Foreword
The MULPIC rapid cooling system, installed after the finishing mill, provides rapid cooling to a certain extent in the temperature range of the austenite-to-ferrite phase transformation, resulting in a finer phase transformation microstructure than simply controlling rolling, thus achieving higher strength steel plate quality. The newly installed MULPIC rapid cooling unit at Jinan Iron and Steel Group's Heavy Plate Plant is beneficial for improving steel plate quality and optimizing product varieties, saving energy and reducing costs, and providing strong support for product development.
1. System Design
The rolling mill's automatic control system obtains basic data such as steel grade, composition, thickness, length, width, and initial and final rolling temperatures of the rolled steel plate from the MES or L3 information management layer, and then automatically rolls the steel plate according to the rolling strategy. After rolling, the steel plate is conveyed into the MULPIC cooling zone. The MULPIC control system performs strategy analysis and automatic control to complete the intelligent cooling of the steel plate.
The MULPIC process control system is divided into several sub-processes, including a control section and a non-control section. These sub-processes exchange data via CORBA messages. The control section contains a mathematical model to describe the cooling process. It mainly consists of four sub-processes: MULPIC model monitor, pre-cooling setpoint calculation, cooling setpoint calculation during cooling, and post-cooling model parameter adaptation. The non-control section generates all other events and data to enable the control section to function, and mainly consists of seven sub-processes: MULPIC area material tracking, MULPIC area setpoint data processing, control area adaptive process, MULPIC application project specific environment calculation, MULPIC area initial data processing, MULPIC area setpoint control, and MULPIC area production data management. The architecture is shown in Figure 1.
(Figure 1: Schematic diagram of the MULPIC control system architecture)
2. Steel plate cooling strategy
To achieve the desired optimal results, each steel plate must be cooled to a different ratio depending on the required grade. This can be categorized into three methods: rapid cooling, direct quenching, and oscillating cooling. The cooling strategy for the steel plate is controlled and implemented by a cooling model. The MULPIC process control program receives instructions from the cooling model and implements them. The steel plate cooling model calculates the settings for each stage of cooling control using a setpoint calculation program. These process values include variables such as the temperature evolution of the steel plate caused by forced convection of the coolant, heat loss due to radiation, and heat generation due to stage deformation. The model's inputs include the steel plate PDI (Purpose Intake Discharge), cooling equipment parameters, and measurements collected during cooling.
Rapid cooling refers to all cooling zones being controlled by elevated water tanks, with the direct quenching booster pump not operating, and a maximum water density of 15 L/s/m². Direct quenching refers to the operation of the direct quenching booster pump, with cooling controlled by different cooling zones and water flow rates. For example, zone A is controlled by the direct quenching booster pump, with a maximum water density of 33 L/s/m². Zones B, C, and D are controlled by elevated water tanks, with a maximum water density of 15 L/s/m², to achieve different cooling effects.
To achieve the desired cooling effect for the special steel plate, it is necessary to select an oscillating cooling strategy. When an oscillating strategy is selected, the acceleration ratio, deceleration ratio, and speed during oscillation are set in the model parameter table. The number of oscillations and the total cooling time are calculated by the model to achieve the desired cooling target. To keep the cooling ratio as constant as possible throughout the entire length of the steel plate, each oscillation is offset from the previous position, so that the cooling during acceleration and deceleration does not concentrate in a particular part of the steel plate.
3. Functional module design
3.1 Steel Plate Tracking Calculation
During the cooling process, the steel plate is tracked in segments to establish dynamic cooling data along its entire length. The HMD detects the steel plate position, and information from the pyrometer and roll speed is obtained by the Level 1 MULPICBA system and transmitted to the MULPIC process calculator. A tracking map is then created to represent the position of the steel plate within the tracking area.
The secondary tracking software system tracks the steel plate exiting the finishing mill stand until it passes through a pyrometer at the hot straightener exit, where the pyrometer measures the final straightening temperature. Production tracking includes the area defined by the following equipment or measuring instruments: finishing mill, finishing mill output, MULPIC, MULPIC exit, hot straightener, and straightener exit.
3.1.1 Tracking Calculations During Cooling
The MULPIC process calculator will calculate each tracking signal, completing the calculation of tracking device extraction error and tracking device release error.
The formula for extracting errors from tracking devices is:
(Where dHEAD = board head position calculated during device extraction, and dDEV = tracking device position)
The formula for the release error of the tracking device is:
(Where dHEAD = board head position calculated during device extraction, dDEV = tracking device position, l = board length)
3.1.2 Tracking calculation after cooling
The MULPIC process calculator compares each calculated length with the plate length predicted from the plate PDI and the average calculated length from all departure signals. Due to these signals, if the plate length error exceeds the specified value, an alarm is triggered. At the mill exit, cooling machine inlet, and hot straightener exit, for one or more calculated plate lengths displayed by the control pyrometer, if the plate length error is within the allowable value, no adjustment is made to the current plate; if the error exceeds the allowable error, the system corrects it.
3.2 Initial setting value calculation
For each steel sheet, initial setpoint calculations are performed by the MUPLIC process calculator upon initial PDI receipt to establish cooling setpoints. If the operator changes the target values for the steel sheet before rolling, the initial setpoints are recalculated. As part of the initial setpoint calculation, the frame height value must be determined and the data transmitted to the MULPIC data storage, allowing ample time to adjust the frame height before the next steel sheet arrives in the cooling zone. The initial setpoints also include the cooling water flow ratio and a speed reference for the cooling machine.
3.3 Setpoint Calculation
During and after rolling, because the position of the steel sheet is tracked throughout the cooling zone, the MULPIC program calculator recalculates the water flow ratio based on actual measurements of the steel sheet temperature and thickness at the mill exit, according to the mill speed. These updated settings are transmitted to the Level 1 MULPIC data storage system. If the steel sheet cooling does not reach the actual steel sheet temperature, the system notifies the operator.
The setpoint generation algorithm is responsible for calculating the target values for cooling water flow rate, reducing steel plate temperature, and the required cooling ratio. The generated setpoints also include calculations for data such as velocity parameters, cooling manifold frame height settings, edge shielding settings, water crown valve settings, head/tail shielding settings, and head-to-tail flow ratio.
3.4 Feedforward Control
Feedforward control calculates the initial setpoints for the steel plate based on the assumption of a constant longitudinal temperature and a target thickness. In reality, the longitudinal temperature of the steel plate is not constant and varies depending on the mill reheating and rolling process, so the steel plate thickness will deviate from the target value.
To compensate for these changes, a feedforward control is required. The MULPIC program calculator performs the feedforward calculations. Actual measurements of the steel plate thickness and temperature are used to recalculate the cooling zone setpoints. The calculations include the thickness of each steel plate, the average surface temperature at the mill exit, the head position of the steel plate center under the pyrometer at the MULPIC inlet, and the plate speed. Once the parameters for each steel plate are obtained, the model server performs the feedforward template calculations and feeds back the calculated water flow rate for each MULPIC cooling zone. At the end of the feedforward calculation, the updated flow rate data is transmitted to the MULPIC PLC. The calculation cycle for the required flow rate in each cooling section is 00 ms. The manifold flow rate calculation is shown below:
(Where, Fn = flow velocity in manifold/part n, FSN = flow velocity in steel plate in manifold/part n, J = total length of steel plate in manifold/part n)
The formula for calculating the steel plate velocity associated with a specific manifold/section is as follows:
;
(Where, FSN = plate velocity in manifold/part n, FSM = plate velocity in manifold/part n calculated by template, LSI = plate length in inner manifold/part n, LS = plate length)
When calculating the feedforward flow rate, the position of each steel plate in the cooling zone is adjusted according to the expected time for adjusting the actual flow rate. The expected time includes the expected time of the MULPIC PLC and the response time of the flow control valve. The expected time will be used to adjust the steel plate position according to the following formula:
:
(Where PSA = expected steel plate position, PS = actual steel plate position, TA = expected time, and V = plate speed)
Feedforward control can be started or stopped by the operator via HMI.
3.5 Water Flow Control
When the steel plate enters the cooling zone, the feedforward controller uses the temperature data detected by the inlet pyrometer and the thickness data measured by the steel plate to recalculate the set value based on the measured steel plate parameters.
The cooling water system is turned on before the steel plate enters the cooling zone, allowing for a period of time before the flow rate stabilizes. The cooling water system is turned off when the steel plate leaves the cooling zone. As the steel plate passes through the cooling zone, the water flow control setpoint is adjusted to achieve initial and final shielding.
4. Follow-up handover with the hot straightening machine area
After the steel sheet leaves the cooling zone, it enters the hot straightening area. If the distance between the MULPIC and the hot straightening machine is short, or if temperature equalization of the steel sheet has not occurred before the hot straightening machine, the MULPIC control system will track the steel sheet to the equalization point. In any case, based on the MULPIC speed reference, the MULPIC exit temperature guarantees that the supplied steel sheet will be transported to the equalization point.
Based on the PDI parameters of the steel plate, the steel plate may not necessarily be straightened, and the number of straightening passes is determined by the operator. Based on process information received from the Level 1 system of the hot straightener, such as the inlet roll gap, outlet roll gap, and the position of the steel plate, the MULPIC model will take into account the expected use of the hot straightener and the actual use of the hot straightener in the setpoint calculation.
Once the steel plate leaves the hot straightener area, it will move to the cooling bed. The hot straightener's Level 1 system receives the straightening signal. When the tail of the steel plate leaves the hot straightener outlet high-temperature timing, it indicates that the steel plate has been moved out of the straightener area. At this point, MULPIC tracking of the steel plate ends, the MULPIC control system terminates all cooling functions for this steel plate, and uploads the data to realize its own program self-learning function, ready to prepare for the cooling task calculation of the next steel plate.
5. Conclusion
The MULPIC rapid cooling system not only meets the requirements of the controlled cooling process after rolling at Jinan Iron and Steel Group's heavy plate plant, but also has the ability to directly quench high-end products after rolling. After controlled cooling after rolling, the strength of the steel plate is greatly improved, reducing production costs and improving the quality of the steel plate, thus providing basic conditions for product development.
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