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Experimental Research on Impact Toughness of Q420C Steel After Fire Exposure
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摘要: 为研究Q420C钢材火灾高温作用后的力学性能变化规律，考察高温和降温对钢材冲击韧性的影响，试验涵盖了200，400，600，800 ℃温度梯度，自然冷却与浸水冷却两种降温方式，在20，0，-20，-40，-60 ℃五种低温环境下进行冲击试验，测定钢材高温冷却后的冲击功值，结合常温条件下Q420C钢材的试验结果，采用Boltzmann函数进行回归分析，系统总结了钢材经处理后韧性的变化规律。同时，通过宏观观察与扫描电镜观察试件的断裂模式，研究表明：在不同试验条件下，钢材展现出各异的表观特征、冲击韧性及断裂模式。1）在400 ℃与600 ℃的高温环境中，钢材表面会形成蓝色氧化膜及碳化物，这些物质在浸水冷却过程中易于脱落；随着温度升高，碳化现象愈发显著，且碳化物更易剥落，采用浸水冷却方式可有效减轻这一现象，使钢材表面更加平整。2）经自然冷却处理的钢材冲击功值随处理温度的升高不断上升，但600 ℃为此趋势的临界温度点，随着温度继续升高（800 ℃）韧性虽有降低但仍大于T=400 ℃时；浸水冷却对材料韧性影响较大，在400 ℃时冲击功值略有上升，略高于未经高温处理情况下，此后持续大幅下降，最低为29 J；对比常温条件下Q420C钢材的冲击试验结果，发现冲击功值的变化呈倍数关系，计算得出材料经不同处理后其冲击功值折减系数，表明冷却方式和承受高温对钢材韧性影响显著，采用自然冷却的效果明显优于浸水冷却。3）多数试件的断裂表面保留着纤维区和剪切唇，说明材料展现出良好的韧性性能，对于较低温度（200，400 ℃）处理后的钢材，冷却方式对其断口形态的影响并不显著，随着温度继续升高至600，800 ℃，自然冷却下钢材在-40 ℃以下才出现明显的脆性断裂特征，经过浸水冷却处理的钢材在更高的环境温度下也出现了脆性断裂特征，标志着材料在更早发生了脆性断裂。
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关键词:
- Q420C钢材 /
- 冲击试验 /
- 高温作用 /
- 冷却方式 /
- Boltzmann函数
Abstract: In order to study the change law of mechanical properties of Q420C steel after fire and high temperature， and to investigate the influence of high temperature and cooling on the impact toughness of steel， the test covers the temperature gradient of 200，400，600，800 ℃， natural cooling and immersion cooling. The impact test was carried out under five low temperature environments of 20，0，-20， -40 and-60 ℃， and the impact energy value of steel after high temperature cooling was measured. Combined with the test results of Q420C steel under normal temperature conditions， the Boltzmann function was used for regression analysis， and the change law of toughness of steel after treatment was systematically summarized. At the same time， the fracture modes of the specimens were observed by macroscopic observation and scanning electron microscopy. The results show that the steel exhibits different apparent characteristics， impact toughness and fracture modes under different test conditions. 1） In the high temperature environment of 400 ℃ and 600 ℃， the surface of the steel will form a blue oxide film and carbides， which are easy to fall off during the immersion cooling process； with the increase of temperature， the carbonization phenomenon becomes more and more obvious， and the carbides are more likely to peel off. The immersion cooling method can effectively reduce this phenomenon and make the steel surface more flat. 2） The impact energy value of steel treated by natural cooling increases with the increase of treatment temperature， but the critical temperature point of 600 ℃ for this trend， although the toughness decreases with the increase of temperature （800 ℃）， it is still greater than that of T = 400 ℃. Immersion cooling has a great influence on the toughness of the material. The impact energy value increases slightly at 400 ℃， which is slightly higher than that without high temperature treatment， and then continues to decrease significantly， with a minimum of 29 J. Compared with the impact test results of Q420C steel under normal temperature conditions， it is found that the change of impact energy value is in a multiple relationship. The reduction coefficient of impact energy value of the material after different treatments is calculated. The cooling method and the high temperature have a significant effect on the toughness of the steel. The effect of natural cooling is obviously better than that of immersion cooling. 3） The fracture surface of most specimens retains the fiber area and shear lip， indicating that the material exhibits good toughness.-
Key words:
- Q420C steel /
- impact test /
- high-temperature effect /
- cooling method /
- Boltzmann function
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