Strain Hardening Behavior and Microstructure Evolution of High-Manganese Steel Subjected to Interrupted Tensile Tests

Self Cite

Detailed studies on microstructure-property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel-17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase. case, the phase transformation kinetics depends significantly on the austenite deformation degree [5,8,9]. In case of the first generation AHSSs, the main advantage of the TMP is the ability to refine the ferrite grain size by controlling the austenite pancaking [10,11]. Moreover, the thermomechanical treatment can increase the amount of retained austenite to obtain the optimal TRIP (TRansformation Induced Plasticity) effect [12,13]. In case of medium manganese steels (third generation AHSS), the TMP is also a good alternative for the cold-rolling process [14][15][16]. However, most publications on medium-Mn steels concern the cold-rolling process and subsequent inter-critical heat treatment.Cold-rolled medium-Mn TRIP steels are susceptible to plastic instabilities associated with dynamic strain aging (DSA) and serrated flow (PLC-Portevin-Le Chatelier) effects due to the heat treatment required after cold-rolling. From a technological point of view, the PLC and DSA effects must be avoided. The DSA phenomenon gives rise to non-homogeneous plastic flow during the sheet-forming processes and may lead to surface defects on formed parts [17][18][19]. Our previous reports on thermomechanically processed medium-Mn sheet steels indicate that the problem can be avoided [7].Newly developed fine-grained ferrite-austenite or bainitie-austenite steels contain manganese in a range of 3-12%, while carbon content is ca. 0.1-0.2%. These steels contain also aluminum and silicon (1-3%) additions which delay the carbides formation during the bainitic transformation. The increased Mn amount leads to obtain the high fraction of retained austenite (~10-30%). The Mn addition also increases the carbon solubility and lowers the cementite precipitation temperature [20][21][22]. Al is added to partially replace silicon due to the problems related to galvanizing, hot-rolling and welding [23][24][25]. However, Gir...

mentioning

confidence: 99%

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Grajcar

Kilarski²,

Kozłowska

2018

Self Cite

“…An Epsilon high temperature extensometer was used in the tensile test to measure the uniform deformation of the specimen, but the stress-strain curve of the material after necking is no longer reliable and is thus calculated by an FE-based inverse modeling procedure in this research. Many researchers [2,4,20] have proposed several FE-based inverse methods respectively and we used a modified inverse method based on the early work of Joun [20].…”

Section: Fe-based Inverse Modeling Proceduresmentioning

confidence: 99%

“…The simulation of metal forming process is a highly non-linear problem involving large material deformation and complex boundary conditions [2] which requires accurate hardening behavior of the investigated sheet metal over a wide range of plastic strains. Till now the most popular method to obtain the stress-strain relationship of metal sheet, represented either in the form of a set of discrete data points or analytical constitutive models, is typically by conducting standard uniaxial tensile test with rectangular cross-section [3,4]. The stress and strain values of the uniaxial tensile test can be obtained by using an extensometer that measures the elongation of a certain gauge length but the calculated stress value is accurate on the assumption that the test specimen is subjected to homogeneous state of uniaxial loading, which means that the stress-strain relationship identified by standard uniaxial tensile test is valid only before the so-called diffuse necking large strains under uniform deformation.…”

Section: Introductionmentioning

confidence: 99%

Evaluation Study on Iterative Inverse Modeling Procedure for Determining Post-Necking Hardening Behavior of Sheet Metal at Elevated Temperature

Mei

Lang

Liu

et al. 2018

The identification of the post-necking strain hardening behavior of metal sheet is important for finite element analysis procedures of sheet metal forming process. The inverse modeling method is a practical way to determine the hardening curve to large strains. This study is thus focused on the evaluation of the inverse modeling method using a novel material performance test. In this article, hot uniaxial tensile test of a commercially pure titanium sheet with rectangular section was first conducted. Utilizing the raw data from the tensile test, the post-necking hardening behavior of the material is determined by a FE-based inverse modeling procedure. Then the inverse method is compared with some classical hardening models. In order to further evaluate the applicability of the inverse method, biaxial tensile test at elevated temperatures was performed using a special designed cruciform specimen. The cruciform specimen could guarantee that the maximum equi-biaxial deformation occurs in the center section. By using the inverse modeling procedure, the hardening curves under biaxial stress state are able to be extracted. Finally the stress-strain curves obtained from the two experiments are compared and analysis studies are provided.2 of 15 point. However, many sheet forming processes usually involve large deformation and can generate strains far beyond the necking point, so the stress-strain curve up to the necking point is not sufficient for numerical simulation procedures. Generally, the available pre-necking stress-strain curves are extrapolated to large strains using different hardening models [5]. It is obvious that the predicted post-necking hardening curve greatly depends on the choice of phenomenological constitutive models and one model that is best fitted to a certain material may not suits for another [6].Recently, numerous investigations have been made to acquire the hardening curve beyond diffuse necking by many researchers [7][8][9][10][11]. The initial attempt was made by Bridgman [7], who derived a set of analytical models for the distribution of stress and strain across the diffuse necking region for a round bar. However, the correction and compensation for stress and stress values of Bridgman's method is based on the measurement of evolving geometrical parameters of the necking area, which requires much experimental effort. Ling [8] and Zhang et al. [9] extended the research of Bridgman by proposing new models for determining the post-necking hardening behavior of strip specimen with rectangular section. Other researchers [10,11] further studied the necking problem experimentally based on Bridgman's work in more sophisticated ways. Currently, a new optical-numerical measurement techniques, i.e., the Digital Image Correlation (DIC) method, is used widely for obtaining full-field information for both in-plane displacements and strains of the test specimen and has been applied by many researchers for determining the post-necking hardening curve of uniaxial test [12,13], Scheider et al. [14] investigat...

“…Moreover, one should note that the rapid degradation represented by a necking-failure phenomena occurred immediately after reaching the tensile strength. Earlier studies reported that microstructural effects such as martensite formation and mechanical twinning contributed to the strengthening of high manganese steel and delayed the onset of necking, which is associated with large uniform elongation [24][25][26][27][28][29]. Based on these features, strength-based design is typically performed for industrial structures and can lead to structural safety problems with respect to excessive deformation and rapid necking-fracture in high manganese steel.…”

Section: Introductionmentioning

confidence: 99%

“…Step 4: the modified calculation of (·) n+1 considering the plastic region through return mapping is carried out as shown in Equations (19)(20)(21)(22)(23)(24).…”

mentioning

confidence: 99%

Numerical Model for Mechanical Nonlinearities of High Manganese Steel Based on the Elastoplastic Damage Model

Kim

et al. 2018

For constructing marine liquefied natural gas (LNG) fuel/storage tanks, high manganese steel is being recognized as an alternative to stainless steel, nickel alloy, and aluminum alloy. In this study, the nonlinear tensile behavior of high manganese steel was investigated and numerically simulated at cryogenic temperatures at which natural gas exists as a liquid. Physical experimental tensile tests were carried out for a flat test specimen at 293 K and 110 K. In particular, the tensile behavior of a flat hole-notched high manganese steel specimen was experimentally obtained. A specimen with a hole was readily fractured compared to one without a hole. Tensile behavior of high manganese steel at the two cryogenic temperatures was compared to that of stainless steel, nickel, and aluminum alloy. In addition, numerical tests were performed for flat tensile specimens under identical experimental conditions. The elastoplastic damage model was derived and implemented using an Abaqus user-defined subroutine to appropriately simulate material behavior and degradation. The influence of some parameters on tensile behavior was investigated. The simulation results satisfactorily replicated the nonlinear tensile behavior of high manganese steel. The proposed numerical method, which is based on the damage-coupled material constitutive model, can be applied to structural analysis on the finite element analysis platform considering mechanical nonlinearities induced by severe conditions such as cryogenic temperature.