Microstructure and Mechanical Properties of Precipitation Strengthened Fire Resistant Steel Containing High Nb and Low Mo

Through the thermo-mechanical control process （TMCP）, a high Nb low Mo fire resistant steel with the yield strength （YS） of 521 MPa at room temperature （RT） and 360 MPa at elevated temperature （ET） of 600 ℃ was developed based on MX （M=Nb, V, Mo; X=C,N） precipitation strengthening. A series of tensile and con- stant load tests were conducted to study the mechanical properties at ET. The dynamic continuous cooling transfor- mation （CCT） as well as precipitation behavior of microalloy carbonitride was investigated by means of thermal sim- ulator and electron microscopy approaches. Results showed that the failure temperature of tested steel was deter- mined as 653 ℃, and the granular bainite was obtained when the cooling rate was higher than 10 ℃/s. In the rolled state, a certain amount of M/A islands was observed. During heating from RT to ET, M/A islands disappeared, and cementites and high dense compound precipitates （Nb, Mo, V）C with size of less than 10 nm precipitated in ferrite at ET （600 ℃）, which resulted in precipitation strengthening at ET.

Effect of Ti/V ratio on thermodynamics and kinetics of MC in γ/α matrices of Ti-V microalloyed steels

Through the solubility product theory of the ternary secondary phase,classical nucleation theory,and Ostwald ripening theory,a model was established to describe the thermodynamics and kinetics of(Ti,V)C precipitates in austenite/ferrite(y/α)matrices.The model was used to calculate the volume fraction,precipitation-temperature-time(PTT)curve,and nucleation rate-temperature(NrT)curve of MC(M=Ti,V)precipitates in γ/α matrices in Ti-V microalloyed steels with various Ti/V ratios,which is verified by hardness tester,transmission electron microscopy and energy-dispersive X-ray spectroscopy.The calculations indicate that,by decreasing Ti/V ratio from Ti4V0 steel to Ti0V4 steel,the complete-dissolution temperature decreases monotonically from 1226 to 830℃,and the equilibrium volume fraction of MC pre-cipitated from austenite decreases from 0.333%to 0.091%at 900℃.Moreover,the maximum nucleation temperature of MC precipitated from α matrix decreases from 748 to 605℃and the fastest precipitation temperature decreases from 844 to 675℃as Ti/V ratio decreases.PTT and NrT diagrams of MC precipitated from α matrices in different Ti-V microalloyed steels all exhibit C-shaped and inverse C-shaped curves.In addition,both theoretical calculation and experimental results show that when tempered at 600℃for 100 h,Ti2V2 steel shows the largest hardness value of 312 HV among the three steels tested because it has a larger volume fraction(0.364%),a larger precipitate density(1689 μm-2),and the smallest average size(8.4 nm)of(Ti,V)C precipitates.The theoretical calculations are consistent with experimental results,which indicates that the thermodynamics and kinetics model for(Ti,V)C precipitates is reliable and accurate.