Cause analysis and improvement measures for ultrasonic flaw detection disqualification of 12MnNiVR plate
In view of the problem of the ultrasonic flaw detection disqualification of 12MnNiVR plates used in large crude oil stor-age tanks,the plates with abnormal detection waveforms and continuous casting billets in the same furnace were sampled and an-alyzed by metallographic microscopy,scanning electron microscopy,and energy spectrum analysis.The reasons for the ultrason-ic flaw detection disqualification of the plate were identified,and corresponding improvement measures were proposed.The re-sults show that there is significant H element segregation in the thickness direction of the 12MnNiVR continuous casting billet,which leads to a decrease in the bonding force of the matrix structure.During rolling deformation,stress concentration is caused by dislocation slip and accumulation,resulting in low stress cracking,formation of microcracks and rapid expansion,leading to flaw detection disqualification of the plate.There is severe alloy element segregation inside the continuous casting billet,and the enriched(Nb,Ti)C in the segregation zone leads to the formation of microcracks inside,which further expand during the subsequent rolling process,resulting in flaw detection disqualification of the plate.Large sized MnS inclusions tend to accumulate in microcracks and porosi-ty of continuous casting billets,seriously affecting the flaw detection results of the plate.There are also high level of porosity defect in the continuous casting billet,which is not fully welded during the rolling process.As the source of cracks,it expands and cracks during the cooling process in heat treatment of the plate,which is also the main reason for flaw detection disqualification of the plate.By exten-ding the VD vacuum treatment time,reducing the pouring supercooling degree,prolonging the LF furnace refining time,and optimizing the TMCP two-stage rolling process,the internal quality of the continuous casting billet has been effectively improved,and flaw detection qualification rate of the plate has been increased from the original 86.7％to 98.8％.
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