Abstract: Taking low-carbon high-strength cold-rolled sheet as the research object,the microstructure of the annealed cold sheet was observed by using optical microscope,field emission scanning electron microscope and the mechanical properties were tested by using automatic tensile testing machine.The results show that when the tested sheet is annealed between 520 ℃ and 580 ℃,the ferrite grain morphology changes little,it is still elongated;the number of cementite distributed in the microstructure is small,and the yield strength and tensile strength change little,yield ratio remains above 0.9,and the elongation is about 1%.When the tested sheet is annealed between 610 ℃ and 700 ℃,undistorted recrystallized grains appear around the deformed ferrite,the recrystallized grains will grow into equiaxed ferrite with the increase of annealing temperature.The spheroidization process of cementite is the main reason for the increase in the number of granular cementite in the microstructure,and the yield strength and tensile strength decrease rapidly,the elongation increases rapidly,the yield ratio shows a downward trend,reaching a minimum of 0.86 at 670 ℃.When the tested sheet is annealed between 730 ℃ and 760 ℃,the ferrite grains grow sufficiently,the grain morphology changes little,there is almost no freely distributed cementite in the microstructure,the cementite aggregates and distributes only in grain boundaries,and the yield strength and tensile strength decrease slowly,the elongation slowly increases,and the yield ratio increases,reaching 0.91 at 760 ℃.Based on the comprehensive consideration,the optimum annealing tmperature of the tested sheet is 670 ℃.

Key words: CSP sheet, annealing heat treatment, microstructure, mechanical properties, yield ratio