增氮对钒微合金化钢连续冷却相变行为的影响

Effect of Nitrogen Addition on Continuous Cooling Transformation Behavior of Vanadium Microalloyed Steels

用热膨胀仪测定了3种不同钒、氮含量试验钢的CCT曲线,观察了在不同冷速下的组织,分析了钒的析出行为,计算了各形核基底与铁素体的晶格平面点阵错配度,研究了增氮对钒微合金化钢连续冷却相变行为的影响。结果表明,增氮促进了铁素体的形成,提高了试验钢的相变开始温度,也提高了形成全贝氏体组织的临界冷却速率;在冷速0.8-1.6℃/s范围内,低氮钢的显微组织为粒状贝氏体+板条贝氏体,而在增氮钢内则有大量的针状铁素体;在低氮钢中钒主要在相变前后析出,析出物以VC为主,增加钒含量只能提高其析出量,不能改变析出温度和析出物的成分;而增氮后钒在奥氏体内析出,以VN为主;在900℃,奥氏体、VC和VN与铁素体的平面点阵错配度分别为6.72%、3.89%和1.55%,VN与铁素体存在近似共格的低能界面,能作为铁素体优先形核位置,有效促进铁素体形成。

Effect of nitrogen addition on continuous cooling transformation behavior of vanadium microalloyed steels was investigated. Therefor, CCT curves of three experimental steels with different vanadium and nitrogen content were measured by thermal dilatometer; the microstructural evolution of the steels with the varying cooling rates was characterized; their precipitation behavior was tracked, and planer lattice misfit degree of the precipitates with the ferrite matrix was calculated. The results show that ferrite transformation is promoted by nitrogen addition, and the starting temperature of transformation and the critical cooling rate for full bainite transformation are increased as well. For cooling rates in a range0.8- 1.6℃/s, the microstructure of steels with low nitrogen consists of granular bainite + lath like bainite,while acicular ferrite also exists; during or after the γ-α transformation vanadium compounds in low nitrogen steels precipitate mainly as VC, the quantity of which increases with the increasing vanadium content. However, for the steel rich in nitrogen, vanadium compounds precipitate as VN in austenite at high temperature. The lattice misfit degree of ferrite with the precipitates of austenite, VC and VN, which occurred at 900℃are 6.72%, 3.89% and 1.55% respectively. It indicates that VN precipitates act as preferential nucleation sites for ferrites and promote the ferrite transformation.