Abstract: Cold bending crack appeared on Q345B thick plate after using direct rolling process. The metallographic structure, inclusions, precipitates in hot charging rolling and direct charging slab were studied, and the results showed that banded structure and MnS inclusions in thick plates were the mainly reasons which caused cold bending crack in direct charging plates. Precipitates in direct charging plate were less which could not give enough strengthening effect. Carbon and other elements generated dendritic segregation during solidification process, and then dendritic segregation transformed into banded structure during rolling process. MnS inclusions were generated during solidification process, which formed in Al-killed steel without other deoxidation elements according to the appearance and chemical composition of inclusions. The banded MnS inclusions have been reduced, and segregation has also been improved accordingly throught sulfur content controlling, electromagnetic stirring application and chemical composition adjustment.

Key words: direct rolling；cold bending crack , metallographic structure；inclusion