Nanocarbides in Ti-microalloyed high strength steel produced by CSP process

Optical microscopy,electron microscopy and electrolytically extracted phase analysis,in combination with thermodynamic calculations,were used to study precipitates and their precipitation behavior in Ti-microalloyed high strength steel produced by CSP process.It is found that many micron-sized cubic TiN particles and a large number of nanometer precipitates exist in the steel.The mass fraction of MX phases(M=Ti,Mo,Cr;X=C,N) in the steel is 0.0927%,in which particles smaller than 10nm account for 26.9%.Thermodynamic calculations show that titanium nitride has been almost completely precipitated from the steel during or before soaking,but titanium carbide hardly precipitates before tandem rolling.Decreasing the contents of nitrogen and sulfide and controlling the coiling temperature can increase the volume fraction of titanium carbide,and decreasing the γ→α transformation temperature can prevent fine carbides from growing.These results indicate that the precipitation hardening resulting from nanometer particles is about 156MPa and the effect can be raised through controlling the chemical composition and rolling parameters.