| The grain size of the metal strip product is an important quality parameter for characterizing the internal microstructure of the material.To some extent,it determines the mechanical properties of materials such as strength,hardness and electromagnetic properties of materials such as magnetic flux,conductivity.The on-line detection of grain size is the key to realize the closed-loop control of the microstructure of metal strip,and it is also the necessary means to realize the intelligent production of strip.However,for a long time,the detection method of the grain size of metal materials has been stuck in the off-line destructive detection,and has the drawbacks of long cycle time and inability to realize on-line detection.In this paper,a method for on-line detection of grain size of metal strips based on laser ultrasonic technology is studied.A laser ultrasonic inspection system for detecting the grain size of moving metal strips is built in the laboratory.The grain size detection of the metal strip in the moving process is realized by the calculation model based on the ultrasonic attenuation coefficient spectrum.The following innovative results have been achieved:(1)A laser ultrasonic inspection system for detecting the grain size of the metal strip was built to realize the long-range ultrasonic excitation,reception,and continuous acquisition and processing during the movement of the strip.Ultrasonic longitudinal wave reciprocatingly propagated inside the strip is detected by the two-wave mixing interferometer with 8 ns width pulsed laser as the excitation source of the ultrasonic wave.High-speed signal acquisition system based on PXIe bus was used to realize high-speed continuous acquisition of ultrasonic signals,providing hardware guarantee for experimental research of grain size.(2)The influence of the propagation direction characteristics of laser ultrasound on the detection results is revealed when the detection spot deviates from the ultrasonic epicenter through simulation analysis and experimental verification.A finite element model for simulating ultrasonic excitation and propagation processes was established.A blade method for monitoring the offset between the detected light and the ultrasonic epicenter is proposed.The influence of laser ultrasonic propagation direction characteristics on ultrasonic longitudinal wave attenuation coefficient is analyzed with the simulation and experimental data.(3)The calculation model of grain size distribution of metal strip based on ultrasonic longitudinal wave attenuation coefficient spectrum is established,and the relationship between grain size distribution width and the average value is analyzed.Based on wavelet transform,baseline drift removal technique and fast Fourier transform,the ultrasonic far-field multiple echo algorithm is used to eliminate the influence of near-field diffraction,and the calculation model of ultrasonic longitudinal wave attenuation coefficient spectrum is established.Based on the EBSD experimental data,the calculation model of grain size distribution was established,and the influence of the error of the grain size distribution width and the average value was analyzed.(4)Considering the potential influencing factors in the online detection process,it reveals the influence of the moving speed of the strip during the detection process,the external jitter and the small angle deflection on the detection results.The compatibility of the application environment for the grain size online testing using laser ultrasonic technique is found out. |
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