Study On Precipitation Behaviors Of V And Nb In Microalloyed Steel Bars

The theoretical calculation method and stress relaxation test were used to study the solution and precipitation behavior of microalloying elements V and Nb.Three different samples were used in the stress relaxation test: sample 1 with none microalloying elements like V or Nb, sample 2 with single V and sample 3 with both V and Nb. The compression deformation amounts are 20%, 50% and 70% respectively. In addition, the deformation temperature range of sample 2 is 800 ?~950 ?, and that of sample 3 is 850 ?~1050 ?. The precipitation-temperature-time(PTT) curves of sample 2 and sample 3 under different deformation amounts have been drawn according to the results of stress relaxation test. It can be seen that the PTT curves of sample 2 and sample 3 both exhibit a “C” type, that is to say the optimum temperature and the fastest precipitate time are existence. Meanwhile, the fastest precipitate times of sample 2 under three deformation amounts are 8.05 s, 7.55 s and 3.15 s, and the optimum temperatures are 830 ?, 850 ?and 850 ? respectively. The fastest precipitate times of sample 3 are 6.75 s, 2.55 s and 1.75 s, and the optimum temperatures are always 900 ?.Microstructure observations of sample 2 and 3 after stress relaxation show that the grain size decreases with the increasing deformation amounts under the same temperature, and the grain size decreases with the decreasing deformation temperatures. In addition, with the increasing deformation amounts, the size of the second phase becomes smaller, and the amount of that becomes larger. Moreover, the grain size is finer and the amount is larger of sample 3 than sample 2.The calculative all-solid-soluted temperatures(TAS) of sample 2 and 3 are 1000.29 ? and 1224.38 ? without the effects of Wangner coefficient. Then, in order to make the second phase solute in austenite totally, the soaking temperatures are selected to be 1150 ? and 1250 ? respectively in the stress relaxation test. Besides, the Nb content has a significant influence on TAS. Increasing Nb content can improve TAS, leading to carbonitrides precipitate at high temperature to prevent the austenite grain growing.Thermodynamic calculation results show that Nb can almost precipitate in austenite totally due to the low solid solubility of Nb in austenite, while a part of V solute in austenite. It is generally accepted that the microalloying elements in steel are used to stimulate precipitation, instead of being solid soluted. It can be found from the calculation results that the utilization of V in austenite has an optimized value. So it's necessary to successfully design these microalloying elements in steel with the aid of theoretical calculations for the purpose of cost saving.Theoretical calculation of stress induce precipitation behavior of carbonitrides, and it is found that the theoretical PTT curves deviate from those got by stress relaxation test slightly, but the variation trend are consistent that the fastest precipitate times are shortened with the increasing deformation amounts. The present study can provide theoretical support for rolling process of steels with the aid of theoretical calculations of free energy change of second phase and PTT curves.