4. draft scheduling for enhancing plate steel property
General Measures for Property Improvement
In today’s rolling practice, steel property enhancement is primarily achieved through controlled rolling temperature, high reduction and application of micro-alloys. The three key approaches are integrated in the production scheduling to fulfill a given product order. Adding microalloys does improve mechanical properties, however, it increases material cost and lowers down product weldability. With the low profit margin in today’s plate production, any other approaches beyond alloy addition would be worthwhile. In the plate rolling, high reduction creates pressure in achieving plate shape, so it should be carefully handled in the Level 2 model.
Application of Micro Alloys
After a sales order is received, production scheduling, mainly metallurgical scheduling, is the first step for the production. The scheduling primarily determines the followings:
- Chemical composition (amount of alloy to use)
- Major temperature values for reheating, rolling, hold, finish, and controlled cooling, etc.
- Draft schedule strategy, e.g. the thickness or temperature in which a high draft is to be applied (advanced scheduling)
The scheduling is based on the general technical level in the given plant, mainly based on experience in the past. The alloy consumption should be minimal, while the required properties should be guaranteed. In order to cover the risk of the property variation, the scheduling should be based on the worst case. Improved property stability through high-quality Level 2 model greatly reduces the alloy consumption.
Application of micro-alloys usually increases recrystallization temperature by 50°C to 100°C. Based on the mill capacity and various production constrains, high reduction may be conducted in the temperature above the recrystallization region or below it. Not only should the temperature set points be optimal, but the Level 2 model on the temperature prediction should be accurate as well.
The temperature of the plate during rolling is affected by two factors contrary to each other: heat loss through heat transfer and heat gain from deformation energy. Any variation of the draft and rolling speed, etc. would lead to temperature variation. When the temperature error is high, the system would miss the metallurgical targets such as those for controlled rolling. Consequently, the product properties would be poorer than usual.
Rolling in High Reduction
Rolling in high reduction is one of the primary strengthening approaches in the rolling process. In the roughing passes, primary limitation is the mill torque limit (except the first one or two passes). Decision should be made on when and how much a high reduction should be applied. Options include whether above the recrystallization temperature region or below it, and in which thickness. In the finishing passes, two contradictory factors should be compromised for a given accuracy of the Level 2 force model. On one hand a low draft is helpful to achieve better flatness (e.g. 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 unless the force prediction has very low error. On the other hand, a high draft (better also 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 steel grade can be used to fulfill an order and to reduce alloy consumption. For a given accuracy of the Level 2 model those two contradictory factors have to be balanced. The high quality Level 2 model makes it possible to schedule a lower-cost grade than usual for a product order, and/or to increase the yield and productivity.