This is a small transverse crack located at the corner of the billet. The reasons for this may be:
1) The taper of the mold is too large.
2) The surface of the mold is scratched.
3) The arc between the outlet of the mold and the zero segment is inaccurate.
Improvement method: adjust the taper of the mold, strictly align the arc, and adjust the secondary cooling so that the temperature of the corner of the billet cannot be less than 800 °C during straightening.