| High-manganese austenitic twinning-induced plasticity(TWIP)steel shows both high tensile strength and elongation during the deformation,and has good application prospects in the automotive field.The mechanical properties of TWIP steel are affected by many factors,among which the alloy composition is a very important factor.As an indispensable alloying element in high manganese austenitic steel,Mn can increase stacking fault energy and change tensile deformation behavior of such steel.However,the effect of Mn content on the twinning behavior and tensile properties of high manganese austenitic steel is not clear in the range of stacking fault energy where only deformation twinning occurs without martensitic transformation.In addition,the microstructure of TWIP steel is single-phase austenite at room temperature and its yield strength is low,which limits its application.In order to solve the above problems,Fe-Mn-C TWIP steel is taken as the research object.Firstly,the effect of the alloying element Mn on the tensile deformation behavior of TWIP steel was observed.Then,TWIP steels were treated by cold rolling and cold rolling annealing,and the effects of these processes on the tensile properties,microstructure and microstructure evolution during tensile deformation were systematically studied.Finally,the strain distribution characteristics of cold-rolled and annealed high-carbon TWIP steel were studied by scanning electron microscope in-situ tensile technology and digital image correlation(DIC)method.The main findings are as follows.The tensile properties,strain-hardening behavior,microstructure evolution and internal relationship of Fe-Mn-C TWIP steels with three Mn contents and two grain sizes were studied.The results show that with increasing Mn content,the yield strength and tensile strength of TWIP steel decrease,while the elongation increases,especially for coarsegrained steel.At lower strains,increasing Mn content inhibits the formation of twins,resulting in a lower twinning rate;however,at medium and high strains,the higher-Mn steels exhibit a higher twinning rate than lower-Mn steels.In addition,increasing Mn content promotes the continuous formation of deformation twins to a higher strain level.The change of twinning behavior with Mn content is more obvious for coarse-grained steels.Moreover,with increasing Mn content,the strain-hardening rate decreases,but the strainhardening rang increases.Compared with fine-grained steel,the strain-hardening range of coarse-grained steel increases faster with increasing Mn content.The tensile properties,initial microstructure and microstructure evolution during tensile deformation of high-carbon steel and medium-carbon steel cold rolled at various reductions were studied.The results show that with increasing the rolling reduction,the yield and tensile strengths of high-and medium-carbon steel increase,but the elongation decreases.Compared with medium carbon steel,the yield strength of high-carbon steel increases more obviously with increasing the reduction,but the elongation decreases more slowly(except that the elongation of high-and medium-carbon steels is approximately same under large rolling reduction of 0.6).In addition,the amount of dislocations and twins in the initial microstructure increase with increasing the reduction,especially for high-carbon steel.Given a reduction,the amount of dislocations and twins of high-and medium-carbon steels increase during tensile deformation,and and gradually reach the saturated state;with the increase of reduction,the tensile strain reaching the saturated dislocation density and the amount of twins decreases.Compared with the cold-rolled medium-carbon steel,the tensile strain of the cold-rolled high-carbon steel to reach the saturated dislocation density and twinning amount is smaller,but the saturated dislocation density and twinning amount are higher.Based on the quantitative analysis of the parametric model,it is found that the contribution of dislocations and twins in the three TWIP steels to the flow stress during tensile deformation is dominant,while the contribution of lattice friction stress and dynamic strain aging is relatively small.The tensile properties,microstructure and microstructure evolution during tensile deformation of cold rolled and annealed high-carbon steel and medium-carbon steel were studied.For cold rolled high-carbon steel,with increasing the annealing time,the yield strength decreases,while the tensile strength and elongation increase.However,after longtime annealing,a large number of carbides precipitate in high-carbon steel,which leads to premature fracture of the sample,and the yield strength,tensile strength and plasticity are greatly reduced.For cold rolled medium-carbon steel,no carbide is produced during annealing.With increasing the annealing time,the yield strength and tensile strength decrease,but the elongation increases.The yield strength,tensile strength and elongation of cold-rolled annealed high carbon-steel are higher than those of medium-carbon steel except cold rolling and long-time annealing.The microstructure observations show that although the dislocation density in cold rolled high-and medium-carbon steels decreases with the increase of annealing time,the amount and size of twins are almost unchanged.,but the amount and size of twins are almost unchanged.During tensile deformation,the dislocation density and the amount of twins increase,and finally reach saturation.With increasing annealing time,the rate of twinning and dislocation multiplication become faster,and can continue to a higher strain level.After anneling for a short time,the dislocation multiplication rate,twinning rate,strain range of dislocation multiplication and twinning in high-carbon steel are higher than those in medium-carbon steel.The results show that the microstructure and tensile properties of TWIP steel can be adjusted in a large range by changing the carbon content and annealing time.The results of in-situ tensile and DIC study on cold-rolled annealed high carbon TWIP steel show that the strain distribution in cold-rolled annealed microstructure is not uniform during tensile deformation,resulting in a series of parallel banded strain concentration.In addition,strain concentration can also be observed at the twin interface.This shows that the fracture mode of cold rolled high carbon steel is not affected by carbides after short time annealing.When cold rolled high-carbon steel is annealed for a long time,a large amount of carbides is precipitated at the grain boundaries and inside the grains.When the carbides are connected into a line,the strain concentration of matrix near the “linear” carbides is obvious.Moreover,when a large number of carbides are precipitated but sparsely distributed in a certain region,the matrix around the “granular” carbides in the region also show obvious strain concentration.It is because of this large amount of carbide that the cold rolled high-carbon steel annealed for a long time will fracture prematurely. |
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