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Improved Level 2 Draft Scheduling for Good Plate Shape and Properties

Primary purpose for a plate mill Level 2 model is to create quality draft schedule for good plate shape and product properties. Three key issues should be well addressed in a Level 2 model. Firstly, the logic in creating draft schedule based on predicted parameters should be effective; secondly, parameter prediction as basis for the draft scheduling should be accurate; and thirdly, metallurgical issues such as retained strain, recrystallization, etc., for model accuracy and rolled steel properties, should be well considered. Level 2 model should achieve equal deformation target, metallurgical temperature target and maximal productivity target. Draft scheduling is also aimed at grain refinement for the finish product. This paper summarizes work results in several plate mills on Level 2 model logic improvement, Level 2 parameter prediction improvement and metallurgical integration into Level 2 model. Features for the next-generation Level 2 model as metallurgical system are also introduced.

Following sections are included in this writing:

  1. Draft Scheduling for Optimal Plate Shape and Property
  2. Accurate Parameter Prediction
  3. Draft Scheduling for Improving Plate Shape (1), (2)
  4. Draft Scheduling for Enhancing Plate Steel Property (1), (2), (3)
  5. Plate Mill Application Examples (Oregon Steel), (Nanjing Steel)
  6. Next-Generation Level 2 System, Summary, References

1.    Draft Scheduling for Optimal Plate Shape and Property

Draft Scheduling in Level 2 Model

Primary purpose for a plate mill Level 2 model is to create optimal draft schedules. The mill Level 2 system consists of Level 2 model and Level 2 management system. The level 2 management system conducts data collection and data communication inside and outside the Level 2 system. Some people also refer the Level 2 management system as the "Shell". In comparison, the "Core" of the Level 2 system is the Level 2 model.

Level 2 model generally performs following functions:

  1. Prediction of rolling process parameters such as force and temperature, as basis for the draft scheduling and production plan. Online model with learning (adaptation) is usually used.
  2. Creation of production schedule, at first the draft schedule, based on the predicted parameters. By considering rolling process and mill equipment (roll bending, AGC, etc.), it creates draft schedule for satisfying product dimension and shape.
  3. Examination of temperature and rolling speed to make sure the temperature requirement is satisfied; if not satisfied, it either adjusts the draft schedule or changes rolling speed within equipment limits, or adjusts certain hardware use such as Frits (high-pressure and low volume cooling/descaling equipment), in order to satisfy the temperature goal. Product property optimization is currently pursued through temperature control.
  4. Optimization of production plan, to improve production arrangement based on given product (grade, dimension, property, etc.). Currently, major functions in this aspect are performed by Level 3 (e.g. for hold temperatures), or by human planning. Level 2 may have certain adjustment to the original plan if there is considerable difference from initial expectation.
  5. Verification of rolled steel properties, as a new feature of the next-generation Level 2 model proposed by Metal Pass. Considering the fact that strain and strain rate also have modification to the temperature effect on the rolled steel property, the new version of the Level 2 model tries to include the strain and strain rate effects on the steel properties.

The requirements for the draft scheduling exist at least in three areas:

  1. targeted finish size, so width, height and length should be achieved within the dimension tolerances;
  2. good shape, such as flatness of the rolled plate should be sufficiently good; and
  3. rolled steel properties, such as tensile strength and yield strength, etc., should satisfy property requirement.

To be mentioned is that the dimension of the plate is included into the shape in this paper?s discussion.

Draft scheduling logics

Level 2 draft scheduling is specific for the production stage. Most systems divide the rolling passes into roughing passes and finishing passes. For roughing passes, it is aimed at applying as high draft as possible below the torque limit, in order to increase the productivity. However, there are some special occasions to be considered. In the first pass (sometimes, the first two passes), due to grain boundary issues, etc., the draft should not be too high. If the steel?s formability is an issue, it should overwrite the maximum torque rule. In the finishing passes, the main purpose is to achieve required shape and properties. It is usually to calculate draft schedule by solving equation system established based on many constrains. In the last two passes, the key issue is to maintain the shape.

Parameter prediction accuracy

Property and shape requirements usually conflict, especially in the finishing passes: high draft helps property but would negatively affect shape. Accurate force and temperature prediction would generally help the both requirements, so for example, if the force/temperature prediction accuracy improves, there is a room to increase the draft in the finishing passes while the shape is maintained.

<To Be Continued>

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