Microstructure, Precipitation, and Strengthening Mechanism of a High‐Strength Q1100 Heavy Steel Plate via Niobium–Boron Microalloying Technology

Abstract

Herein, a niobium–boron‐bearing high‐strength Q1100 steel plate is developed through chemical composition designing and thermomechanical control process combined quenching and tempering. The microstructure, precipitation, and mechanical properties of the experimental steels for various conditions including as‐quenched and tempered at 300, 350, and 600 °C are investigated. Results show that the microstructure of the as‐quenched specimen is all martensite. The regular lath structure is still observed in the 300 and 350 °C tempering conditions, while equiaxed grains appear in the 600 °C tempering condition. With an increase in tempering temperature, the grain size of Q1100 steel slightly decreases. As tempering at 300 and 350 °C, the short rod cementites precipitate, while at 600 °C, MX (M = Nb, Ti, Mo; X = C, N) spheroidal carbide precipitates. Grain refinement strengthening and dislocation and precipitation strengthening are the main strengthening mechanisms for Q1100 steel tempered at low temperatures. Interestingly, a yield platform appears in Q1100 steel after tempering, which is mainly attributed to the combined effect of decreasing dislocation density and grain size.