## 1 Introduction  
Steel is an important material for the manufacture of products, such as precision machinery. It is easy to develop fractures caused by early fatigue damage and microcracks when exposed in harsh environments, such as high pressure, high temperature, and high speed for a long-term [1]. It threatens the safe operation of equipment and the safety of people's lives. EMATs excite and receive acoustic waves through the electromagnetic coupling, and are widely used in the non-destructive testing field of metallic materials [2; 3; 4; 5]. Compared with piezoelectric acoustic transducers, the detection efficiency is improved due to EMATs' characteristics of non-contact and strong environmental adaptability. However, there are some problems in EMATs, such as low signal-to-noise ratio of received signals [6; 7] and low transduction efficiency [8]. The signal is 20-40 dB lower than that of piezoelectric acoustic transducers [9]. In the detection of ferromagnetic materials, the magnet of EMATs generates a magnetic force to attract the steel, which brings inconvenience to the detection. Although the conventional EMATs do not contact the tested material directly, it is limited by the small lift-off distance. In addition,the detection requirements of the ferromagnetic material for the higher lift-off distance cannot be satisfied in extreme environments such as high temperatures. In view of conversion efficiency and signal to noise ratio performance of EMATs, scholars have proposed some methods such as EMATs structure optimization [10; 11; 12; 13; 14]. In different metal materials, the main transduction mechanism of EMATs is different in generating ultrasonic waves. The Lorentz force is the main mechanism in non-ferromagnetic metal materials. However, in ferromagnetic materials, the Lorentz force, magnetizing force, and magnetostrictive force work together [15]. Compared with the Lorentz force and magnetostrictive force, the contribution of the magnetizing force is little in the generation of ultrasonic waves in steel materials and can be ignored [16]. [PERSON] found that the magnetostrictive force is affected by the direction and strength of the static bias magnetic field [17]. [PERSON] proposed that the Lorentz force is the main mechanism of ferromagnetic materials in the normal bias field (vertical magnetic field) [18]. [PERSON] found that when the external static magnetic field is a horizontal magnetic field, the main transduction mechanism of EMATs can be the magnetostrictive force [19]. In the horizontal magnetic field, [PERSON] proved that the contribution of the low-carbon steel magnetostrictive mechanism for the EMATs efficiency increases with temperature increasing by simulation and experiment [20]. The existing research mainly focused on the optimization of the EMATs structure, whereas there is inadequate research in the improvement of the excitation power of EMATs. [PERSON] established a simulation model of an electromagnetic acoustic excitation transducer without a bias magnetic field. The simulation and experiment results showed that acoustic waves can be effectively excited by high current in a circular coil [21; 22]. A compact EMAT with a solid-state circuit and no permanent magnets was proposed by [PERSON], which operates at MHz frequencies and detects through pulse echo detection method in narrow environments [23]. [PERSON] demonstrated a new high-power phase-controlled EMAT that detected artificial internal defects of cast steel samples by a high amplitude superposition signal, composed of the signals excited by phased array coils [24]. A simple high-power EMAT without an external bias magnetic field was proposed by [PERSON] to detect defects of metal cans with liquid through the echo mode of longitudinal ultrasound [25]. [PERSON] designed a multiple-rail acoustic excitation source with reproducible pulse power output and the switching noise is effectively reduced [26].