The Reasonable Measures To Steel Plate Control Delamination Defects

For Q345D medium and thick steel plates, delamination defects sometimes appear, resulting in inconsistent flaw detection. The analysis believes that the root cause of the inconsistent flaw detection is the internal segregation of the cast slab. In this case, the subsequent reasonable control of cooling and optimization of the ring cooling can greatly alleviate this problem. On the contrary, the improper cooling rate after rolling leads to the formation of bainite structure and induces microcracks; while the slow cooling of the steel plate after straightening is not timely, the release of internal stress is insufficient, which leads to the expansion of internal microcracks, resulting in unqualified inspections.

For this reason, reasonable measures are:

1. Centralized furnace system

The product structure of medium and heavy plate companies generally has problems such as heterogeneous steel plates, different specifications, multiple batches, and small batches. In actual production, the length and width of the rolled steel plate are quite different. When the steel plate is slowly cooled, the stacking is not neat, which leads to the failure of the inspection of the edge of the ultra-wide plate or the failure of the inspection of the head of the ultra-long plate.

Through the establishment of a centralized furnace system, arranging contract centralized furnaces with similar length and width of rolled steel plates to ensure the neat stacking of subsequent steel plates. The batch of centralized loading furnace ensures that the width difference between the upper and lower steel plates in the same stacking position is not more than 500mm, and the length is not more than 2000mm. When necessary, use asbestos cloth to wrap around the steel plate to enhance the thermal insulation effect.

2.Optimized control cooling

Research has shown that the appearance of bainite in steel plates is generally caused by the excessive cooling of steel plates. In the past production of Q345D, the cooling rate was often controlled at 6-10m/s during controlled rolling and cooling. In order to reduce the bainite structure produced by rapid cooling, the number of ACC pole tube opening groups was reduced, when the cooling rate was reduced to 3-5℃/ At s, the segregation zone in the core of the steel plate no longer appears bainite structure, forming a conventional pearlite structure. After the optimization of controlled cooling, the stress of the segregation zone in the core of the steel plate is significantly reduced, and the microcracks caused by the stress of the structure are greatly improved.\

3.Optimize slow cooling time

Optimizing the slow cooling process of steel plates mainly focuses on the stacking temperature and stacking duration of steel plates.

• 1) Grab temperature and stack. After the rolled steel plate thickness is not greater than 80mm, the residence time in the cooling bed shall not exceed 5min after hot straightening; the thickness of the steel plate greater than 80mm can be appropriately extended to 10 minutes to ensure that the temperature of the steel plate is controlled above 500℃ before stacking.
• 2) Strictly control the stacking slow cooling time. The slow cooling time of the steel plate with a thickness of not more than 50mm (based on the sealing time) is 12h; the slow cooling time of the steel plate with a thickness of more than 50mm is 24h, and it is strictly forbidden to unstack and finish in advance.

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