Control Of Acicular Ferrite In Nb-V Microalloyed Fire-resistant Steel With Low Mo Content

With the increasing market demand and the increasingly high demand of construction steel, it’s urgent to accelerate the development of fire-resistant steel with high strength and optimize the performance of fire-resistant steel by controlling the composition of microstructure. This paper mainly deals with the thermal simulation of static and dynamic continuous cooling process of Mo-Nb-V fire-resistant steel, the cooling rate, final cooling temperature, deformation and deformation temperature on the effect of microstructure transformation were studied, the formation rule of acicular ferrite under different conditions was also analysed. The results are as follows:(1) The cooling rate has a great impact on the microstructure of fire-resistant steel. As the cooling rate increases, the microstructure gradually shift from polygonal ferrite to the Hybrid organization of quasi-polygonal ferrite, acicular ferrite and bainite ferrite; The number and distribution of acicular ferrite are optimistic when the cooling rate is 10 to 30℃/s, and the lath width is about 1μm.(2) Heat preservation at final cooling temperature promotes more precipitations of Nb, V carbonitride, fine acicular ferrite and granular bainite were obtained at final cooling temperature of 600℃ with 300s, it is the other holding temperature can’t be compared with, but the lath width is wider and the number has been declined after 300s isothermal heat, so isothermal holding time should be controlled reasonably.(3) Deformation in austenite non-crystallization zone refined acicular ferrite and its lath width is about 0.5μm. Acicular ferrite has a high density of dislocations and sub-structure, some small micro-alloy carbonitrides distribute in the matrix were also obtained.(4) With the increase of deformation temperature, acicular ferrite lath width and the number of quasi-polygonal ferrite increased; With the increase of deformation, the size and the number of pro-eutectoid ferrite nucleated in the original austenite grain boundary increased, the lath width of acicular ferrite nucleated inside the austenite is narrowed, but the number has been declined.