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Improved Level 2 Draft Scheduling for Good Plate Shape and Properties


3.    draft scheduling for improving plate shape (2)

(5) Mill Capacity and Productivity

As long as the steel property and shape requirements are satisfied, the Level 2 model would schedule as few passes as possible within the mill capacity limit, in order to achieve high productivity. Usually, a smaller draft makes it easier to achieve the equal deformation across the width; this is at least a partial reason that the finishing passes are usually with smaller draft than rest passes. To be noted is that for some alloy steels, amount of draft should also be below the formability limit to avoid possible crack.

(6) Roughing Pass Specific

For roughing passes, it is aimed at pressing steel as hard as possible in order to reach high productivity, so constrain is primarily the equipment capacity, mainly the allowed maximum torque of the equipment. Equipment should not be damaged and the roll should not be broken. Roughing passes are also with some requirements for width target. In addition, geometry deformation would be carefully checked to eliminate potential problems (e.g. fish tail). In creating draft schedule, high draft in certain thickness or certain temperature range would be applied, so that the region in the middle of the height receives sufficient reduction.

(7) Finishing Pass Specific

In the finishing passes, due to a short contact length, the rolling torque is usually not a limiting factor; instead, the roll force (except the last pass) should be carefully examined to make sure it is below force limit. Roll crown and roll deflection, etc., should be predicted as accurate as possible in order to achieve the equal draft over the plate width. Full range of mathematical models, usually over 100 counts, may be applied, including those to quantitatively predict certain defects such as center buckle and edge wave.

Later finishing passes, such as the last two passes, should mainly focus on guaranteeing the finish shape. However, too low reduction would negatively impact the rolled steel property. Therefore, shape and property requirements are contradictory and have to be balanced. See the discussion in the next section, the "DRAFT SCHEDULING FOR ENHANCING PLATE STEEL PROPERTY". In this case, increasing model prediction accuracy for force prediction and roll deformation prediction improves both the shape and property.

In certain situation, the finish pass (last pass) rolling may be actually conducted in the austenite/ferrite two-phase region due to the temperature error, though the rolling was initially scheduled above the two-phase temperature region. This could lead to severe shape defect, due to the force model failure. Force model improved by Metal Pass has function to handle this issue so to maintain good plate shape.

(8) Rolling Process Oriented

In a more comprehensive manner, algorithm to determine number of passes depends on a great number of factors and may need to be under frequent modification. For example, sometimes an even number of passes plus a dummy pass would be much better than odd number of passes. Often, a set of intermediate dimension targets are of great help. Not every Level 2 system is capable of saving good draft schedules or portions of schedules for future use. Even the established Level 2 model, such as the one from former Tippins, there are still a great number of improvement opportunities in the draft scheduling logics.

Much more factors need to be taken into account if metallurgical principles are to be implemented in the Level 2 model.

(9) Linear and Nonlinear Algorithm

Most Level 2 models use linear algorithm to perform draft scheduling. Nonlinear algorithm would be preferred for draft scheduling. However, higher technical skills would be required to handle various issues in nonlinear scheduling.

<To Be Continued>

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