Effects of Niobium Microalloying on the Microstructure and Properties of Low Carbon Aluminum-killed Steel Sheet

Effects of microalloying of 0.017% niobium on the microstructure and properties of low carbon aluminum-killed steel sheet were investigated by analyzing its microstructure and properties in as hot-rolled and as annealed conditions as well as its recrystallization behavior during annealing at different coiling temperatures.The results showed that the ferrite grain of the hot-rolling band of the low carbon aluminum-killed steel tends to be nonpolygonal at low coiling temperature.However its properties is little affected by the hot-rolling coiling temperature.The ferrite grain size of the low carbon aluminum-killed steel is finer in as annealed condition than that in as hot-rolled condition,and hence its strength in as annealed condition is greater than that in as hot-rolled condition when the annealing temperature is low.On the other hand,lower coiling temperature would result in higher strength.The ferrite grain of hot band is obviously refined and hence the strength of hot band is significantly increased whereas the recrystallization annealing is prohibited after niobium microalloying.It was also found that the strength of the niobium-treated steel in as annealed condition is almost equal to that in as hot-rolled condition when the annealing temperature is low and tends to be equivalent to that of the low carbon steel annealed at lower temperature when the annealing temperature is high as a result of the coarsening of niobium precipitates and ferrite grains.