Experimental Research on Impact Toughness of Q420C Steel After Fire Exposure

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.