采用激光超声非接触检测技术，选用各向异性较小的热轧钢板作为研究对象，通过对标准试样的检测分析对平均晶粒尺寸计算公式进行标定。通过分析完整的激光超声一次回波和二次回波的衰减来计算试样的平均晶粒尺寸，并采用电子背散射衍射技术对平均晶粒尺寸检测的结果来验证计算的准确性。针对在实际测量中超声波信号会受到设备稳定性、试样表面状态、检测环境的变化和试样内部组织的不均匀性等的影响，提出利用超声回波时域信号能量衰减来计算衰减系数的方法，即能量衰减特性分析方法，可以有效地提高平均晶粒尺寸的计算精度。

Laser ultrasonics, a technique based on the generation of ultrasonic waves by a pulsed laser and on their detection by a laser interferometer, can be used to determine grain sizes in steels. The absolute values of the average grain size can be calculated directly from the attenuation measurements of ultrasonic longitudinal bulk waves. The general methods of attenuation measurements are computed based on the amplitude of longitudinal waves. In fact, the amplitude as only one value is infected by the noise. Besides, the attenuation should be considered as the energy reduction. The method of time domain energy is proposed to compute the attenuation. The results indicate that time domain energy can improve the accuracy of the grain size prediction. The laser ultrasonic technique may be incorporated online for direct measurements of grain size during steel production.