含Nb-Ti微合金钢连铸过程中TiN析出行为研究

Precipitation behavior of TiN in Nb-Ti alloyed steel during continuous casting process

针对控制含Nb微合金钢铸坯角裂缺陷的增Ti固N控制工艺,通过热力学计算和试验分析手段,详细研究了钢中TiN颗粒的形成条件。经过研究得到结论:Ti元素可以稳定Nb在钢中的固溶状态,所生成的TiN颗粒在2μm左右,属于非金属夹杂物范畴。随着冷却速率的增加,钢中生成的TiN颗粒位置逐渐由原奥晶界转为晶内析出。基于Ohnaka微观偏析模型计算凝固TiN凝固析出行为,当固相率达到0.95以上凝固前沿的[Ti][N]浓度积高于平衡浓度积,开始析出TiN。根据凝固前沿溶质元素偏聚程度排序:Scheil>Ohnaka>B-F>C-K>V-B>Level。在钢液中析出的Al_2O_3类的高熔点氧化物降低了TiN形核势垒,促进了TiN颗粒的析出行为。

Transverse crack on the slab comer became server defect for Nb-containing steel due to precipitation of NbN particles along the prior austenite grain boundary. By adding strong nitride element Ti the precipitation of NbN can be inhibited instead by TiN particle that keep the Nb as solution condition in steel. In the present research the formation mechanism of TiN was studied by thermodynamics calculation and experiments in lab. It came to the conclusions as follow. With the increase of cooling rate of molten steel, the precipitation location of TiN particles transfers from -/grains bound- ary into grain inner. Basing on the Ohnaka micro-segregation model, the precipitation behavior of [Ti], IN] and [Ti][N] were investigated under various solid fraction during solidification process. As the solid fraction is larger than 0.95, the [Ti][N] value was higher than the one under equilibrium state and TiN particles precipitated from solidifying front. Fur- thermore, the high melting element such as non-metal inclusion AhO, plays an important role for decreasing nucleation potential barrier of TiN, which enhanced formation of TiN particles in molten steel. For micro-segregation model, ac- cording to segregation degree at solidification front, they can be sorted as following sequence, Scheil model 〉 Ohnaka model 〉 B-F model 〉 C-K model 〉 V-B model 〉 Level model .