升温过程中Nb和Nb-Mo微合金化钢中碳化物的析出行为研究

PRECIPITATION BEHAVIOR OF CARBIDE DURING HEATING PROCESS IN Nb AND Nb-Mo MICRO-ALLOYED STEELS

利用Gleeble热模拟实验机、Vickers硬度计、SEM,HRTEM及DSC研究了淬火态含Nb和Nb-Mo微合金钢在升温过程中碳化物的析出行为.利用经典形核长大理论及Avrami方程对淬火态钢中MC型碳化物的析出动力学进行了计算.结果表明,含Nb和Nb-Mo微合金钢淬火后以20℃/min的速率加热至不同温度水冷,在300和700℃,由于e-碳化物和MC型碳化物析出而出现了硬度峰值.MC型碳化物在650℃左右析出,由于析出强化而硬度上升,与理论计算得到的MC型碳化物的析出鼻子点温度约650℃的结果相一致.Mo进入Nb C中降低了Nb C与铁素体基体的错配度,从而减小了析出相与铁素体基体间的界面能,使得(Nb,Mo)C析出动力学加快,所以Nb-Mo钢中析出相粒子分布更为密集,尺寸更为细小,具有较高的析出强化作用.

As an important carbide forming element, Nb plays an important role in steel. Precipitated Nb can restrain the austenite grain growth during soaking process and provide precipitation strengthening after g /a phase transformation. Precipitated or dissolved Nb can inhibit recrystallizaton of deformed austenite. Recently, both Nb and Mo are added in steel to enhance the role of Nb. However, these kinds of researches mostly focused on continual cooling process of g /a transformation or isothermal process during tempering, and precipitation behavior of MCtype carbide in steel containing Nb and Mo during reheating process and the effect of Mo on precipitation of Nb C in ferrite were rarely reported. Therefore, in this work, precipitation behaviors of MC-type carbide and the synergis-tic effect of Nb and Mo in steel containing Nb or Nb-Mo during reheating process at the heat rate 20 ℃/min were investigated by means of Vickers hardness test, SEM, HRTEM and DSC. The results show that both Nb and NbMo steels have hardness peaks at 300 and 700 ℃, which are attributed to the precipitation of e-carbide and MCtype carbide, respectively. The MC-type carbide precipitates at about 650 ℃ during reheating process, which is in a good agreement with the nose temperature of MC-type carbide calculated by Avrami equation.（Nb, Mo）C particle forming in Nb-Mo steel during precipitation has a small mismatch with ferrite matrix compared with Nb C, leading to the decrease of interfacial energy. Thus, the precipitation kinetic of MC-type carbide in Nb-Mo steel is faster than that in Nb steel, which results in the denser and finer MC-type carbide and higher precipitation strengthening effect.