Analysis of transverse corner cracks and mould flux optimisation in the continuous casting of 34CrNiMo6 steel blooms

34CrNiMo6 steel is widely used in heavy-duty transmission components, and improving its bloom surface quality is important for increasing product yield and reducing downstream processing costs. To address transverse corner cracking during continuous casting, mould flux evaluation, optimisation and industrial validation were conducted based on the steel's solidification characteristics, thermophysical properties and casting conditions. Calculations indicated a relatively low risk of peritectic transformation shrinkage, but low thermal conductivity increased corner shell sensitivity to non-uniform cooling and heat-flux fluctuations, demanding continuous liquid slag film lubrication and stable interfacial heat transfer. The existing Flux A exhibited a long melting time and high viscosity, break temperature and crystallisation temperature, resulting in a thin liquid slag layer and a shortened effective lubrication range that contributed to frequent cracking. Following the concept of ‘stable lubrication and controlled heat transfer’, Flux B was developed by reducing basicity, Al ₂ O ₃ and carbon contents, increasing Na ₂ O content and introducing a small amount of Li ₂ O. Optimisation reduced the melting temperature from 1185 °C to 1081 °C and the viscosity measured at 1300 °C from 0.59 to 0.36 Pa·s. Shorter melting time and lower break and crystallisation temperatures indicated improved melting and rheological properties and controlled high-temperature crystallisation. A parallel two-strand comparative trial over two consecutive heats showed that replacing Flux A with Flux B increased the liquid slag layer thickness from approximately 5 to 9 mm and mould flux consumption from 0.35 to 0.68 kg·t ⁻¹ . The number of transverse corner crack indications detected by magnetic particle inspection in the sampled blooms decreased from 20 to 3, representing a reduction of approximately 85%. These results demonstrate that mould flux optimisation based on steel solidification characteristics and thermophysical properties provides an effective approach to improving the bloom surface quality of Cr–Ni–Mo quenched-and-tempered steels.