Precipitation Behavior And Microstructure-property Control In Ti Microalloyed Plate Steel

Microstructure and mechanical properties of Ti microalloyed steels are specially related to the precipitation behavior of Ti-bearing particles. The interphase precipitation of carbides in transformed ferrite during continuous cooling plays an important role in strengthening matrix. The precipitation behavior and characterization of nano-sized carbides have a significant effect on the mechanical properties during tempering. Quenching-tempering and deformation at γ+α region ca(?) both influence the precipitation characteristics of carbides and change the microstructure of matrix simultaneously, thus achieving improvement on combination of strength and toughness.Nano-sized precipitates were effective in improving the strength of the ferrite matrix, and the precipitation behavior of carbides were investigated in Ti microalloyed steels during continuous cooling. The results show that a large amount of nano-sized Ti-bearing carbides formed as interphase precipitation in ferrite matrix, and the occurrence of interphase precipitation was largely related to the cooling rate and Ti content. Addition of Mo results in a delay for eutectoid ferrite formation and facilitates nano-sized carbide precipitation, which plays an effective role in precipitation strengthening.Effects of tempering process on the interphase precipitation and supersaturated precipitation were studied. It is found that the variation of yield strength had relation to the carbide precipitates evolution during tempering. These Mo-bearing nano-sized carbides exhibited excellent thermal stability under longtime holding at high temperature, and interphase precipitates presented superior coarsening resistance compared to supersaturated precipitates during tempering.By quenching and tempering, low temperature toughness exhibited significant improvement due to the improvement of matrix, microstructure. Quenching and tempering improved toughness due to the formation of lath bainite and facilitated precipitation of nano-sized carbides in the ferrite matrix, furthermore, the nano-sized carbides formed in as-rolled steel were largely retained after quenching and tempering, and thereby increased the strength of steel.In order to make use of fine-grain strengthening and precipitation strengthening adequately, the effect of finish rolling temperature on the microstructure and precipitation behavior of carbides were investigated. When the second stage deformation was decreased to y+a region, it refines the microstructure and promotes the formation of mixture of equiaxed grains and subgrains. The performance of low temperature toughness was enhanced due to the grain refinement. By decreasing deformation temperature, the interphase precipitation was suppressed but supersaturated precipitation in ferrite at low temperature was promoted because of strain accumulation, which plays an important role in precipitation strengthening effect.