Application of Pulse Compression Technology in Electromagnetic Ultrasonic Thickness Measurement of High-Temperature Continuous Casting

In the process of high-temperature continuous casting, the non-contact non-destructive testing technology is used to realize the real-time measurement and monitoring of the thickness of billet shell and the end position of liquid core, which is of great significance to improve the quality grade and speed of continuous casting and avoid the interrupt of the production line. Aiming at the problem of poor detection echo signal-to-noise ratio（SNR） caused by the decrease of the energy conversion efficiency of the electromagnetic acoustic transducer（EMAT） in high-temperature castings and forgings, a finite element model of the racetrack coil EMAT detection process based on chirp signal and 13-bit Barker code sequence transmitting pulse is established. The SNR and spatial resolution of the two modulated signals are compared after pulse compression and sidelobe suppression. A 190 mm thick continuous casting slab with rough surface and coarse grain as the research object, the SNR and spatial resolution of pulse compression signals obtained by pulse compression techniques with different modulated signals in reflection and transmission modes are analyzed. A water-circulation type high-temperature EMAT probe is developed, the SNR and main lobe width of chirp and Barker code pulse compression technology are compared with 600-750 ℃ continuous casting billet. The results show that at room temperature, the larger the duration of the excitation signal, the higher the SNR of pulse compressed signal is. Higher SNR and smaller main lobe width can be obtained by using the transmission thickness measuring method. When the pulse compression method is used to detect the high attenuation continuous casting billet at high temperature, a higher SNR can be obtained by the phase coding pulse compression signal, and the linear frequency modulated pulse compression signal has a higher spatial resolution. Compared with single-frequency sinusoidal signal excitation at 750 ℃, the SNR of pulse compressed signal obtained by using pulse compression technique is improved at least 7.0 dB.