N对高强度压力容器钢组织转变的影响

Effect of nitrogen on microstructure transformation of high strength pressure vessel steel

利用扫描电镜、电子背散射衍射、透射电镜及VASP第一性原理模拟等方法研究了氮含量对微合金化压力容器钢组织转变及力学性能的影响。结果表明:当氮含量从60×10^(-6)增加至93×10^(-6)时,试验钢中的铁素体体积分数变化较小,珠光体占比从5.8 vol%增加至18.4 vol%,粒状贝氏体组织从49.4 vol%减少为28.0 vol%;当氮含量增加至150×10^(-6)时,试验钢组织由47.6 vol%的珠光体与52.4 vol%的铁素体组成。氮含量增加使试验钢在强度变化较小的基础上,-40℃低温冲击吸收能量和伸长率提升,这主要是因为高氮(150×10^(-6))试验钢中析出更多的氮化物颗粒,增加了相变过程中的形核位置,有效促进了珠光体转变;热轧后的低氮(60×10^(-6))试验钢在正火后由于奥氏体中C的过饱和度增加,较高温度下连续冷却发生了贝氏体转变,形成粒状贝氏体组织,恶化了试验钢的塑韧性。

Effect of nitrogen content on microstructure transformation and mechanical properties of microalloyed pressure vessel steels was studied by means of scanning electron microscopy,electron backscattering diffraction technique,transmission electron microscopy,and VASP first principles simulation.The results show that when the nitrogen content increases from 60×10^(-6)to 93×10^(-6),the volume fraction of pearlite increases from 5.8 vol%to 18.4 vol%,the granular bainite decreases from 49.4 vol%to 28.0 vol%,and the volume fraction of ferrite changes little.When the nitrogen content increases to 150×10^(-6),the microstructure of the experimental steel is composed of pearlite(47.6 vol%)and ferrite(52.4 vol%).With the increase of nitrogen content,the elongation and low-temperature impact absorbed energy at-40℃of the experimental steel are improved while the strength changes little,mainly because more nitride particles are precipitated in the experimental steel with high nitrogen content(150×10^(-6)),which increases the nucleation sites in the transformation process and effectively promotes the pearlite transformation.For the hot-rolled experimental steel with low nitrogen(60×10^(-6)),due to the increase of supersaturation of C in austenite after normalizing,bainite transformation occurs during continuous cooling at higher temperature,resulting in the formation of granular bainite structure,which deteriorates the plasticity and toughness of the experimental steel.