Application of Infrared Thermometer in Steel Rolling Production
1 Introduction
In the modern steel rolling production process, in order to ensure the physical quality of the steel plate, the controlled rolling and cooling of the steel plate requires certain means of temperature measurement and detection. The characteristics of high precision and strong reliability of infrared thermometer can provide effective, accurate and reliable temperature measurement of steel plate, so as to improve product quality, reduce consumption and increase productivity.
2. The composition of the infrared thermometer
Infrared thermometers, also known as infrared radiation thermometers, determine the temperature of the measured object by measuring the electromagnetic radiation of the object, which comes from the energy contained in the object. For industrial applications we are concerned with infrared radiation extending from the shorter wavelengths of visible light to infrared light up to 20 μm. Therefore, an infrared thermometer (radiation thermometer) is a device that quantifies radiant energy and uses electrical signal output to express its corresponding temperature.
2.1 Optical system
The optical system is an important part of the infrared thermometer. Its main functions are: the convergence of radiant energy, the aiming at the target to be measured, the determination of the field of view of the thermometer, and a certain sealing effect on the inside of the thermometer.
2.2 Infrared detector
The infrared detector is the core part of the infrared thermometer. The infrared detector receives the radiant energy of the measured object through the objective lens, converts the radiant energy into an electrical signal, and finally obtains the surface temperature of the measured object through subsequent processing.
2.3 Signal Processing
The infrared detector converts the infrared radiation into an electrical signal, which is sent to the signal processing part, and is input to the microprocessor through the preamplifier and A/D conversion. At the same time, the ambient temperature compensation signal is also input to the microprocessor, which is linearized by the microprocessor. After processing, environmental compensation and emissivity correction, the corrected output signal is obtained.
2.4 Display output
In practical applications, the temperature signal provided by the processor is used in two ways: one is to display it through the display; the other is to send the temperature signal to the industrial control system to realize the control of the production process, and there are also two ways to use it at the same time.
Different types of thermometers can display real-time values, maximum values, minimum values, average values, and differences, and can also display emissivity set values, alarm set values, etc., and can also display temperature curves and heat maps after software processing wait. The most commonly used thermometers are 0-20mA or 4-20mA current output. If a voltage signal is required, the current signal can also be converted and scaled.
3. Selection of infrared thermometer
In industrial applications, there are often some media between the pyrometer and the measured target, which can weaken or even completely block the radiation of the surface energy of the measured target, and the pyrometer can only measure the target it "sees". Our commonly used fixed thermometers mainly include the following categories:
① Broadband thermometer, or broadband thermometer, its spectral response range is limited by the optical system, mainly used to measure low temperature, equipped with a detector with a wide spectral response range.
② Select the band thermometer, its response wavelength is limited by the filter, and the response band of the detector can be selected according to the needs of the application.
③ The short-wave thermometer can reduce the measurement error when the emissivity changes. The short wave mentioned here is relative, and it can be a wavelength of 0.6 μm at a temperature of 1500K, or a wavelength of 3 μm at a temperature of 300K.
④ Colorimetric thermometers, also known as two-color thermometers, have better measurement results when used in "very dirty atmospheres".
In the selection of the thermometer, in addition to the required temperature range, the two parameters of the thermometer "temperature change percentage" and "emissivity change percentage" are also very important for the accurate selection of the thermometer:
① The temperature change percentage of the thermometer refers to the change of the output value of the object due to the temperature change. For infrared thermometers, the greater the percentage of temperature change, the higher its sensitivity.
② The change percentage of emissivity refers to the change of the output value of the instrument when the emissivity of the measured target changes. Since the emissivity of the steel plate changes randomly within a certain range at a certain wavelength and temperature during the steel rolling process, the change in the output value of the thermometer caused by the change in emissivity is not the real temperature change of the target. Therefore, it is also necessary to adjust the emissivity change percentage.
4. Specific application
Take the temperature detection of Jinan Iron and Steel Plate Plant during controlled rolling and controlled cooling in the roughing mill process as an example: a total of four sets of LAND infrared thermometers are installed after the descaling box, before the roughing mill, and before and after the water curtain cooling device after the roughing mill . Descaling chambers provide the perfect opportunity to measure the temperature of unscaled steel plates. Before the steel billet enters the rolling mill, almost all the iron scale etc. are washed away by the high-pressure water spray, which provides a clean surface for the rolling process. The probe starts to measure the real temperature on the surface of the steel plate to ensure that this temperature is within the rolling limit and to set the rolling parameters.
The main problems encountered are: (1) determine the reasonable position of the non-contact probe so that the influence of the spray from the descaling box and the presence of oxides is minimized; (2) the probe and the mill stand should also be kept at a certain distance to prevent The splashing of oxides during the rolling process of the steel plate will cause damage to the probe; (3) water and residual scale can form a cooler area on the surface of the billet, resulting in changes in readings.
The principle of radiation temperature measurement is: the thermometer can only measure the target it "sees". There are two ways to solve the absorption of radiation by gas. One is to use a peep tube and air purger to provide a wireless Obstacles to the visual path; the other is to choose an operating band that is not affected by the medium. In response to these problems, we have selected M1/R1 short-wave probes in the LAND product SYSTEM system with high quality and reputation - so as to avoid the influence of water vapor absorption; small target size and fast response function - will aim at the oxidation on the surface of the billet A hot target between iron sheet and "black water" and makes the signal processor use the peak hold function to ensure the accuracy and continuity of temperature measurement to the greatest extent, even if the target is partially obscured or completely out of view, temperature measurement The result will also meet the requirements, so that the system output can track the real temperature of the steel plate; the high-level probe output weakens the influence of electronic interference, and this output can be directly used as the display of the final temperature; the position of the probe should be as far as possible As close as possible to the entrance of the mill, this avoids disturbances from cooling water spray and movement during opening.
