Sometimes the stratification of square billet will occur, which will lead to incompatibility of flaw detection. To prevent this defect, we can take the following three measures: (1) Optimizing slow cooling time: optimizing slow cooling process of tube slab, mainly for stacking temperature and stacking duration. After hot straightening, the residence time in the cold bed should not exceed 5 minutes for the rolled steel plate whose thickness is not more than 80 mm. Thickness of steel plate over 80 mm can be appropriately extended to 10 minutes to ensure that the temperature of steel plate before stacking is controlled above 500 C. The slow cooling time of steel plate with thickness less than 50 mm is 12 hours; the slow cooling time of steel plate with thickness greater than 50 mm is 24 hours. It is strictly forbidden to disassemble and stack in advance.
(2) Optimizing Controlled Cooling 方管板坯出现贝氏体一般是由于冷速过快造成的。当冷却速度降低至3-5℃/s时，钢板心部偏析带不再出现贝氏体组织，形成常规的珠光体组织。控制冷却优化后，板坯心部偏析带组织应力明显减少，因组织应力产生的微裂纹得到大大改善。
Bainite in square tube slab is usually caused by too fast cooling rate. When the cooling rate is reduced to 3-5 C/s, the bainite structure is no longer present in the segregation zone of the steel plate core, and the conventional pearlite structure is formed. After controlled cooling optimization, the microstructural stress of segregation zone in the center of slab decreases obviously, and the microcracks caused by microstructural stress are greatly improved.
(3) Centralized charging system
In actual production, the length and width of rolled steel plate are quite different. When the steel plate is slowly cooled, there is irregular stacking, which results in unqualified edge inspection or head inspection of ultra-wide plate. By establishing the centralized charging system, arranging the contract centralized charging of rolling steel plate with similar length and width to ensure the orderliness of subsequent stacking of steel plate. When necessary, asbestos cloth is used to wrap around the square tube slab to enhance the insulation effect.
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