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Research paper : Toward the integrated optimization of steel plate production process (K. Nishioka et al.)−98−Synthesiology - English edition Vol.5 No.2 (2012) cooling speeds after rolling, and it has been a fundamental technology in the development of new quality steel plates in recent years. However, it is difficult to manufacture TMCP steels that maintain the correct shape and the uniform cooling of a huge steel plate with several meters in length and width. Furthermore, where the manufacturing of high-quality steels is concerned, processes such as heat treatment, coating, etc. are implemented, causing problems of increased loads (process capacity vs. volume of required processing) in the finishing processes, lowered integrated production capacity, prolonged manufacturing lead time, and increased inventory, etc.3 Comparison with the automobile industry’s lean production system What kind of production control system is the automobile industry’s lean production system? Firstly, as is typical of the automobile industry, the effective use of time on the assembly line was set as an objective. To achieve this, various methodologies conceptualized as automation, just-in-time, etc. were invoked, thus eliminating muda (waste) on the shop floor thoroughly, and a production system that could respond flexibly to the changes of market trends and production processes was completed.[4]-[7] The essence of this production system is the pull-type production control that makes the upstream processes provide the final market with whatever the downstream needs in the exact amount required and at optimal timing.[2][8] In some of the assembly industries, mostly among electric equipment manufacturers, systems other than the assembly-line system, for example, the cellular manufacturing system, have been introduced.[9] These share their objective of adopting the lean production system in that they also control time effectively.Now let's determine why this production system that has been successful in assembly industries has not worked well in the steel industry. When considering the reasons, it becomes necessary to understand that the structures of processes between assembly industries and process industries are intrinsically different. The lean production system is based on the assumption that the mainline is fully leveled evenly, and it will become effective when synchronized with sub-lines. In contrast, the steel industry has had as given the integrated processes of the blast furnace-converter-rolling mill, and has pursued the improvement of efficiency by constructing larger equipment/ facilities. A plate mill is located in the midst of a push-type process structure whose upstream processes pursue economies of scale, and application of the lean production system was difficult because it assumes the existence of a pull-type process structure that pursues minimization of stock and production lead time by coordinating each work process in order to satisfy the needs of the downstream processes.In the steel industry, to improve efficiency, it is important to group together as many individual orders as possible, so that their manufacturing lot size in the upstream processes of the semi-finished product – steelmaking is increased. As a result, it used to be considered essential to have a process “to group orders into a larger lot”, i.e., to manufacture steel in a large lot in the upstream processes and to divide it in the downstream processes to meet individual orders. The pursuit of scale in the upstream processes leads to the production of intermediate goods with common steel types and sizes, and because the production lot is normally larger than the delivery lot of products, it also contains the intermediate goods of products that differ in delivery timing; this flows down the downstream processes spasmodically. Such process structure causes stock to increase, and shortening the production lead time simply by studying the lean production system was difficult.4 Efforts made for integrated optimization in manufacturing steel plates4.1 Past effortsThe Japanese iron and steel industry, amid the long-running structural economic depression after the 1973 oil shock, reproduced on a diminishing scale and promoted rationalization, concentrating its efforts on improving labor productivity. Where the fields of shipbuilding, buildings, bridges, tanks, line pipes, etc. are concerned, i.e., major customers of steel plates, they realized weight saving, higher functionality and combined characterization, demanding the steel industry to deliver products, for example, with higher strength and toughness, or high environmental-resistance. New structural steels with additional functions were developed using the TMCP technology, but steels by accelerated cooling had lower rolling efficiencyTerm 3 compared with steels for general use. Furthermore, as the application of the TMCP technology advanced, the load of the correction (flattening) processTerm 4 became heavier and the state of heavy load in the finishing processes became increasingly commonplace as the product mix shifted to a mix of more sophisticated products. In such a situation, the necessity of reinforcing the finishing capacity became increasingly recognized, but it was a common understanding that, where steel plate manufacturing was concerned, it was difficult to improve the integrated efficiency of the plant as a whole. Solving the bottleneck problem in the production system had not been prioritized as a management objective of high priority, partly because the shorter production lead time could not demonstrate a short-term profit improvement effect.Furthermore, because the upstream processes of ironmaking-steelmaking occupy a significant part in the cost structure of an integrated steelworks, steelworks management with a cost-oriented consciousness tended to be more interested in them.

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