Effect Of Annealing Process On Microstructure And Yield Ratio Of Cold-Rolling Q195 Steel Sheets

The cold-rolling Q195 steel sheets are widely used in light industry,mechanical industry and transportation industry and holds a significant share in steel market,due to their excellent plasticity,ductility and processability.However,the yield ratios of Q195 steel sheet products are too high.Yield ratio(YR=ReI/Rm)is an important performance indicator to estimate the potential of cold deformation and uniform extension capability.Steels with higher yield ratio have smaller deformation capacity from the start of plastic deformation to final fracture,thus high yield ratio is detrimental to sheets' processability and manufacturing safety.Therefore,it is essential to study the influential factors of yield ratio to control the steel yield ratio and develop low yield ratio steels.This project focused on Q195 steels produced at 1780(sheet width<1780mm)and 950(sheet width<950mm)production line of Taishan steel.A combined analysis of microstructure and mechanical properties is carried out.The main cause of the high yield ratio of Q195 steel sheets is discussed,and the relationship between annealing parameters and microstructure and yield ratio of cold-rolling Q195 steel sheets is established to investigate whether the high yield ratio can be lowered by adjusting the annealing process.The mechanical property test of Q195 steel sheets produced in Taishan steel shows that the yield ratios of Q195 steel range from 0.68 to 0.83,which is higher than the requirement of general carbon steels(0.60--0.65).And the high yield ratio results mainly from the high yield strength(281-338MPa).Based on the analysis of the microstructure and inclusion content,it is concluded that the high yield ratio of Q195 steel sheets produced at 1780 production line is caused by a combination of several reasons,that is the excessive inclusions and the inhomogeneous dispersion of the steel microstructure in the thickness,the small grains in the center,and coarse grains on the surface.For the Q195 steel sheets produced at 950 production line,the high yield ratio is due to the small grains and excessive inclusions.The hard Al2O3 in the steel and complex inclusions are dispersed in the matrix,which lead to grain and dispersion strengthening,thereby increasing the yield ratio.The influence of annealing temperature and soaking time on the microstructure and yield ratio of Q195 steel sheets was investigated through simulated annealing experiment.The result shows that the 1780-cold-rolling Q195 steel sheets finish recrystallization at 610? during isochronic annealing for 3h.As the annealing temperature increases from 610? to 760?,the homogeneity of microstructure has been improved,while the grains are still fine,thus the yield strength and the yield ratio is still high.The recrystallization temperatures of cold-rolling Q195 steel sheets from 950 production line with cold reduction ratios of 58.5%and 86.1%are 580?and 550? respectively.The recrystallization temperature is lower and the recrystallization grains are finer with the higher cold reduction ratio.As the annealing temperature increases from 640? to 760?,the microstructure homogeneity of 950-cold-rolling Q195 steel sheets has been improved,and the grains are coarse to some extent,thus the yield ratio decreases and the yield ratio decreases from 0.92 to 0.84,which is still higher than the requirement of general carbon steels.The cold-rolling Q195 steel sheets from both production lines finish recrystallization in 5min during isothermal annealing at 670? and 730?,which indicates that the cold-rolling steel sheets can finish the recrystallization quickly when annealed above the critical temperature.The hardness drops quickly when soaked during the first 30min,while the hardness declines and the grains grow slightly with the soaking time extends from 30min to 7h.In conclusion,the high yield ratio caused by excessive inclusions and fine grains of Q195 steel sheets cannot be lowered effectively only through adjusting the annealing process.The melting process of both production lines need to be optimized to improve the cleanness of steels.Besides,the hot-rolling parameters of 1780 production line need to be optimized to improve the homogeneity of microstructure.