脉冲压缩技术在高温连铸电磁超声测厚应用研究

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

在高温连铸过程中,采用非接触无损检测技术实现高温连铸坯壳厚度和液芯末端位置的实时测量与监测,对提高连铸质量等级和连铸速度,避免生产线中断具有重要的意义。针对电磁超声换能器(Electromagnetic acoustic transducer,EMAT)在高温连铸件的检测回波信噪比差这一难题,建立了基于chirp信号、13位巴克编码序列激励的跑道线圈EMAT检测过程有限元模型,比较了两种调制信号经脉冲压缩和旁瓣抑制后脉压信号的信噪比(Signal-to-noise ratio,SNR)和空间分辨率。以表面粗糙、晶粒粗大的190 mm厚连铸坯为研究对象,分析了反射和透射模式下不同调制信号脉冲压缩技术所获得的检测回波信噪比和空间分辨率。开发了水循环式高温EMAT探头,以600~750℃高温连铸坯为检测对象,比较了chirp和巴克码脉冲压缩技术实施后的检测回波信噪比和主瓣宽度。结果表明:常温下,选用的激励信号时宽越大,脉压信号信噪比越高,且采用一发一收透射式测厚能获得较高的信噪比和较小的主瓣宽度;高温时,采用脉冲压缩方法检测高声衰减连铸坯时,相位编码脉压信号获得较高的回波信噪比,线性调频脉压信号具有较高的空间分辨率。在750℃高温下,与单频正弦信号激励相比,采用脉冲压缩技术后得到的脉压信号,SNR至少提高7.0 dB。

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.