Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： 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 TiC particles is greater than the recovery and softening of matrix. With the tempering time from 300 s to 10 h, nano-sized TiC particles precipitate more and more and the mass fraction of TiC with the size less than 5 nm increases, owning to the precipitation hardening produced by tiny TiC 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 TiC 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 TiC 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 TiC speeds the decrease of hardness and is also an important factor resulting in the decrease of hardness.
Subjects: Materials Science >> Materials Science (General) submitted time 2017-04-10 Cooperative journals: 《金属学报》
Abstract：摘要：利用热模拟试验机、扫描电子显微镜(SEM)、高分辨透射电子显微镜（HRTEM）及能谱仪（EDS）研究了Ti-Mo和Ti-Mo-Nb低碳微合金钢的连续冷却转变规律，探讨了Nb对Ti-Mo微合金钢组织及性能的影响。结果表明：Nb元素能够提高钢的Ac1和Ac3温度，降低冷却过程中奥氏体的分解温度，缩小铁素体-珠光体相区，使贝氏体相区向左下方移动。此外，Nb的添加能够细化Ti-Mo-Nb微合金钢中的组织，提高硬度。利用HRTEM对冷速为50 ℃/s的样品进行分析，发现：Ti-Mo和Ti-Mo-Nb微合金钢中均存在少量应变诱导析出的碳化物，分别为(Ti, Mo)C和 (Ti, Nb, Mo)C粒子，呈随机分布。两种析出物均为NaCl型结构，其晶格常数分别为0.432和0.436nm，平均粒径分别为12.11 nm和8.69 nm。Ti-Mo-Nb微合金钢中析出相体积分数更多，尺寸更小，是其组织细化，硬度提高的主要原因。
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