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In this paper, the fluctuation of impact energy for Q690 steel plate with large thickness of 80 mm are analyzed. The low temperature impact tests of samples taken from different positions of plate thickness are carried out as well as their fracture and microstructure are compared and analyzed with scanning electron microscopy and metallographic microscope. The differences of impact energy at different positions of plate thickness are great, which are 14.9 J in the center, 27.2 J at the quarter part and 95.4 J near the surface. The corresponding microstructures are coarse tempered bainite and a small amount of tempered sorbite structure with orientation of martensite in the center, tempered sorbite and bainite structures with orientation of martensite at the quarter part as well as fine tempered sorbite structure with orientation of martensite near the surface. The analysis results showed that the main cause for fluctuation of impact energy was the differences of microstructure and grain size. The experiment showed that the higher the proportion of bainite in microstructure, the larger the grain size and the lower the corresponding impact energy. Based on the CCT(continuous cooling transformation) curves of Q690 steel, it is analyzed and judged that insufficient cooling during quenching process causes insufficient cooling speed from the quarter part to the center of steel plate due to the larger thickness specification of Q690 steel plate so that the whole martensitic structure could not be obtained after quenching, microstructure in the direction of plate thickness is uneven, especially different proportions of bainite structure appear at the quarter part and in the center of plate thickness, which cause the fluctuation of impact energy. The uniformity of microstructure for Q690 steel plate with large thickness specification is improved so that the impact energy and its stability are improved by optimizing on-site quenching and tempering process.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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