Abstract: In view of the problem of large grain NbC grain boundary precipitation and proeutectoid ferrite film induced continuous casting slab cracks in Nb containing low alloy steel, the solidification heat transfer model was established by using Matlab and the target temperature of each secondary cooling section was optimized. The secondary cooling water volume of continuous casting was improved by back calculation, which effectively increased the cooling rate of slab edges within the NbC precipitation temperature range and the slab temperature before straightening, and enhanced the hot plasticity of narrow edges of the slab before straightening. The research shows that the NbC in Q345 steel is basically completely precipitated in the upper temperature range of A_e3 (1 108～905 ℃). Under the original secondary cooling system of continuous casting, the cooling rate of the slab in the NbC precipitation range is low (4.33 ℃/s), which makes it difficult to avoid the precipitation of large particles of NbC at the austenite grain boundary. In addition, the temperature of the narrow edge of the continuous casting slab before straightening is lower than the temperature of the transformation A_e3, resulting in the formation of proeutectoid ferrite film at the austenite grain boundary, both of them can induce the formation of narrow edge cracks. Through the adjustment and calculation of the target temperature of the secondary cooling section 3-7 and the edge water volume, the narrow edge cooling rate of the slab is achieved to 5.92 ℃/s, which is conducive to the fine dispersion and precipitation of NbC. At the same time, the temperature before the narrow edge straightening of the slab is higher than A_e3 temperature, which avoids the formation of the proeutectoid ferrite film, can significantly improve the thermoplasticity of the Nb containing low alloy steel, and provides a theoretical reference for controlling the edge defects of the Nb containing low alloy steel under the CSP process.

Key words: niobium containing low alloy steel, thin slab continuous casting, narrow edge cracks, optimization of secondary cooling, NbC precipitation regulation