Effect of finish rolling temperature on microstructures and mechanical properties of 1000 MPa grade tempered steel plate for hydropower station

With the decreasing of finish rolling temperature, the microstructure of the hot rolled steel was changed from lath martensite to complex microstructures composed of lath martensite, ferrite, granular bainite and lath bainite, unrecrystallized prior austenite grains are the main reason for the formation of complex microstructures. After tempering, the steels are mainly composed of ferrite and cementite. For the steel with high finish rolling temperature, the proportion of high misorientation boundary is less, and it has rod-like precipitates with the length of 25-85 nm and punctate precipitates with the size of 4-16 nm, the intensities of {111}< 110 > and {001}< 110 > textures are enhanced. The experimental steel has the best combination with strength and toughness as yield strength of 996 MPa, tensile strength of 1023 MPa, total elongation of 19.1 %, impact value at -60 degrees C of 125 J. Grain refinement, higher density of dislocation, fine and dispersed precipitates could enhance the yield strength and tensile strength. Stronger intensity of {332}< 113 > texture and weaker intensity of {001}< 110 > texture could improve the impact toughness.

Automated summary

The study found that decreasing finish rolling temperature can lead to the formation of complex microstructures, primarily composed of ferrite and cementite after tempering. The experimental steel exhibited excellent mechanical properties due to grain refinement, higher dislocation density, and dispersed precipitates, demonstrating a good combination of strength and toughness.