## 1 Introduction
Electromagnetic ultrasonic inspection is a promising technique for detecting flaws. We chose the electromagnetic ultrasonic transducer with a periodic permanent magnet as the test device to create the ultrasonic guided wave in grounded flat steel. [PERSON] et al. proposed a novel technique for rapid in situ detection of rail cracks through ultrasonic B-scan imaging. They developed an SH guided-wave electromagnetic ultrasonic transducer (EMAT) with a center frequency of 0.154 MHz for detecting straight cracks. Ultrasonic B-scan imaging was then carried out [22]. [PERSON] and colleagues carried out an insightful investigation into the transient phase characteristics of transverse horizontal guided waves, exploring the impact of varying defect depths and widths. By integrating the electromagnetic and elastic wave fields, these researchers developed a highly refined electromagnetic ultrasonic transducer model, enabling them to achieve unparalleled accuracy in predicting corrosion defect depths [23]. [PERSON] and colleagues developed a novel method for detecting defects in metal plates using electromagnetic ultrasonic surface waves. Through simulation, they demonstrated that optimizing the excitation parameters of the sensor using an orthogonal experimental design can enhance the amplitude of the generated signal by approximately 80% [24]. [PERSON] et al. analyzed the structure and principle of EMAT. Building on this foundation, they simulated EMAT with COMSOL multi-physics field finite element software to obtain the distribution characteristics and electromagnetic and force fields of EMAT during ultrasonic propagation, providing a theoretical basis for the design and optimization of EMAT [25]. [PERSON] et al. conducted a finite element analysis on Electromagnetic Acoustic Transducers (EMAT) and explored their efficacy in detecting cracks in ferromagnetic and non-ferromagnetic materials. Their findings evidenced that the model was proficient in detecting cracks in both types of plates, which are commonly used in aircraft [26]. [PERSON] et al. developed a flexible thin-film electromagnetic ultrasonic transducer for nondestructive testing and verified the transducer's performance by simulation and experiment [27]. [PERSON] et al. introduced a novel Electromagnetic Ultrasonic Transducer (EMAT) that comprises a Periodic Permanent Magnet (PPM) and a Return Misalignment Curve Coil. They conducted two-dimensional finite element simulation analysis and experiments to evaluate its performance. The outcomes demonstrated that the newly devised Rayleigh-wave EMAT enhanced the received signal by 57.9% compared to the conventional Rayleigh-wave EMAT [28]. [PERSON] et al. completed the design, fabrication, and testing of a guided ultrasonic array transducer for structural integrity monitoring of large plate structures. Using the first antisymmetric Lamb wave mode, experimental measurements were carried out on steel plates with various defects [29]. Exciting and propagating ultrasonic guided waves in grounded flat steel is simple due to their regular structure, which resembles a narrow plate structure. Based on this, we utilized ultrasonic guided waves to inspect grounded flat steel. Using COMSOL Multiphysics software, we established a 3D finite element simulation model for electromagnetic ultrasonic defect detection of flat steel, studied the propagation characteristics of ultrasonic SH waves in grounded flat steel, focused on the effects of the depth, length, and width of the corrosion defects of the flat steel, and then conducted experimental verification of the simulation results.