Study on microstructure transformation behavior of Ti-Nb microalloyed high-speed guardrail steel under continuous cooling
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摘要: 利用热模拟试验机、OM、TEM等试验设备，研究了Ti-Nb微合金化高速护栏钢的连续冷却组织转变规律，建立了试验钢的CCT曲线。研究结果表明：当冷速为0.5 ℃/s时，试验钢中的奥氏体发生铁素体-珠光体相变；当冷速大于1 ℃/s时，开始发生贝氏体相变；当冷速为10～20 ℃/s时，既发生铁素体-贝氏体相变又发生马氏体相变；当冷速≥30 ℃/s时，发生贝氏体-马氏体的相变。随着冷速的增加，试验钢的硬度也随之增大。在不同冷速下钢中均存在（Ti, Nb）C析出物，且在钢中呈弥散分布，在低冷速条件下，钢中析出物的体积分数较大，尺寸较小，具有一定的析出强化效果。Abstract: The thermal simulation testing machine, OM, TEM and other experimental equipment were used to study the continuous cooling transformation behavior of Ti-Nb microalloyed high-speed guardrail steel in the experiment. The results show that when the cooling rate is 0.5 ℃/s, the austenite of the test steel emerges ferrite-pearlite phase transformation. When the cooling rate is greater than 1 ℃/s, the bainite phase transformation begins to occur. At the cooling rate of 10～20 ℃/s, both ferrite-bainite phase transformation and martensite phase transformation come out. While the bainite-martensite phase transformation appears when the cooling rate rises to 30 ℃/s. As the cooling rate increases, the hardness of the test steel also improves. There exists (Ti, Nb) dispersed precipitates in the steel at different cooling rates. In the condition of low cooling rate, the volume fraction of the precipitates in the steel is larger, and the size is smaller, which has a certain precipitation strengthening effect.
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Key words:
- high-speed guardrail steel /
- Ti-Nb microalloying /
- CCT curve /
- hardness
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图 2 试验钢在不同冷速下的温度-膨胀量曲线
Figure 2. The temperature-expansion increment curve of the steel at different cooling rates
图 3 试验钢在不同冷速下的金相显微组织
Figure 3. The metallographic microstructure of the steel at different cooling rates
图 4 试验钢在0.5 ℃/s 和50 ℃/s冷速时的TEM形貌
Figure 4. The TEM images of the steel at the cooling rate of 0.5 ℃/s and 50 ℃/s
(a) 0.5 ℃/s；(b) 50 ℃/s
图 7 试验钢析出物体积分数随温度的变化
Figure 7. The variation of the volume fraction of precipitates in the steel with different temperature
图 8 试验钢在0.5 ℃/s和50 ℃/s冷速时析出物的形貌和EDS形貌
Figure 8. The morphology and EDS images of precipitates in the steel at the cooling rate of 0.5 ℃/s and 50 ℃/s
(a) 0.5 ℃/s；(b) 50 ℃/s
表 1 试验钢的主要化学成分
Table 1. The target composition of the steel
% C Si Mn Ti Nb N Cr S P 0.050～0.070 0.10～0.30 1.40～1.60 0.10～0.15 ≤0.01 ≤0.005 0.30～0.50 ≤0.005 ≤0.010 
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表 2 试验钢不同冷却速度下的相变温度和组织类型
Table 2. The transformation points and microstructures of the steel at different cooling rates
冷却速率/(℃·s−1） 转变温度/℃ 组织 A→F+P A→F+P+B A→F+B A→F+B+M A→B+M 开始 结束 开始 结束 开始 结束 开始 结束 开始 结束 0.5 802 650 F+P 1 792 589 F+P+B 5 760 513 F +B 10 696 509 F+B+M 20 699 466 F+B+M 30 646 412 B+M 50 624 395 B+M
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