Cause analysis and control of cluster blister defect in automobile high strength steel
In order to reduce the cluster blister surface defect of 590 MPa grade automobile high strength steel plate,the causes and mechanism of the defect were analyzed and investigated.The results show that the defect originates from the subsurface micro-cracks of the billet which locates in the area of 1/4 and 3/4 width of the upper surface.The main reasons why these kinds of steel are more sensitive to cracks are that the precipitation of NbCN and AlN under the temperature of straightening zone causes grain boundary embrittlement in Nb-containing peritectic steel and high alumina steel.Uneven solidification shell of peritectic steel and deep oscillation marks of high aluminum steel caused by slag-steel modification reaction further increase the stress concentration and crack sensitivity.Ac-cording to the solidification characteristics of steel,measures were taken respectively,such as controlling mass frac-tion of N element in molten steel not higher than 0.004 0％,adding Ti according to w([Ti])/w([N])=4-5,desig-ning and developing new mold slag,and optimizing the continuous casting process.The number of cracks per meter decreases from 12-20 to 0.03-0.20,and the incidence rate of cluster blister is reduced from 29.8％to less than 0.5％.
high strength steelcluster blistercracksgrain boundary embrittlenessprecipitatedeep oscillation mark
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