Level 2 Draft Scheduling Issues

Rolling mill draft scheduling is one of the primary task for the Level 2 model. Level 2 model is an expended roll pass design program for flat rolling (currently the shape rolling roll pass schedules are primarily generated by design engineers rather than Level 2 models). The requirements for the draft scheduling exist at least in three areas: (1) targeted finish size (width, high and length) should be achieved within the dimension tolerances; (2) flatness of the rolled strip/plate should be sufficiently high; straightness of the rolled product should also be good; and (3) rolled steel properties, such as tensile strength and yield strength, etc. should satisfy the property requirement. All of the above-mentioned requirements, except the straightness (Camber issue), are directly or indirectly related to the draft scheduling.

Draft scheduling Constrains

Flat rolling draft scheduling is under many constrains, such as equipment capacity, productivity and product quality. For roughing passes, it is aimed to press steel as hard as possible in order to reach high productivity, so the restrain is primarily the equipment capacity such as the maximum force and 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 the finishing passes, two contradictory factors should be compromised for a given accuracy of the Level 2 model in order to achieve the proper product quality. On one hand a low draft is helpful to achieve better flatness (low levels of the center buckle and edge wave) because a high draft is more liable to a draft difference between the width center and width sides. On the other hand, a high draft (often accompanied with a low temperature) can increase mechanical properties for a given steel grade; or for given requirements of the rolled steel properties a lower-cost grade can be used to fulfill an order and to save large amount of money. For a given accuracy of the Level 2 model those two contradictory factors have to be balanced. However, with a higher accuracy of the Level 2 model, a higher draft and lower temperature can be used while the flatness of the rolled steel remains good. The high quality Level 2 model makes it possible to schedule a lower-cost grade than regular one for a product order, and/or to increase the yield and productivity. Usually in the finishing pass, the force and torque limits are not a constrain factor for the draft scheduling.

Roll deformation and temperature accuracy issues

Flat rolling draft scheduling is primarily based on force prediction together with force induced roll deformation (e.g. deflection, flattening), etc. During the rolling, due to the existence of the roll separating force, there must be the roll deformations such as the roll deflection and roll flattening. Due to the high requirement for the flatness of the rolled steel, roll crown (both thermal crown and mechanical crown), roll deflection and roll flattening, etc. have to be accurately determined. Failure to achieve so would cause center buckle (if the draft in the middle is too high) or edge wave (while the draft in the two edges is too high), etc.

One of the key function for the Level 2 model is to select the needed roll deflection, etc. for a given roll crown, which is also calculated by the Level 2 model with the initial mechanical crown, roll thermal crown and roll wear, etc. If the Level 2 model cannot accurately determine those roll deformations and the roll crowns, for whatever reason (force inaccuracy, poor deformation model or poor crown model, etc.), the deformation over the steel strip/plate width is not equal. If the difference is over a certain limit, defects such as center bucket or edge wave would be caused. See the discussion on the cause center buckle and edge wave in the Significance of Accurate Level 2 Force Prediction section.

How we Improve your Draft Schedule

In the first step we would improve your model prediction accuracy for the roll separating force and temperature. See related documents such as:

• Improvement on Level 2 Model Force Prediction
• Temperature Model Improvement

For the roughing passes, we would carefully check whether the roll separating force and rolling torque are over the equipment's maximum values, if not, whether there is room to increase draft in order to achieve higher productivity. Mean while, geometry deformation will also be carefully check to eliminated potential problems (e.g. fish tail). We would apply high draft for certain passes also for the purpose of the finish product properties.

In the finish passes, the roll deflection, roll flattening, roll crown, thermal crown, mechanical crown, etc. will be carefully determined in order to achieve the equal draft over the strip width. We improve the Level 2 model by applying our rolling process models in predicting each of the above-mentioned roll deformation and roll crowns.

We may apply our mathematical model to quantitatively predict certain defects such as center buckle and edge wave.

Many Level 2 model use linear algorithm to perform draft scheduling. If you prefer, we may introduce nonlinear algorithm for your draft scheduling. However, we would suggest that you improve your model predictions for the force and temperature first, especially the force prediction model.

Our accurate model would increase your potential to reduce your slab cost in fulfilling a given product order. This would save you large amount of money because the slab cost is truly the  primary part (about 50-60%) of your expense.

See the profile for the primary consultant Dr. Benjamin Li. You may also view our company and personnel profiles. Please contact us via email bli68@qq.com or by phone (0086) 13400064848 for top quality consulting services.