Abstract: M2 steel was melted by a 25 kg high-frequency vacuum induction furnace and was cast to ingots with a cross section of 100 mm×50 mm by the water-cooled copper mould and the sand mould.The effects of cooling rate on the ingot's secondary dendrite arm spacing(λ₂),permeability,and size and distribution of grains and carbides were studied.The results show that fast cooling rate can effectively decrease the λ₂,permeability and size of grains and network carbides,and improve the distribution and uniformity of network carbides and grains during solidification.The λ₂ values of ingots cast with the sand mould and the water-cooled copper mould are 42.5 μm and 21.6 μm,the cooling rate is 1.06 K·s^-1 and 12.5 K·s^-1,and the permeability is 1.3×10^-1 μm² and 3.5×10^-2 μm²,respectively.Fast cooling rate can effectively reduce the center carbon segregation degree.The area fraction of carbides is 0.46 and 0.30 respectively at the center of ingots cast with the sand mould and the water-cooled copper mould,and increases by 38.7%and 2.2% respectively compared with their average values.The average grain size of ingots cast with the water-cooled copper mould and the sand mould is 41.3 μm and 72.6 μm,respectively.Comparing with the sand mould ingot,the grain size at the center of the water-cooled copper mould ingot reduces by 43.2%,and the grain size of the water-cooled copper ingot is more uniform.The relationship between grain size and cooling rate was presented in this paper.