A Study On The Microscopic/Macroscopic Mechanisms And The Weakening Approaches Of The Plastic Instability Phenomena In A 7MnCA Medium Mn Steel

Medium Mn steel is regarded as a promising candidate for automotive lightweighting strategy since its combination of excellent mechanical properties and relatively low cost.However,there exists a serious problem to be solved,which is the plastic instability phenomena.These plastic instability phenomena,frequently manifested as the Lüders band or PLC band propagation,will give rise to the surface roughness and deteriorate the sheet forming performance,thus impeding the large-scale industrial application of this steel.In this study,an intensive research on these plastic instability phenomena in a cold-rolled medium Mn steel 7MnCA（provided by Baosteel）is conducted,which can be divided into three parts.Firstly,the macroscopic features of the plastic instability phenomena（Lüders band and PLC band）under different deformation conditions in7MnCA were systematically characterized.Secondly,comprehensive and self-consistent explanations were proposed to elucidate the mechanisms of these plastic instability phenomena from the perspectives of the dislocation mechanism-based crystal plasticity and the coupon-level continuum mechanics.Thirdly,two approaches,macroscopic loading mode optimization and microstructure design,were undertaken to weaken the plastic instability phenomena in 7MnCA.The main conclusions are listed as the following:1)The different deformation conditions（deformation temperature,nominal strain rate and pre-deformation）will exert significant influences on the macroscopic features of these plastic instability phenomena.Firstly,these exists distinct competition/coordination between Lüders band and PLC band under different deformation temperatures.While these two kinds of bands coexist under some temperatures（25℃and 55℃）,one kind of band will prevail during the whole plastic deformation process under some other temperatures（100℃and 250℃）.Secondly,these plastic instability phenomena are similar when deformed at different nominal strain rates(within the quasi-static range,from 2*10^-4/s to 3*10^-3/s),which are all manifested as the Lüders propagation followed by type-A PLC bands.The main difference lies in the extent of Lüders strain,which increases with the increase of nominal strain rate.Thirdly,the effect of pre-deformation on the macroscopic features of these plastic instability phenomena depends on whether the aging process is applied after pre-deformation.If the pre-deformation is followed by the aging process（such as 24 hours at room temperature）,then the spread area of type-A PLC band will be affected.Otherwise,no difference will be observed.2)From the perspective of the dislocation mechanism-based crystal plasticity,a microscopic interpretation framework for explaining the macroscopic features of these plastic instability phenomena,is proposed.Firstly,the microscopic origin of Lüders band formation is the lack of enough mobile dislocations in the initial microstructure,while that of PLC band formation is attributed to the dynamic strain aging.Either the formation mechanism of Lüders band or that of PLC band,would take the dominant role at different deformation temperatures or plastic strains,thus leading to the distinct competition between Lüders band and PLC band.Secondly,enough work hardening ability was believed to be necessary for suppressing the further growth of the local plastic instability induced by Lüders or PLC band and supporting band propagation.For the Lüders band,referring to the XRD-measured two-stage transformation kinetics of retained austenite and the TEM-characterized microstructure evolution within the tensile Lüders strain range,it could be inferred that the enough work hardening ability is most likely provided by martensitic transformation in an indirect way.The“indirect way”means that the dislocation strengthening which originates from the obtained hard martensite and the geometrically necessary dislocations（GNDs）in the adjacent ferrite introduced by martensitic transformation will contribute to provide the work hardening ability.In addition,for PLC band,the dynamic strain aging is believed to provide the main contribution.The above interpretations could be utilized to qualitatively explain the observed variations of Lüders strain and band propagation behaviors with respect to these different deformation conditions.3)From the perspective of the coupon-level continuum mechanics,the rules of Lüders band propagation and the method of extracting the true stress-strain curve of7MnCA,are studied.To explore the rules of Lüders band propagation,the four-parameter（?,₀,K,n）true stress-strain model is firstly proposed.This model can be divided into two parts,the softening part and the hardening part.The softening part is described by the parameter?and₀.?is the true stress difference between the true upper stress and the true lower stress,and₀ is the true strain at which the softening part ends.The hardening part is described by the parameter K and n,which is the work hardening coefficient and the work hardening exponent,respectively.With this four-parameter model and a rapid Lüders band behavior prediction method,Lüders band behaviors in thousands of materials could be analyzed.And finally,two rules of Lüders band propagation are achieved,which reveals that in what materials the Lüders band can propagate and in what materials the Lüders strain is large.Firstly,the Lüders band can propagate in materials with the parameter K and n not being too small,and K being large if n is large.Secondly,the Lüders strain is large in materials with the parameters n,?,₀ being large and K being small.To extract the values of these four parameters which describe the true stress-strain curve of 7MnCA,an estimation was firstly extracted from the engineering stress-strain curves and these estimation values were further optimized by virtue of the Nelder-Mead algorithm.4)Two approaches,macroscopic loading mode optimization and microstructure design,are utilized to weaken the plastic instability phenomena in 7MnCA.The macroscopic loading mode optimization is realized by introducing the special-shaped tensile specimen,which can produce remarkable stress gradients in the specimen during the tensile test.These stress gradients result in the relatively large geometrical constraints,thus contributing to weakening these plastic instability phenomena.Furthermore,it is also found that the larger stress gradients will bring about more weakening effect.The microstructure design is realized by applying a novel two-step intercritical annealing treatment,which additionally introduce one step of high-temperature（slightly lower than Ac3）and short-time intercritical annealing to the as-received 7MnCA.During this additional annealing process,the grain size of austenite increased appropriately and that of ferrite exhibited almost no increase.This kind of microstructure will contribute to the relatively excellent mechanical properties,and eliminate the Lüders band by the stress-induced martensitic transformation（before the initiation of plastic deformation in ferrite）as well.Finally,it should be emphasized that this study is of great significance.It not only provides a systematic,multi-scale and profound understanding of these plastic instability phenomena in medium Mn steel in the field of academic research,but also offers some guidance on sheet forming and heat treatment process design for practical application.