The influence of second-phase particles on austenite grain growth behavior in Nb-Ti microalloyed steel
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摘要: 对Nb-Ti微合金化钢在
1050 ～1250 ℃条件下分别进行5～240 min等温奥氏体化的热处理试验，研究其奥氏体晶粒长大行为，利用Thermo-calc及TEM对不同加热工艺下的第二相粒子进行分析，通过HUMPHREYS理论对晶粒长大方式进行了预测。结果表明：随着奥氏体化温度的升高，NbC粒子中的Nb/Ti质量比逐渐减小，1050 ～1100 ℃保温1 h时晶粒尺寸细小均匀，温度升高至1150 ℃时发生晶粒异常长大现象，1250 ℃加热时晶粒尺寸粗化明显，各工艺下晶粒的实际长大方式与HUMPHREYS模型预测相同。综合考虑加热过程中奥氏体晶粒尺寸及微合金元素的溶解与析出规律，试验钢的最佳加热温度为1200 ℃左右。-
关键词:
- Nb-Ti微合金钢 /
- 奥氏体晶粒尺寸 /
- 晶粒长大动力学 /
- 第二相 /
- HUMPHREYS模型
Abstract: The heat treatment experiments were conducted on Nb-Ti microalloyed steel at 1050-1250 ℃ for 5-240 minutes to study the austenite grain growth behaviour. Thermo-calc and TEM were used to analyze the precipitation particles under different heating processes, and the grain growth mode was predicted using the HUMPHREYS' theory. The results showed that as the austenitization temperature increased, the mass ratio of Nb/Ti in the NbC particles gradually decreased. The grain size was small and uniform when the temperature was kept at1050 -1100 ℃ for 1 hour. As the temperature increased to1150 ℃, abnormal grain growth occurred. When heated at1250 ℃, the grain size coarsened significantly. The actual grain growth mode under each process was the same as predicted by the HUMPHREYS' model. Considering the austenite grain size and the dissolution and precipitation patterns of microalloyed elements during the heating process, the optimal heating temperature for the experimental steel is around1200 ℃. -
图 1 试验钢在不同加热工艺下的显微组织与晶粒尺寸分布
Figure 1. Microstructure and grain size distribution of the test steel under different heating processes
(a)1 050 ℃/60 min; (b)1 100 ℃/60 min; (c)1 150 ℃/60 min;(d)1 200 ℃/60 min; (e)1 250 ℃/60 min; (f)1 200 ℃/120 min;
图 2 试验钢在不同加热工艺下的晶粒尺寸分布
Figure 2. Grain size distribution of the test steel under different heating processes
(a)1 050 ℃/60 min; (b)1 100 ℃/60 min; (c)1 150 ℃/60 min;(d)1 200 ℃/60 min; (e)1 250 ℃/60 min;
图 3 不同加热温度下奥氏体晶粒尺寸随保温时间变化曲线
Figure 3. The variation curves of austenite grain size with holding time at different heating temperatures
图 6 Thermo-calc热力学计算结果
(a)(b)各相含量随温度变化关系；(c) TiN中组元含量随温度变化关系；(d)NbC中组元含量随温度变化关系
Figure 6. Thermo-calc thermodynamic calculation results
图 7 试验钢在不同加热温度下保温 60 min 的第二相形貌及能谱
Figure 7. The morphology and energy spectrum of second-phase particles held for 60 minutes at different heating temperatures
(a)(a1)1 050 ℃; (b)(b1)1 100 ℃; (c)(c1)1 150 ℃; (d)(d1)1 200 ℃
图 9 不同工艺下的晶粒长大方式预测
Figure 9. Prediction of grain growth patterns under different processes
表 1 试样化学成分
Table 1. Chemical composition of test material
% C Si Mn P S Cr Ni Mo Nb Ti Al Fe 0.059 0.21 1.60 0.0076 0.0012 $\leqslant $0.6 0.050 0.014 0.033 余量 
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表 2 奥氏体晶粒在不同温度下的长大规律
Table 2. Growth rule of austenite grains at different temperatures
温度/℃ n K 温度/℃ n K 1050 0.19189 4.14076 1200 0.13806 15.57769 1100 0.19568 6.25400 1250 0.1315 23.54515 1150 0.18785 7.73186
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表 3 Nb/Ti质量比随温度变化关系
Table 3. The relationship between Nb/Ti mass ratio and temperature
温度/℃ Nb/Ti质量比 温度/℃ Nb/Ti质量比 1050 7.73 1150 1.36 1100 4.85 1200 0.58
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表 4 HUMPHREYS模型中的参数
Table 4. Parameters in the HUMPHREYS model
加热温度/℃ 保温时间/min 不均匀因子 平均晶粒直径/μm 析出相直径/nm 1050 30 3.05 20.16 18.56 1100 30 2.72 32.13 23.16 1150 30 5.5 35.69 36.69 1200 30 3.89 48.13 68.83 1250 30 2.29 63.61 0
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