The Improvement of Inner Quality of High-Strength Structural Plates

by Chao-Yun Wang, Chun-Hung Wu

Publisher - China Steel Corporation, Taiwan

Category - Engineering & IT

High-strength structural plates applied to bridges or skyscrapers usually bear large loading that the inner quality is critical to security. In China Steel Corporation(CSC), ultra-sonic test(UT) is used to ensure the inner quality of plates. The major internal defects of plates are micro-cracks appeared in the thickness center which usually accompanied with needle shaped MnS and brittle texture(Martensite). The forming mechanism of cracks are higher hydrogen content in liquid steel or hydrogen pick-up at the beginning of casting, more hydrogen atoms captured by hydrogen traps after solidification, redundant hydrogen atoms resulted in plates’ inner hydrogen-induced cracks(HIC) due to insufficient diffusion after rolling. Several countermeasures were adopted, including increasing maintenance frequency of roller gap, limiting superheat and casting speed and proper diverting rules. On the other hand, RH vacuum efficiency and snorkel status were reviewed severely to ensure lower hydrogen content in liquid steel. After the implementation of above countermeasures, the rejects from ultra-sonic test failures decreased to 1/3 of original, and significantly reduced the rejects cost and shortened the delivery time.

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