Introduction of Dynamic Secondary Cooling Water System

Introduction of Dynamic Secondary Cooling Water System

1. The overview of the system 

After the introduction of the continuous casting computer control technology from Demag and VAI, our company has accumulated rich experience in the understanding of the dynamic secondary cooling water model through the study and debugging of the on-site technicians, and developed the dynamic secondary cooling model which is more suitable for on-site production and various parameters required.

Firstly, different cooling models are established according to the thickness of the cast products and the cooling characteristics of the steel grades, and the cooling method of the cast products is determined by the cooling model. When setting the water volume, the online thermal tracking model in the dynamic secondary cooling model is used for online tracking calculation of the cast products, which is also the core of the whole system. The thermal tracking model adopts the sheet movement method and the finite difference method. According to the two-dimensional unstable conduction equation, the characteristics of molten steel, the temperature of the tundish, the heat dissipation of the cast product’s surface and the movement of the cast product, the temperature curve along the casting direction is calculated and formed. The sheet movement method, that is, a cross section of the cast product moves continuously along the arc of the continuous casting machine with time, so as to obtain the temperature field of the cast product at different times and spaces. The temperature curve includes the surface temperature curve of the cast product, the center temperature curve and the thickness curve of the solidified shell. Then, for the setting value calculation, according to the temperature curve provided by the thermal tracking model and the determined cooling model, the amount of water required for each secondary cooling zone can be calculated. The result should make the surface temperature of the cast product close to the temperature of the cooling model itself average slab temperature specified by the cooling model itself for each cooling zone. This result can be the average temperature of the cast product specified for each cooling zone.

The dynamic secondary cooling water system includes two parts: the online production system and the offline setting system. The online production system includes process data tracking and set value calculation (based on dynamic speed, dynamic solidified shell or temperature model), and the relevant information can be seen on the human-machine interface, and allows manual change of water distribution mode. The offline setting system supports offline development, simulation, maintain the cooling water meter. 

Ø The offline setting system

The process engineers can predefine cooling water meters (each meter can be adapted to one or one type of steel grades) through the offline setting system. The offline setting system allows to create, delete, modify and copy cooling water meters, develop and maintain dynamic water distribution systems, adjust the online water distribution. The offline setting system also has the simulation function of simulating the actual production situation, and the simulation system can simulate the water distribution situation under different casting conditions.

Ø The online production system

The core of the online production system is the dynamic cooling model. The dynamic cooling model cyclically calculates (5s) the water flow of each zone according to the different cooling modes used. The calculation mode can be defined in each zone. The dynamic cooling model collects online real-time data and the cooling water meter used to meet the quality requirements of different steel grades, including dynamic velocity model, dynamic temperature model and dynamic solidified shell model. At the same time, each model can correct the set value of water volume in each area. 

1）Dynamic velocity model

The model cyclically calculates the running time of the cast product, tracks the selected cooling mode to calculate the set value of the secondary cooling water volume in each zone, and mainly considers: the average speed, the actual drawing speed and the relevant flow of secondary cooling zone.

2）Dynamic shell model

The dynamic shell model includes cyclic calculation of shell thickness and residence time in the sector, tracking the selected cooling mode to calculate the set value of each zone, comprehensively considering: shell thickness, current actual pulling speed, relevant zone flow and complete freezing point of casting stream.

3）Dynamic temperature model

The dynamic temperature model calculates the surface and center temperature of the cast product according to the actual water flow, casting speed, steel grade and superheat degree. According to the selected cooling water meter, the set value of each zone can be calculated by the temperature model. The bulging restrictions (considering shell thickness, molten steel static pressure and roll spacing), surface temperature control (surface temperature per zone), current actual drawing speed, flow rate in the relevant zone and casting stream heat transfer status are all considered.

In the actual casting process, there are many factors that affect the heat transfer. In order to improve the billet quality and make the secondary cooling more reasonable, the factors such as billet bulging, nozzle maximum/minimum flow, roll cooling medium, open casting and capping must be considered for correction in the actual given secondary cooling water distribution. The set value of the secondary cold water volume in each zone can be derived from the first calculation value combined with the basic set value, and the set value can be adjusted through the following reasonable processing to make the set value more reliable and practical.

At the same time, the system also provides the following functions:

1) Provide basic steel grade classification properties ( there are 8 types of steel grades, which fully cover the design and production steel grades and include the steel grades that may be produced in the future, and can be expanded infinitely).

2) The water meter is automatically selected. When the molten steel enters the mold, the water distribution model automatically selects the water meter according to the steel grades.

3) Manual selection of water meter. At any time, a cooling water meter can be manually selected manually. The manually selected water meter can be put in immediately or changed when the next batch of molten steel enters the mold.

4) Output surface temperature and center temperature curves, shell thickness curves and water distribution status information in each area on the man-machine interface.

2. Guarantee conditions

In order to ensure that the dynamic control model of the secondary cooling water distribution of the continuous casting machine designed by the bidder can achieve the design function on the basis of automation, the technical documents shall meet the following guarantee conditions for the process equipment and control system of the buyer under the operation conditions of the pouring operation.

Warranty conditions for workmanship and equipment:

Ø The secondary cooling water flow and pressure are adjusted to the allowable range of the set value

Ø Adjust the compressed air pressure to the allowable range of the set value

Ø The nozzle distribution meets the technical requirements with low nozzle clogging rate.

Ø Requirements for molten steel composition [S] < 0.015%

In terms of equipment, such as basic automation system and network system requirements are as follows:

n The basic automation side needs to establish a communication connection with the upper management or process computer (including hardware configuration and software programming), the specific scheme is explained in the preliminary design.

n The communication monitoring signal between the upper computer and the basic automation part is the changed bytes sent by the upper computer according to a certain cycle. Basic automation needs to collect and monitor these numbers. Once the maximum time required for communication delay is exceeded, it is regarded as communication interruption. Once the communication is interrupted, the basic automation should start the water distribution of the accident water meter to prevent the abnormal quality of the continuous casting billet.

n The basic automation part needs to set the window for selecting and displaying the operation mode on the HMI of the basic automation side, so as to monitor and switch (manual or automatic) various operation modes.

n In the process of putting the dynamic model of the host computer into operation, the operator should select the water meter stored in the basic automation database corresponding to the steel type to be poured before the start of pouring. At the same time, the water meter of the basic automation should automatically track according to the changes of the casting length and drawing speed, but it is not involved in the control during the pouring. Once the communication of the upper computer is interrupted, the basic automation can be automatically put into operation to realize the seamless switching of water meters.

n The data exchange must be established between the upper computer and the basic automation. The main signals sent from the basic automation to the secondary computer are:

Ø Monitoring signal for communication with the host computer

Ø Current actual control method

Ø The signal that the water distribution mode of the upper computer has been selected

Ø The water meter number that is selected by the basic automation

Ø The actual water volume of each zone

Ø The adjustment method of the control valve in each zone under the water distribution mode of the host computer

Ø The adjustment method of the control valve in each zone under the basic automatic water distribution mode

Ø The adjustment method of the control valve in each area under the operator's water distribution mode

Ø The adjustment method of the control valve in each area under the manual water distribution mode

Ø Switching status of shut-off valve in each zone

Ø Pouring speed

Ø Width of casting slab (with on-line mold width adjustment function)

The main signals sent from the host computer to the basic automation are:

Ø Host computer communication monitoring signal

Ø The upper computer control mode is effective

Ø Flow setting value of each cooling zone

Ø Switch command of shut-off valve in each zone

n System safety protection measures

There are isolation protection measures between the computer control system and external systems such as office management to prevent media or network from being infected with viruses, restrict illegal access, and avoid malicious attacks by hackers.