回火时间对高Ti微合金化淬火马氏体钢组织及力学性能的影响

EFFECT OF TEMPERING TIME ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF HIGH Ti MICROALLOYED QUENCHED MARTENSITIC STEEL

利用TEM,XRD和Vickers硬度计等研究了回火时间对高Ti微合金化马氏体钢组织及力学性能的影响,阐明了高Ti微合金化马氏体钢在回火过程中析出强化和组织软化之间的交互作用规律.结果表明,高Ti钢在600℃不同时间回火,硬度表现出不同的趋势.10~300 s回火,硬度不断升高,是由于Ti C的析出强化作用远大于基体回复而导致的软化作用;300 s^10 h回火,硬度保持长时间的平台,是由于细小Ti C粒子的不断析出,且5 nm以下的粒子所占比例提高,不断增加的细小Ti C粒子所产生的强化抵消了由于基体组织软化导致的硬度下降;10~20 h回火,硬度快速降低,且降低速率高于不含Ti钢,Ti C粒子的平均尺寸由10 h的2.76 nm粗化到20 h的3.15 nm.计算表明,Ti C粒子的粗化引起硬度降低11.94 HV,基体软化引起硬度降低24.56 HV,表明基体软化是硬度降低的主要因素,而Ti C粒子的粗化加速了高Ti钢硬度的降低,是导致硬度降低的又一重要因素.

With the development of Ti microalloying technology, the application and theory research of Ti in microalloyed steels are becoming more deeply and widely. However, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched martensitic steel has been rarely touched upon, meanwhile, it has long been inconclusive whether precipitated phases coarsening or the recovery and softening of martensitic matrix is the dominant role resulting in the decrease of hardness along with long time tempering of microalloyed steel. In this work, the effect of tempering time on the microstructure and mechanical properties of high Ti microalloyed quenched steel was systemactically investigated by TEM, XRD and Vickers-hardness test,and the interaction between precipitation hardening and microstructural softening of the high Ti microalloyed steel was also studied. The results indicate that the hardness increases for Ti microalloyed steel with tempering time 10∽300 s, which is attributed to the fact that the precipitation hardening by nano-sized Ti C particles is greater than the recovery and softening of matrix. With the tempering time from 300 s to 10 h, nano-sized Ti C particles precipitate more and more and the mass fraction of Ti C with the size less than 5 nm increases, owning to the precipitation hardening produced by tiny Ti C which offsets the hardness decrease due to the gradual softening with recovery of matrix, and therefore, the hardness can keep a long platform; in addition, with the tempering time 10∽20 h, the hardness decreases significantly and the deacreasing rate of hardening for steel with Ti microalloying is higher than that for steel without Ti microalloying. The average particle size of Ti C increases from 2.76 nm at 10 h to 3.15 nm at 20 h. Calculation results show that the decrease of hardness caused by coarsening of Ti C is 11.94 HV, while caused by recovery of matrix is 24.56 HV. It is shown that the recovery of matrix is the dominating factor for reduction in hardness, but coarsening of tiny Ti C speeds the decrease of hardness and is also an important factor resulting in the decrease of hardness.