Level 2 Model as a
Controlled rolling with hold
A Level 2 model usually contains
a module for the controlled rolling, in which metallurgical principles are
applied to establish the controlled rolling schedules. The controlled rolling
schedules are designed either by temperature or by the steel thickness.
In order to control the rolling material microstructure that is mainly influenced by the deformation temperature, the controlled rolling process can largely be divided into the three stages. At first the steel is rolled to a certain temperature such as 1000°C (1850°F) from a reheating temperature (e.g. 1120°C, or 2050°F). Then it is held on the table and is cooled by either water (e.g. as the thickness over 60mm) or air, until its temperature drops to about 850°C (1550°F). After the hold, the slab is completed rolling to final gauge by a finish temperature such as 850°C (1550°F). The temperatures for the controlled rolling should be different from grade to grade, and the values could be studied and stored in the expert system for the Level 2 to use.
Besides the deformation temperature, enough deformation amount should be pledged. Generally, the deformation should be more than 30% of the total thickness reduction.
Some plants use the hold thickness as the four times of the finish gauge. In order to further improve the comprehensive performance and properties of the finish steel, Level 2 should assign a heavy draft in the finish stage. For simplicity, this heavy draft could be applied as the slab reaches the one-half of the initial thickness. In principle, this heavy draft should be applied in the pass(es) in which there is no significant grain growth exists and the heavy draft won’t cause shape problem. Usually, the preferred value for the
hold thickness and/or hold temperature, as well as the preferred pass in which
the high draft applies, could be stored in the expert system for a given plant with a designated product range, to make it available for the Level 2 system.
In the hold period, there are microstructure evolutions. As a result, the flow stress changes. Many Level 2 models have no reflection of this change in the flow stress model.
Figure 2 Flow Stress Error vs. Resume temperature
Studies based on the history data of a Level 2 system showed that there were significant force errors in the resume passes (Figure 2). The force error was usually over 20%. The primary reason was due to the
microstructural change during hold (recovery, recrystallization, precipitation, etc.) which was not taken into account in the flow stress formula of the Level 2 system.
<To be continued>
Part 1 |
Part 2 |
Part 3 |