Research On Microstructure Evolution And Mechanical Properties Of High-Strength Medium Mn Q&P Steel

Recently,the Q&P process has been successfully applied to the medium Mn steel and obtained ultra-high-strength medium Mn Q&P steel.But it's still lack of systematic research,especially on the microstructure evolution and brittle fracture.In this paper,a medium Mn steel with a composition of 0.2C-5.05Mn-1.56Si was invstigated.Q&P process on the influence of microstructure evolution and mechanical properties was studied through process simulation,theoretical simulation and related annealing experiments.The main results are presented as follows:(1)The phase transition behaviors and diffusion behaviors of the tested steel were investigated by dilatometer and DICTRA.The results are as follows:The austenitisation is fully achieved when the steel was held at 780? for 500s.With the increase of austenitising time,the Ms keeps increasing from 247? to 268?.With the increase of austenitizing time or temperature,the degree of homogenization of Mn increases.The diffusion rate in martensite is significantly faster than the diffusion in austenite;and there exist short-range diffusion of Mn at the interface.(2)With the increase of deformation temperature and the decrease of deformation rate,the dynamic recrystallization of 0.2C-5Mn-1.5Si steel is more likely to occur.With the increase of deformation temperature and pass time,the static recrystallization become more sufficient.By determining the Z parameter in the dynamic recrystallization model,the activation energy of dynamic recrystallization is 276.881 kJ/mol.(3)The effects of austenitizing time and temperature on the mechanical properties were investigated.When the austenitizing time is 3600s,the best mechanical properties with the largest PSE(22GPa·%)are achieved.And the tensile strength and elongation are 13 12MPa and 17.1%,respectively.when the austenitizing temperature is 800?,the optimal mechanical properties are achieved,which the product of strength and elongation(PSE)is 28GPa·%.The block-shaped microstructure transformes to lath-shaped microstructure when the austenitizing time or temperature increasing.The distribution of Mn element casued by intermediate annealing is the main factor leading to the evolution of micro structure morphology,and the mechanism of microstructure evolution was analized.(4)The types of fracture gradually transition from the low-energy intergranular fracture to the ductile fracture as the austenitizing time increases.There are two reasons for the low energy intergranular fracture of experimental steels.Firstly,block-like secondary martensite with a large size is prone to strain and stress concentration,resulting in microcracks.And the flat interface facilitates crack propagation.On the other side,the TRIP effect cannot be effectively used at the later stage of deformation since the block austenite has a much lower stability.(5)Lath micro structure can be obtained by decreasing quenching temperature or adding the process of prequencing.The adverse effect of block-shaped microstructure can be eliminated.The optimal mechanical properties with different quenching temperature is 25.6GPa·%was obtained.When the prequenching process was applied to Q&P process,the PSE was increased from 6GPa·%to 14GPa·%and 16GPa·%,respectively.