Niobium (Nb) microalloying can improve the material properties of H13 steel (0.4C-5Cr-1.2Mo-1V steel), but it also affects the natures of the primary carbides. Therefore, the effect of Nb content and cooling rate on the behavior of primary carbides in H13 steel was studied in this paper. The matrix structure was obtained by chemical etching, and then the formation location of primary carbides was identified by electron probe microanalysis (EPMA). The three-dimensional (3D) characteristics, including morphology, number density, and size, were obtained by a non-aqueous electrolysis method. The enrichment of alloying elements in the last-to-solidify region leaded to the formation of primary carbides during the solidification. The Ti4C2S2 phase precipitated first, and then the Mo-Cr-rich carbide was formed around the Ti4C2S2 phase. During the cooling process, the Ti4C2S2 phase partly transformed into Nb-rich carbide and then further partly transformed into V-rich carbide. There is a huge difference between the two-dimensional and three-dimensional morphologies of the primary carbides. As the Nb content increased, the size of last-to-solidify region decreased gradually and the size and number density of primary carbides in the 3D observation increasingly increased. However, as the decrease of the cooling rate, the size of primary carbides increased rapidly and the number density of primary carbides decreased markedly. The thermodynamic and kinetics calculation results agreed well with the experimental observations.

铌（Nb）微合金化可以改善H13钢（0.4C-5Cr-1.2Mo-1V钢）的材料性能，但也会影响初生碳化物的性质。因此，本文研究了Nb含量和冷却速度对H13钢中一次碳化物行为的影响。通过化学蚀刻获得基体结构，然后通过电子探针微量分析（EPMA）确定初生碳化物的形成位置。通过非水电解方法获得三维 (3D) 特征，包括形态、数密度和尺寸。最后凝固区合金元素的富集导致凝固过程中初生碳化物的形成。Ti4C2S2 相首先析出，然后在 Ti4C2S2 相周围形成富 Mo-Cr 碳化物。在冷却过程中，Ti4C2S2 相部分转变为富 Nb 碳化物，然后进一步部分转变为富 V 碳化物。原生碳化物的二维和三维形貌之间存在巨大差异。随着Nb含量的增加，最后凝固区的尺寸逐渐减小，3D观察中初生碳化物的尺寸和数量密度逐渐增加。但随着冷却速度的降低，初生碳化物的尺寸迅速增大，初生碳化物的数量密度显着下降。热力学和动力学计算结果与实验观察结果吻合良好。原生碳化物的二维和三维形貌之间存在巨大差异。随着Nb含量的增加，最后凝固区的尺寸逐渐减小，3D观察中初生碳化物的尺寸和数量密度逐渐增加。但随着冷却速度的降低，初生碳化物的尺寸迅速增大，初生碳化物的数量密度显着下降。热力学和动力学计算结果与实验观察结果吻合良好。原生碳化物的二维和三维形貌之间存在巨大差异。随着Nb含量的增加，最后凝固区的尺寸逐渐减小，3D观察中初生碳化物的尺寸和数量密度逐渐增加。但随着冷却速度的降低，初生碳化物的尺寸迅速增大，初生碳化物的数量密度显着下降。热力学和动力学计算结果与实验观察结果吻合良好。初生碳化物尺寸迅速增大，初生碳化物数密度明显下降。热力学和动力学计算结果与实验观察结果吻合良好。初生碳化物尺寸迅速增大，初生碳化物数密度明显下降。热力学和动力学计算结果与实验观察结果吻合良好。