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Application of Control Charts at Tata Steel's Cold Rolling Complex

Ideally control charts are applicable for manufacturing situations where it is not practical to test/inspect 100% of products for quality, since it is based on “grouping” and “sub-grouping” during analysis. In flat products manufacturing, one lot or batch of production can be interpreted as one heat cast during steelmaking. However, several slabs (6 or 7) are cast from each heat during continuous casting and typically, every slab gets rolled to one hot rolled coil, not necessarily in the same sequence as they were cast. Further downstream, in the cold rolling unit, hot rolled coils are de-scaled, cold rolled to final thickness, annealed/ galvanised and skin passed to result in final cold rolled and coated flat steel products. The flow of the manufacturing process is depicted in Figure 4. The original sequence in which the slabs were cast gets jumbled up when the final cold rolled coils are produced and finished in the Cold Rolling Complex. In addition, each cold rolled coil can be several kilometers in length and it is not practical to test many samples of finished coils for quality attributes. Therefore, one sample is taken from each coil, mostly from the coil ends and it is assumed to represent the full coil. Each sample represents typically, 0.01% or so or the final coil. This forms the basis for statistical quality control for cold rolled products.

Quality of cold rolled steel is measured and assessed on different aspects: (a) surface, (b) metallurgical, (c) dimensional and (d) shape. Various parameters on each of the above aspects form part of the quality norms pre-defined based on end application, specific customer requirements national/international standards and past history. These parameters form the basis of "Technical Delivery Conditions" or TDCs designed to meet the customer requirements. As an interval control measure, it is vital to monitor and track the trends or various parameters of TDCs to ensure that the process is under control. The control charts' visualisation developed in this work has been applied to tracking the TDC parameter of cold rolled and galvanised products.

The manufacturing process in the Cold Roll Mill Complex is equipped with the latest control systems and sophisticated automation. The Cold Rolling Mill is a state-of-the-art rolling mill with the best in automation and information technology. The plant is automated by 3 levels of automation.

Level-I

These systems have been supplied by the original equipment suppliers (like Hitachi, Flat Products India, CMI, LOI, etc.) and control the Mill operation. They do the Mill set-up based on the Level-2 input, capture the processed data and send the same back to Level-2. The Level-1 systems do not have any buffering mechanism.

Level-2

These systems are of higher level automation and are responsible for process control and process monitoring of the process lines. Level-2 has limited capability of information buffering. It also has the restricted capability of filtering out unwanted data from either Level-1 or Level-3. The work instructions received from Level-3 by Level-2 are scrutinised and passed onto Level-1 for the Mill operation. Information like production, quality parameters, mill shutdown, and actual processing parameters are also received from Level-1 and sent to Level-3 for recording and reporting.

Level-3 (CRM-IS: Cold Rolling Mill - Information System)


Coils in the Cold Rolling Mill

These systems are responsible for planning, scheduling and running of the CRM. It receives the order information from Level-4 and does the production scheduling for all the eight process lines. The schedule of production is then downloaded onto the Level-2 systems for production. The material productions in the Mill are then recorded back into Level-3. CRM-IS, Cold Rolling Information System, was developed in-house to support the Cold Rolling Mill. CRM-IS was designed in technical consultation with Posdata. The system is a traditional OLTP that takes care of production planning and scheduling, production recording, progress control as well as interfaces with Level-2 and Level-4. The system was developed and implemented simultaneously with the commissioning of the Cold Rolling Mill. The team of CRM-IS has since then been involved in maintaining the system and developing applications suited to the specific needs and requirements of CRM process engineers from various user groups like Mill Operations, Quality and Technical Services and Flat Product Technology Group (FPTG).

On-line Visualisation of Critical TDC Parameters

"On-line Visualisation of Critical TDC Parameters" was developed to study trends of process variables at CRM. The requirement was to have an application that would enable the process engineer to view process trends and take preventive measures rather than detect process deviations and take corrective measures.

Challenges

Control charts were first created through Microsoft spreadsheets and displayed. However, this was not on-line and was restricted to the desktop of the individual process engineer. The requirement was to have a more dynamic application that could display the trends on-the-fly, as and when the process engineer selected the TDC parameter that he wanted to monitor or control. A total of approximately 250 TDCs are there and developing Excel templates for each TDC parameter was not feasible. Form Builder and Graphic Builder were tried to build applications for a few selected and critical TDC parameters. However, these were also static in nature and not reusable. A graphic display was restricted to a particular process variable.

Forms at CRM-IS are developed on Form Builder Forms 5.0/6i of Developer 2000. Designing the trends charts was easy; however, translating the charts to an on-line screen was difficult and needed merging of several graphic elements to form one composite display. The individual graphic elements were created using Graphic Builder. Graphic Builder is used to produce interactive graphical displays. A display is a combination of data and behavior, encapsulated in a self-contained graphical application. This enables reuse of displays by embedding them in a form or a report. An application created in Graphic Builder is called a display. A display contains all components used in the application, including data source definitions, visual elements, and behavior. Each display contains a layout, on which the display's graphical objects are drawn. The layout consists of one or more layers containing individual visual elements of the display. At runtime, you can hide, show, and rearrange layers to present different graphical views to the user. All other components of the application, such as data queries and PL/SQL constructs, are also part of the display. There are over fifty predefined common chart styles, and one can also use a full set of drawing tools to create custom graphical displays such as interactive maps and import a wide range of image formats.

The Work-around

These applications were static and not what the process engineer wanted. A work-around was to have an application that could be dynamic in nature and also re-useable. The solution was simple. It was to have a standard graphical display that referenced statistical data in the Database. The statistical data was refreshed for each new TDC parameter.

Figure 5 shows the data flow of the system deployed to view on-line trends. An application was designed that allowed the process engineer to select the TDC, the parameter and the dates. The application then computed the statistics and populated the Tables. This meant coding the statistical formulae using a simple PL/SQL block of code. The PUSQL programme then would generate the UCLS, CLs, LCLs for the range and the process means. These are then inserted in the database Tables V-XBARVALS and V-RBARVALS (Table 1).

These Tables host the data temporarily when the trends are being generated and viewed through a simple graphic display. This meant:

  • Less coding as it would be a simple case of building the display using the chart wizard.

  • Standardised display that could be called upon for different process variables and TDC parameters.

  • The time to generate the display would be extremely small as the statistic tables would have minimum records.

  • Portable as the application could be used for any process variable.

  • In fact, a similar reusable application has been available as part of the main menu of CRM-IS that can be used to develop individual and MR charts for any process data.

Figure 6 shows the control chart for the TDC CGO1 and for the parameter ZCQTTM for top Zinc coating. CRM-IS has since developed I & MR chart as part of the standard CRM-IS menu (Figure 7) that allows the process engineers to view individual trends and moving range charts for any process variable. The advantage of this Menu Tool is that it is reusable and portable too and can be moved to any similar system developed on Oracle 8 and Developer 2000 with just the minimum changes.