Study on solid solution behavior of second phase particles and austenite grain growth law of Nb-Ti high strength steel
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摘要: 采用透射电子显微镜、光学显微镜以及热力学计算，对Nb-Ti微合金化钢中纳米碳化物在高温加热过程中的固溶行为及奥氏体晶粒长大规律进行了研究。结果表明，在Nb-Ti微合金钢中，奥氏体晶粒尺寸和析出相的大小均受加热温度和保温时间的影响，奥氏体的晶粒尺寸随温度升高和保温时间延长而逐渐增大。与保温时间相比，加热温度对晶粒粗化率的影响更大。温度从950 ℃加热到1300 ℃，奥氏体晶粒尺寸从28.4 μm长大到94.6 μm，晶粒长大迅速；当加热温度达到1200 ℃，保温时间由10 min逐渐延长至120 min，奥氏体晶粒尺寸从79.4 μm逐渐增加到88.5 μm，晶粒长大速度缓慢。Nb-Ti微合金钢中的小尺寸析出相随着加热温度的升高逐渐溶解到奥氏体中，钢中碳化物总数量减少。未溶的析出物均为较大尺寸的(Ti,Nb)C。钢中未溶碳化物中Ti/Nb的原子比随加热温度的升高而增大。就热稳定性而言，碳化物中的Ti高于Nb，其回溶温度更高。随着温度的升高，Nb、Ti和C的固溶量逐渐增加，Nb-Ti微合金钢中析出相的体积分数逐渐降低。随着加热温度的升高，钢中细小的第二相粒子溶解，大大削弱了其对奥氏体晶界的钉扎强度，因此奥氏体晶界开始迁移，奥氏体晶粒开始迅速粗化。Abstract: The solid solution behavior of nanocarbide and austenite grain growth in Nb-Ti microalloyed steel during high temperature heating were studied by means of transmission electron microscope, optical microscope and thermodynamic calculation. The results show that the austenite grain size and size of precipitated phase in Nb-Ti microalloyed steel are affected by heating temperature and holding time. The austenite grain size increases with the improvement of temperature and the extension of holding time. Compared with holding time, heating temperature has a greater influence on grain coarsening rate. The temperature is heated from 950 ℃ to 1300 ℃, and the austenite grain size grows obviously from 28.4 μm to 94.6 μm. When the heating temperature reaches 1200 ℃ and the heat preservation time is gradually extended from 10 min to 120 min, the austenite grains are slowly increasesing from 79.4 μm to 88.5 μm. The small size precipitates in Nb-Ti microalloyed steel gradually dissolves into austenite with the increase of heating temperature. The total amount of carbides in the steel decreases, and the undissolved precipitates are (Ti, Nb) C with a larger size. The atomic ratio of Ti/Nb in the undissolved carbide in steel increases with the increase of heating temperature. As far as thermal stability is concerned, Ti content in carbide is higher than Nb, and its redissolution temperature is higher. The solid solution content of Nb, Ti and C gradually increase with the increase of temperature, and the volume fraction of precipitates in Nb-Ti microalloyed steel gradually decreases. The fine second phase particles in the steel dissolve with the increase of temperature, which greatly weakens their pinning strength to the austenite grain boundary. Therefore, the austenite grain boundary begins to migrate, and the austenite grains begin to coarsen rapidly.
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图 1 不同奥氏体化温度保温30 min的奥氏体晶粒形貌
Figure 1. Austenite grain morphology at different austenitizing temperatures for 30 min
图 2 1250 ℃不同保温时间的奥氏体晶粒形貌
Figure 2. Austenite grain morphology after holding at 1250 ℃ for different time
图 3 奥氏体化温度和保温时间对晶粒尺寸的影响
Figure 3. Variation of austenite average grain size with heating temperature and holding time
图 4 温度对试验钢中Ti，Nb，C平衡固溶量的影响（a）和奥氏体中析出相体积分数与温度的关系（b）
Figure 4. Effect of temperature on equilibrium solid solution amount of Ti, Nb and C in experimental steel (a) and relationship between volume fraction of precipitated phase in austenite and temperature (b)
图 5 不同奥氏体化温度下保温30 min后时钢中析出相形貌
Figure 5. Morphology of precipitates in steel at different austenitizing temperatures for 30 min
图 6 不同奥氏体化温度下保温30 min后钢中析出相尺寸和Ti/Nb的比例
Figure 6. Precipitate size and Ti / Nb ratio in steels with different austenitizing temperatures
表 1 Nb-Ti微合金化钢的主要化学成分
Table 1. Main chemical composition of Nb-Ti microalloyed steel
% C Si Mn Al Cr Nb Ti 0.1 0.3 2.0 0.6 0.25 0.02 0.02 
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表 2 不同奥氏体化温度下保温30 min后钢中夹杂物成分和形状
温度/℃ 夹杂物类型 Ti/% Nb/% C/% 形状 950 高Ti低Nb 47.8 8.3 44 方形 950 高Nb低Ti 2.2 8.4 89.4 方形 1050 高Ti低Nb 49.6 16.5 33.9 方形 1150 高Ti低Nb 57.6 14.4 28 方形 1250 高Ti低Nb 49.9 13.2 36.9 方形
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