Abstract: The crack rate of S355J-3-B steel slab produced by Nanjing Iron and Steel Co.，Ltd. was high in the production process. Based on the component of S355J-3-B steel, the existing forms of boron in steel S355J-3-B steel were investigated using the chemical analysis, raman spectrum and field-emission transmission electron microscope (TEM). The results of chemical analysis showed that when the total boron mass fraction was 15×10-6 and the acid soluble boron was 9×10-6 in S355J-3-B steel，BN precipitates were found using raman spectrum and TEM. The thermodynamic calculation indicated that aluminum should be added in liquid steel firstly for sufficient deoxidation in steelmaking; then titanium was added for nitrogen fixation; finally, boron was added into liquid steel. In the straightening stage of continuous casting process, the pinning effect caused by fine BN precipitates on austenite grain boundaries is the significant reason resulted in the occurrence of surface crack on the slab. However, it is favorable for improving the hot ductility of boron-bearing steel with titanium addition. The crack rate of S355J-3-B steel slab decreased from 10% to 3% with increasing the titanium content.

Key words: boron steel, boron nitride, continuous casting, titanium nitride, hot ductility