Study On The Method Of Electromagnetic Ultrasonic Of Solidified Shell Thickness In Continuous Casting

The thickness of solidified shell at the end of secondary cooling zone in continuous casting is of great significance for subsequent rolling.Therefore,the thickness of shell becomes the detection target,which provides an important basis for subsequent optimal cooling design.At this time,the shell thickness measurement has a complex detection environment,and the traditional process thickness measurement can not meet the detection requirements.The electromagnetic ultrasonic transducer has the advantages of non-contact and no coupling agent,which just meets the environmental requirements of billet detection.Therefore,EMAT is used to detect the thickness of continuous casting billet shell.The thickness of billet shell at the tail end of continuous casting at 500 C was measured by electromagnetic ultrasonic shear wave,and the energy transfer mechanism at this temperature was analyzed.In this paper,the model of cylindrical permanent magnet and helical coil electromagnetic ultrasonic transducer is established by using finite element software COMSOL based on the mathematical model of electromagnetic ultrasonic transducer for Q235 billet with 30 mm shell thickness.The dominant factors of energy exchange mechanism at 500 C billet surface temperature are analyzed.Under the same bias magnetic field intensity and excitation current,the dominant factors are analyzed by comparing the magnitude of displacement amplitude under the action of Lorentz force and the magnitude of displacement amplitude under the superposition of Lorentz force and magnetostrictive stress.The existing experimental conditions are used to verify the dominant factors,and the excitation frequency is also analyzed.The influence on the energy transfer mechanism.The results show that ferromagnetic materials can reduce the excitation difficulty of electromagnetic ultrasonic transducers.When the shell surface temperature is 500 C,with the increase of frequency,the transducer mechanism of shear wave excitation changes from two mechanisms to Lorentz force.When the excitation frequency is 0.8MHz,the two mechanisms work best together.On the basis of transducer mechanism,the parameters of permanent magnet are optimized with constant frequency,which makes EMAT provide more static bias magnetic field intensity on the surface of the tested piece.At the same time,it is further found that the coil in the transducer structure has some influence on the static magnetic field provided by permanent magnet andweakens the magnetic field intensity in the transient simulation of multi-physical field.Using the optimized magnetic field strength,the conductivity and relative permeability decrease with the increase of temperature,and the receiving current density decreases.At the same time,the peak current density in the first echo is extracted and linearly correlated with the temperature.At this time,the surface temperature of the tested piece can be reflected by the peak current density.The Young's modulus of shell thickness can be measured by EMAT,so the average velocity of ultrasonic shear wave propagation can be accurately measured.The shell thickness can be obtained by echo time when the shell thickness is measured by electromagnetic ultrasonic transducer,which provides a new idea for measuring the shell thickness.Finally,the experiment verifies that the ferromagnetic material enhanced echo signal in the simulation is detected,and the temperature of resistance furnace is controlled by a single loop negative feedback of UP550 regulator.Aluminum rod instead of steel rod is used to verify the feasibility of simulating thickness measurement under EMAT temperature measurement,which provides a theoretical basis for the design of EMAT circuit system.