Impact of Multi-Level Microstructures on the Strength and Yield Ratio of Extra-Thick Ultra-High-Strength Steel

The inhomogeneous microstructures and mechanical properties in thickness direction are clearly present for extra-thick ultra-high-strength steel, due to the cross section effect. In this work, multi-level structure and its effect on strength and yield ratio for extra-thick steel are investigated, and the relationship between the microstructures and strength was discussed by the Hall-Petch formula. The results show that the size of prior austenite grain, martensite packet, block and lath are all gradually increasing from the surface to the center, and corresponding strength and yield ratio are decreasing. Simultaneously, the high-angle boundary of the martensite block and the low angle boundaries of the martensite lath have a stacking effect on the dislocation movement, and they act as the “effective grain size” to control the strength and yield ratio. Furthermore, the strain hardening rate is high and decreases rapidly in the first stage in the work hardening behavior, whereas, it is low and reaches a dynamic equilibrium in the second stage.