Study On The Effect Of Dissolution And Precipitation Behavior Of Boron On The Properties Of Low-Carbon Steel

Controversial opinions on the influences of boron on hot ductility and grain size of low-carbon steel have not been settled, currently. Employing the method of theoretical calculation, the segregation of trace boron at solidifying front, the precipitation thermodynamic and kinetics of BN and the effect of cooling rate on the growth of BN were calculated. With different alloying elements addition, the influencing mechanism of boron on high temperature mechanical properties was revealed. The influence of cooling rate on high temperature mechanical properties was evaluated. The effect of boron on the phase transformation and strengthening mechanisms were revealed. This study also analyzed the existing forms of boron and their effects on the properties of low-carbon steel. The results in this study can be used as theoretical principle for developing boron-containing steels. This study can be used as the theoretical foundation for expanding the application of boron in steelmaking process, improving and stabilizing the mechanical properties of boron-containing steel.Firstly, the segregation of boron at solidifying front, the formation of BN and the effect of cooling rate on the growth of BN were evaluated based on theoretical calculation. The calculated results were proved by the experimental findings. The results show that BN can not form at solidifying front without considering solute segregation. Under the condition of solute segregation, the segregation of boron occurred apparently at the solidifying front when solute segregation was included in calculation. The controlling step of BN formation is the diffusion of nitrogen. The cooling rate has no influence on the segregation of boron and nitrogen at solidifying front. The extension of clotting time results in the coarsening of BN with the decreases of cooling rate.The hot tensile test was conducted to evaluate the high temperature mechanical properties of low-carbon steel and low-carbon Nb-Ti-containing steel. The influencing mechanism of boron on the high temperature mechanical properties of low-carbon steel and low-carbon Nb-Ti-containing steel was discussed by analyzing the precipitates, fracture surfaces and microstructure in the studied steel. The results show that the effects of soluble boron on the development the dynamic recrystallization and suppression of the formation of ferrite are favorable for improving the hot ductility of slab. The nitrogen fixation effect of boron decreases the amount of fine nitrides. The hot ductility of slab improves due to the acceleration of dynamic recrystallization. The nitrogen fixation effect of boron can not improve the hot ductility of low-carbon Nb-Ti-containing steel because the amount of fine (Ti,Nb)(C,N) does not decrease. The effects of cooling rate and Nb-Ti-addition on the hot ductility of low-carbon boron-containing steel were also investigated. The results show that the increase of cooling rate and Nb-Ti-addition lead to the decline of hot ductility.The continuous cooling transformation curves of low-carbon steel with different boron content were determined using hot simulation test. The microstructures of low-carbon boron-containing steel change with the different rolling parameters were also evaluated. The effect of boron on the phase transformation and the effect of rolling parameters changes on the microstructures were analyzed. Through rolling simulation test, the effect of boron on the strengthening and toughening of ferrite-pearlite low-carbon steel were studied. The results show that the strength and hardness of low-carbon boron-containing steel is lower than low-carbon boron-free steel under the condition of total boron reacted with nitrogen. Dynamic recrystallization was accelerated with the improvement of soluble boron amount when the amount of boron is higher than that needed for fixing total nitrogen. The hardness and strength of ferrite-pearlite low-carbon steel improves due to the grain refining. The microstructure refines with the increase of cooling rate and finish rolling deformation amount. It is also favorable for microstructure refinement by finish rolling temperature and finish cooling temperature decrease. The microstructure of low-carbon boron-containing steel change from ferrite and pearlite to bainite and martenite with the increase of cooling rate and the decrease of end-point cooling temperature.