Thickness and crown measurement technology of Baosteel's heavy plate rolling mill

Introduction

The thickness gauge and convexity gauge of Baosteel's thick plate mill are the measuring devices of the control system of the main product quality characteristics (thickness and convexity) of the thick plate rolling process. The designed product width range is up to 5 m and the thickness range is 5~125 mm. Due to the product accuracy requirements, there are higher functional and accuracy requirements for thickness and convexity control, and there are corresponding requirements for the real-time and accuracy of the thickness gauge and convexity gauge of the measurement system. Therefore, the use of appropriate measurement methods and compensation technology for possible influencing quantities of the measurement process are the characteristics of this measurement system. At the same time, the technical characteristics of the measurement device itself also ensure its reliability.

1 Composition of the thickness and convexity measurement system

The thickness gauge and convexity gauge of Baosteel's thick plate mill are radioactive isotope thickness measurement systems introduced from IMS, Germany. Compared with the thickness gauges used by other companies and Baosteel before, the new equipment of IMS uses Interbus, measurement signal digitization and computer network, and measurement channel redundancy technology. The architecture of the entire system is clear, compact, simple and efficient, representing the latest development direction of thickness gauges and other special instruments.

1.1 Thickness and convexity measurement principle

When the gamma rays generated by the nuclear decay of radioactive isotopes (137Cs) penetrate the object to be measured, the intensity is attenuated due to scattering and absorption. The degree of attenuation is related to factors such as the type and thickness of the object to be measured. The relationship is as shown in Formula 1:

Where: I represents the intensity of the ray when it passes through the object being measured; I0 represents the intensity of the ray when it does not pass through the object being measured; μ represents the mass absorption coefficient of the object being measured; ρ represents the density of the object being measured; dh represents the thickness of the object being measured.

In actual application, different steel grades will lead to different densities (ρ), and the corresponding mass absorption coefficient (μ) will also change. For a specific steel grade, as long as the chemical composition content is known, the alloy compensation coefficient can be calculated using formula (4).

Where: Ai is the steel alloy compensation coefficient; Ai(ρ) is the steel density compensation coefficient; Ai(Z) is the steel mass absorption coefficient compensation coefficient; ρFe=7.853 8g/cm3, the standard density of iron; ρx is the density of element X; μx is the mass absorption coefficient of element X; μFe is the mass absorption coefficient of iron; G%(x) is the content of element X in the strip.

In the thick plate rolling process, the measured thick plate is actually in a high temperature state, and the product thickness needs to be output by the thickness gauge in the cold state. Therefore, the hot thickness needs to be converted into the cold thickness according to formula (5).

(5)

Where: dh is the thickness of the thick plate at temperature t (hot thickness); d0 is the temperature of the thick plate at temperature t0 (usually room temperature, cold thickness); α: linear expansion coefficient of the thick plate.

Combining equations (1), (4), and (5), the cold thickness of the steel plate after compensation is:

1.2 System composition

The entire thickness and convexity measurement system consists of two devices, namely a single-point thickness gauge and a convexity gauge. The main components of each device are the detection unit installed on site, including a 50 Curie 137Cs radiation source, an ionization chamber, a drive device, etc., and a central control system. The thickness gauge and the convexity gauge share a central control system including a central processing unit, an Interbus bus module, etc. In addition, there are some auxiliary equipment, such as a cooling water control unit, a blower, and a compressed air filter. The single-point thickness gauge on-site detection unit is directly installed between the rolling mill roll and the vertical roll. Due to the limitation of the installation space, a fixed installation method is adopted. The convexity gauge on-site detection unit is concentrated in the drivable C-frame and installed at the mill exit, about 10 m away from the rolling mill frame. Three sets of radiation sources and detectors are used, as shown in Figure 1, which are defined as the CS side, CL side and DS side respectively. The CL detector is fixed to measure the center of the thick plate. The CS side and DS side detectors can be set to measure the edge thickness of the thick plate, or use continuous scanning to measure the thickness from the edge to the center of the thick plate. The convexity and wedge of the thick plate are formed according to the measurement results of the three sets of detectors.

The single-point thickness gauge has 8 measurement channels (ionization chamber) for one detector group, and the convexity gauge has 4 measurement channels for each detector group, for a total of 12 measurement channels. When the ray passes through the thick plate and enters the ionization chamber, the microcurrent signal generated is amplified in two stages inside the measuring head and converted into a digital voltage signal. [page]

The various parts of the thickness gauge and convexity gauge system are connected through a network. In order to ensure the independence and integrity of the system, the thickness gauge and convexity gauge have built an internal independent network. The various parts of the system communicate through the internal Switch, and only the server and the user Switch are connected to the outside. The main functions of the server are to receive the setting data from L2 and route it to the client, collect the convexity data of the thickness gauge and convexity gauge and send it to L2, store various variables and parameters required for the client to run, store and analyze the thick plate rolling thickness data for a long time, diagnose the system, and provide various data for the operation station and engineer station.

Figure 2 Network structure of thickness gauge and convexity gauge

The client is the core unit for processing and control in the entire measurement system. The control logic and mathematical processing of the thickness gauge and convexity gauge are all completed by the client. It uses the WIindow CE operating system, communicates with the server through TCP/IP, controls the bus module through the bus controller, receives control commands from L1 through UDP messages, and sends the system status and real-time measurement data to L1, participating in closed-loop control.

Figure 3 Interbus module block diagram

The bus controller installed in the client is responsible for the control, status collection and diagnosis of the bus modules of the thickness gauge and convexity gauge. Interbus is a field bus researched and developed by Phoenix Contact in Germany, and became the IEC61158 international standard in 2000. The Interbus bus system is a data ring structure. This data ring consists of a bus control board and I/O devices with Interbus bus interfaces. The bus controller is the central device of the data ring control, which realizes the interaction between control data and peripherals.

IMS uses LogiCAD software as the programming and control software for thickness gauges and convexity gauges. LogiCAD software is a graphical programming software that provides users with a friendly interface and can easily configure, monitor and diagnose the functional modules of the system (including Interbus modules). The software can be used to easily design a complete structure of a measurement system and configure all devices connected to it. During the operation of the system, the process data of the connected devices can be displayed on the software or controller display, and the process data on the site can be read and intervened in real time. It can not only identify faults that occur in the system, but also locate the fault type, error location and error cause, and provide corresponding solutions through functional expansion.

2 System Performance Index

Measurement range: 5-125mm
Repeatability: ≤±0.1% target thickness (thickness range: 5~80 mm)
≤±0.2% target thickness (thickness range: 80~125 mm)
Linearity: ≤±0.1% target thickness (thickness range: 5~80 mm)
≤±0.2% target thickness (thickness range: 80~125 mm)
Drift: ≤±0.1%/8 hours
Statistical noise:
Plate thickness: Noise value:
5 mm ≤± 0.015 mm
10 mm ≤± 0.018 mm
20 mm ≤± 0.024 mm
30 mm ≤± 0.033 mm
50 mm ≤± 0.060 mm
60 mm ≤± 0.082 mm
80 mm ≤± 0.117 mm
100 mm ≤± 0.231 mm
125 mm ≤± 0.655 mm

3 Conclusion

Baosteel's 5m wide and heavy plate mill is the first ultra-wide modernized thick plate mill in China's thick plate field [1]. Thickness gauges and convexity gauges play a very important role in the control of plate thickness and convexity. Due to the special environment of the rolling site, the measurement of thickness gauges and convexity gauges still needs to be improved and enhanced. For example, the vibration of the rolling mill will cause random errors in the measurement; secondly, the thickness range of the thick plate is between 5 and 150 mm, and the steel plate has a temperature distribution in the thickness direction. Although the thickness gauge is equipped with a high-temperature radiometer, it measures the surface temperature of the steel plate and cannot truly reflect the actual temperature distribution, which will affect the accuracy of thickness measurement; other factors such as moisture, dust, and high-temperature radiation on the measurement channel will also affect the measurement accuracy. These will be reduced through proper maintenance.

References
1 Yuan Jianguang, Yang Min, He Dalun. Technology and Equipment Used in Baosteel's 5m Wide and Heavy Plate Mill [J]. Baosteel Technology, 2004, (2): 5-8. (end)