Abstract: Severe transverse cracks had appeared on the surface of boron microalloyed steel billets produced by a certain company. The reasons for the formation of cracks in the billet were studied by macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis, and thermoplastic analysis. The results show that factors such as excessive carbon content in the billet, unreasonable secondary cooling system, low casting rate of the casting machine and improper precision control of the casting machine equipment had resulted in transverse cracks on the surface of the billet. Adopting methods such as optimizing the chemical composition of raw materials, improving the casting rate and controlling the accuracy of the casting machine could avoid transverse cracks in the casting billet.