relevant process variables. A decision support concept is implemented through a software tool, which facilitates exploitation by plant managers and operators. The system has been tested on site. The results show its effectiveness in improving the control of the thermal evolution of the strip with respect to the standard operating practice.

The paper proposes a Nonlinear Model Predictive Control strategy for the control of steam turbines rotor thermal stresses, which exploits the approximation of the turbine rotor as an infinite cylinder subjected to external convection. The Nonlinear Model Predictive Control allows optimizing the control strategy in the long term, by significantly reducing the machine start-up time during the power up ramp. This study proposes two different control strategies: the former one is based on the control of the Heat Transfer Coefficient, correlated to the inlet valve stroke. The latter one is based on the control of Heat Transfer Coefficient and the boiler steam temperature reference. Both strategies achieve good results in shortening the start-up time. The overall approach is validated and currently under development on Programmable Logic Controller platforms to the aim of code optimization.

Reaching the maximum yield of produced stainless flat products is an important aim to be successful in the competitive market. This paper describes a solution to optimise the allocation of orders to coils taking into account the actual product quality as well as the quality expected by the customer. In opposite to already available solutions also the appearance of over-quality products is taken into account. Especially for high quality stainless flat products the re-allocation of over-quality products to orders with higher demands can increase the added value distinctly. Another topic is the processing of Manufacturing Specifications (MS). A concept for the digitization of usually freely formulated MS is presented as well as a systematic monitoring, if the MS are really considered by the plant operators. The shown use cases were implemented at the German sites of Outokumpu Oyj, the Finnish producer of stainless flat products sited in Espoo.

The paper describes an application of optimization techniques for the minimization of the specific electrical energy consumption related to the production of steel for a steelworks situated in Italy. The major electrical consumption derives from two internal plants: the Electric Arc Furnace and the Ladle Furnace. This work addresses the problem of understanding the best settings (based on predefined models) to produce a specific steel, which is mainly characterized by its steelgrade and quality, with the minimum energy consumption.

Over the past few decades the issues related to the energy consumption and the climate change have been increased and they have achieved a significant position on the sustainability agenda of the steel industry. Steel production is among the largest energy-intensive industrial processes in the world, as well as one of the most important CO2 emission sources. However, the major role of steel utilisation in the modern society is undeniable. The challenges of industrial energy systems aim at achieving CO2 minimization, without neglecting energy efficiency as well as the development of effective models and strategies for process optimization. The application of Process Integration (PI) methods to the integrated steelmaking route, aims at achieving a reduction in the CO2 emission by optimizing material and energy systems. The work presented in this paper is devoted to the development of a model for optimal exploitation of energy resources and by-products in integrated steelworks through application of multi-objective optimisation techniques. Cases of exploitation of the system within the management of the process gases are presented in a retrofit scenario and compared to the case of nominal operation.

The paper presents a novel method for steel cutting optimization, which is performed by means of a two dimensional bin packing algorithm, with bins of different sizes. Our model extends previous mixed integer programs for bin packing, accounting for many of the issues arising in the steel manufacturing industry. In particular the proposed method can satisfy customer requirements on product quality. The developed algorithm can also deal with parameters variability occurring after the rolling processes. The final problem is formulated as a mixed integer non linear program (MINLP). This model directly applies to steel cutting, but it can be easily adapted in all of its parts, or just some of them, to other (industrial) rectangular cutting problems on different surfaces.

The European steel industry is constantly promoting developments, which can increase efficiency and lower the environmental impact of the steel production processes. In particular, a strong focus refers to the minimization of the energy consumption. This paper presents part of the work of the research project entitled “Optimization of the management of the process gas network within the integrated steelworks” (GASNET), which aims at developing a decision support system supporting energy managers and other concerned technical personnel in the implementation of an optimized off-gases management and exploitation considering environmental and economic objectives. A mathematical model of the network as a capacitated digraph with costs on arcs is proposed and an optimization problem is formulated. The objective of the optimization consists in minimizing the wastes of process gases and maximizing the incomes. Several production constraints need to be accounted. In particular, different types of gases are mixing in the same network. The constraints that model the mixing make the problem computationally difficult: it is a non-convex quadratically constrained quadratic program (QCQP). Two formulations of the problem are presented: the first one is a minimum cost flow problem, which is a linear program and is thus computationally fast to solve, but suitable only for a single gas network. The second formulation is a quadratically constrained quadratic program, which is slower, but covers more general cases, such as the ones, which are characterized by the interaction among multiple gas networks. A user-friendly graphical interface has been developed and tests over existing plant networks are performed and analyzed.

We reflect upon an analogue of the cut locus, a notion classically studied in Differential Geometry, for finite graphs. The cut locus C ( x ) of a vertex x shall be the graph induced by the set of all vertices y with the property that no shortest path between x and z , z ? y , contains y . The cut locus coincides with the graph induced by the vertices realizing the local maxima of the distance function. The function F mapping a vertex x to F ( x ) , the set of global maxima of the distance function from x , is the farthest point mapping. Among other things, we observe that if, for a vertex x , C ( x ) is connected, then C ( x ) is the graph induced by F ( x ) , and prove that the farthest point mapping has period 2. Elaborating on the analogy with Geometry, we study graphs satisfying Steinhaus' condition, i.e.?graphs for which the farthest point mapping is single-valued and involutive.

The paper describes a vision system equipping a CNC machine tool for cutting marble and similar stones, which allows a semi-automatic detection of surface defects and then computes an optimal distribution of the workpieces to cut by avoiding the defective parts of the slab. In order to address the workpiece allocation, a nesting algorithm is exploted which solves the so-called irregular packing problem with a limited computational time by largely reducing the time required to prepare the cutting program. The system has been fully implemented and tested on a real CNC machine tool of a marble manufacturer: the experimental setup and some exemplar results of the workpiece allocation procedure are described and discussed.

Steel production is among the most energy-intensive industrial processes and is a very relevant source of CO2 emissions. Steelworks, like most process and manufacturing industries, are facing the complex challenge of minimizing CO2 emissions while improving energy efficiency and developing effective strategies for process optimization. To this end, Process Integration methods can be successfully applied to the integrated steelmaking route with the purpose of achieving a reduction in the CO2 emissions, while optimizing material and energy systems. The work presented in this paper is finalized at the development of a model for optimal exploitation of energy resources in integrated steelworks, through the application of multi-objective optimization techniques, which can be beneficial both in new design and as a retrofit. Using these techniques, a series of solutions can be found representing optimal trade-offs among different objective functions and the designer can select the best one or specify further criteria. Combining these methods, a retrofit was devised which optimized off-gas exploitation. This retrofit was then compared to the current operation.

The dicycle transversal number t(D) of a digraph D is the minimum size of a dicycle transversal of D, i. e. a set T of vertices of D such that D-T is acyclic. We study the following problem: Given a digraph D, decide if there is a dicycle B in D and a cycle C in the underlying undirected graph of D such such that B,C are disjoint. It is known that there is a polynomial time algorithm for this problem when restricted to strongly connected graphs, which actually finds B,C if they exist. We generalize this to any class of digraphs D with either t(D) not equal to 1 or t(D)=1 and a bounded number of dicycle transversals, and show that the problem is NP-complete for a special class of digraphs D with t(D)=1 and, hence, in general., IMADA-preprint-math, 20 pages

A quasi-hamiltonian path in a semicomplete multipartite digraph D is a path which visits each maximal independent set (also called a partite set) of D at least once. This is a generalization of a hamiltonian path in a tournament. In this paper we investigate the complexity of finding a quasi-hamiltonian path, in a given semicomplete multipartite digraph, from a prescribed vertex x to a prescribed vertex y as well as the complexity of finding a quasi-hamiltonian path whose end vertices belong to a given set of two vertices {x,y}. While both of these problems are polynomially solvable for semicomplete digraphs (here all maximal independent sets have size one), we prove that the first problem is NP-complete and describe a polynomial algorithm for the latter problem.