Study On Mechanical Properties And Control Of Original Structure Before Annealing Of Al Microalloyed Medium Manganese Steel

Lightweight automobiles,and the development of advanced high-strength steels with high strength and high plasticity are the main development trends of the future automotive industry.The comprehensive mechanical properties of Al microalloyed manganese steels are more prominent,which has led many researchers to devote themselves to this research.The thesis takes 0.2C-5Mn-0.5Si-(1.0/2.5)Al medium manganese steel as the research object,comprehensively uses Fact Sage(?)FSstel8.0 thermodynamic calculation,salt bath annealing,X-ray diffraction,scanning electron microscope,transmission electron microscope and electron universal experiment machine et al,studied the phase transformation law,intercritical annealing process,microstructure and mechanical properties of 0.2C-5Mn-0.5Si-(1.0/2.5)Al manganese steel,and on this basis analyzed the Al content,tensile rate and original structure before intercritical annealing on the mechanical properties of medium manganese steel is analyzed.Through research,the conclusions are as follows:Adding Al can significantly expand the temperature range of the intercritical zone and increase the intercritical temperature,especially the A₃temperature.Adding1.0 Al(wt.%)can expand the intercritical zone temperature range from 207°C(0Al)to 338°C,and the A₃temperature from 718°C to 805°C;adding 2.5Al(wt.%)can increase the intercritical zone temperature range from 207°C(0Al)expanded to 506°C,and A₃temperature increased from 718°C to 1016°C.It is worth noting that?-ferrite will appear in the structure after adding 2.5Al(wt.%).The expansion of the intercritical temperature range and the increase of the intercritical temperature help to shorten the intercritical annealing time,so as to achieve good mechanical properties under a short time(1-3min)intercritical annealing.The designed Al microalloying medium manganese steel can obtain good mechanical properties under short-time intercritical annealing(1-3min).The shorter the intercritical annealing time,the more prominent the advantages of Al microalloying.Even when the intercritical annealing time is only 1min,CR-2.5Al cold-rolled medium manganese steel can also obtain excellent mechanical properties:tensile strength of 982.50 MPa,elongation of 42.96%,and strong plastic product of42.00 GPa%.Taking CR3-2.5Al cold-rolled medium manganese steel as the object,the evolution of retained austenite in different stretching stages was studied,and it was found that the retained austenite hardly transformed into martensite during the elastic deformation stage;it entered yield.At this stage,the retained austenite begins to undergo martensitic transformation;entering the uniform deformation stage,the retained austenite accelerates to transform into martensite,showing the TRIP effect;before the sample fractures,the retained austenite transforms into martensite The rate drops and the TRIP effect weakens.By studying the influence of the stretching rate,it is found that as the stretching rate increases,the transformation rate of retained austenite decreases,which leads to a decrease in both the tensile strength and the elongation.Study the effect of hot-rolled and cold-rolled as the original structure before intercritical annealing on mechanical properties.It is found that cold-rolled steel is used as raw material to obtain better mechanical properties and better matching of strong plasticity,but the tensile curve shows obvious yield extension phenomenon;while hot-rolled steel is used as a raw material,although the mechanical properties are slightly worse,the tensile stress-strain curve shows continuous yield.Taking 2.5Al(wt.%)cold-rolled steel as the research object,the influence of different original structures on the mechanical properties was studied through preheating.It was found that the structure of the sample pretreated at 1100?for 1min was?-ferrite and Martensite can obtain ideal mechanical properties after intercritical annealing.The tensile strength is 1106 MPa,the elongation is 39.10%,the strong plastic product is 43.00 GPa%,and the tensile curve has no obvious yield extension phenomenon,basically showing continuous yield.