Investigation On The (Ti, Mo) C Precipitation Behavior And The Mechanical Properties In Low Carbon Martensitic Steels

Low carbon martensitic steel has high strength because of the effect of carbon solution strengthening and dislocation strengthening. Moreover, the strength of low carbon martensitic steel can be further improved by the precipitation strengthening of carbides. The strengthening effects of carbides, such as Mo2C and VC, have been widely researched these years. However, the precipitation behaviour of TiC during the direct quenching and tempering processes and their strengthening effects in low carbon martensitic steel have hardly investigated. In the present study, the precipitation behaviours of (Ti,Mo)C during different processes and their effects on mechanical properties in low carbon martensitic steel were investigated by SEM, TEM, EBSD and phase analysis method. These results can provide the theoretic foundation for the mechanical properties improvement of low carbon martensitic steel.To elaborate the precipitation behaviors of (Ti,Mo)C nano-precipitates in steel, the size, amount and controlling factors of precipitates during different stages were investigated by performing TEM and phase analysis method combining with the calculation of thermodynamics and kinetics. There are two dominant stages as follows: large amount of nano-precipitates with the diameter in5-36nm were formed during the the austenite deformation between the nose temperature and almost850℃, the whole mass fraction of Ti precipitation can reach47.0%. Furthermore, lots of nano-precipitates with the diameter in5-18nm were also formed during the reheating process, and the whole mass fraction of Ti precipitation can be33.0%to46.6%.To describe the microstructures evolution and the effect of (Ti,Mo)C on the grain refinement, the SEM, TEM, EBSD were performed. The results show that the (Ti,Mo)C can hardly inhibit the migration of y/a interface during reverse transformation. However, the (Ti,Mo)C can refine the grain size by the pinning effect on the grain boundary. Meanwhile, obviously shorter martensite lath, increment of high angle grain boundaries and increment of orientation difference between adjacent blocks were found in Ti-Mo steel after reheating process. The martensite became pancake and the austenite nucleation driving force was enhanced by the stored energy after being rolled at low temperature in unrecrytallized austenite region. Therefore, the orientation of sub-structure was inherited and the grain size was further refined after reheating. Effects of (Ti,Mo)C and microstructure heredity on the mechanical properties of low carbon martensitic steel were systematically investigated.1000MPa-1700MPa high strength martensitic steels had been obtained by the grain refinement and the modification of strengthening mechanisms induced by the (Ti,Mo)C. The impact energies of martensitic steels above had obvious improvement comparing to the usual low carbon martensitic steels. Furthermore, the ductile-brittle transition temperatures of low carbon martensitic steels were also decreased by the grain refinement induced by the (Ti,Mo)C.