TWIP Steel Research Articles

On-line quality control and tool wear evaluation in trimming process by data analytics techniques

Sheet metal is an extremely common form of use in broad engineering sectors and represents the main consumption of steel and aluminum in Europe. All production processes where sheet metal is used require cutting operations. Therefore, it is an extremely relevant procedure, common and of great economic importance in material-intensive sectors such as automotive, construction and engineering. In a competitive environment where the demand for quality increases continuously and using harder to process high-performance materials, online monitoring and quality control become an attractive prospective. In this regard, application of data analytics techniques offers a promising toolset for fast and efficient analysis. This work presents a pilot demonstration of an instrumented sheet metal cutting line that integrates online quality control and predictive maintenance concepts, namely detection of wear in tools as well as problems arising from the raw material and process. Automated sequential cutting was performed using a straight trimming die mounted on a hydraulic press to cut High-Mn TWIP steel strip. Three piezoelectric force sensors were located in the trimming tool, collecting cycle data. Tests performed included studying the effect of blunting on the M2 (1.3343) tool, as well as the influence of correct lubrication. The resulting data were analyzed through the extraction of several parameters from each cycle to correlate die wear-induced force response evolutions with the shear-induced deformation in High-Mn TWIP steel sheets. Extracted parameters were the maximum and minimum force of each cycle and the impulse (integrated force over time) was computed and segmented by plastic deformation and crack contributions. Our analysis show that advanced algorithms can be used to analyze sensor data on an automated installation, revealing OK/NOK conditions and anomalous events.

Mechanical properties and deformation behaviour of TWIP steel at different strain rates

The mechanical properties of TWIP at different strain rates ranging from 5 s−1 to 500 s−1 were investigated by a VHS 160/100-20 high-speed tensile testing machine. The Johnson-Cook (J-C) dynamic constitutive model of TWIP steel was established based on the test results and verified by numerical simulation. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyse the microstructures of TWIP steel before and after tensile deformation. The findings demonstrate that under the tested strain rate, both the tensile strength and yield strength of TWIP steel increase with increasing strain rate, whereas the elongation decreases first and then increases. The strain rate of 100 s−1 is the turning point of the mechanical properties. The stack fault energy (SFE) is 26 mJ m−2 at room temperature, and the microstructure of TWIP steel before and after deformation is composed of single austenite, which demonstrates that only TWIP effect occurs. Dislocations accumulate along grain boundaries, forming structures such as dislocation cells and dislocation walls, causing stress concentration and promoting the formation of microcracks. With increasing strain rate, the dislocation cells are more obvious, and the thickness of the deformation twins decreases, resulting in an increase in the stress required for dislocations to cross the twin boundaries and material strengthening. The formation of nanoscale twins can not only effectively hinder the dislocation motion, but also provide space for dislocation slip and change the grain orientation, so that the slip system which is not conducive to deformation has a new orientation, resulting in a synergistic effect on strength and plasticity of TWIP steel. This work elucidates the relationship between the microscopic deformation mechanism and the macroscopic mechanical properties of TWIP steel at a specific strain rate, and a dynamic constitutive model is established, which can provide a reference for the application of TWIP steel in the design of anti-explosion and anti-impact structures.

Procedures for microstructurally conditioning an Fe-22Mn-0.6C-1.5Al TWIP steel for optimal mechanical behaviour

Procedures for microstructurally conditioning an Fe-22Mn-0.6C-1.5Al TWIP steel for optimal mechanical behaviour

Enhancement of hydrogen embrittlement resistance in a Fe-18Mn-0.6C twinning induced plasticity steel by copper alloying

The effect of Cu alloying on hydrogen embrittlement (HE) in TWIP steel was studied using the low-speed linearly increasing stress test (LIST) with simultaneous cathodic hydrogen charging. Cu alloying influenced HE resistance by the following mechanism. (i) Cu alloying increased the stacking fault energy, which canceled out the embrittling effect of hydrogen on promoting local plasticity and mechanical twins/dislocations concentration. (ii) Cu alloying refined the austenite grains, especially after low-temperature (700 °C) annealing and thus delayed mechanical twinning that causes an increased hydrogen concentration. (iii) Cu alloying caused the formation of a Cu-rich surface which reduced the diffusible hydrogen content, particularly for the steel annealed at 900 °C. (iv) Cu alloying dispersed mechanical twins by Cu precipitate/Cu-rich clusters, which homogenized the diffusible hydrogen and decreased the local hydrogen concentration at the HE vulnerable interfaces. Mechanism (iv) in the 700 °C annealed Cu-alloyed TWIP steel produced the greatest increase in HE resistance.

Limitations of meta-atom potential for analyzing dislocation core structure in TWIP steel

In recent years, the scheme of meta-atom interatomic interaction has been proposed and employed as a simple alternative to the complex potentials for multi-component alloys. Although this potential model has primarily been utilized for large-scale simulations of plastic deformation, the interpretation of structures and behaviors of individual line defects still needs to be explored. The major issue arises from the fact that the meta-atom model, and other such homogenization approaches, represent the real system only in a statistical sense over a critical length scale. As the descriptive spatial parameters for the structure of a dislocation may span from a few angstroms to tens of nanometers, it necessitates the examination of the transferability of meta-atom potential in this context. Here we analyze the core structures of dissociated and twinning dislocations in TWIP steel using the meta-atom force-field and variational Peierls–Nabarro model. In particular, we compute the core widths, stacking-fault widths, and Peierls stresses of dislocations. In addition, the impact of Escaig stress on the stacking fault ribbon has been explored. The analyses reveal the limitations of the meta-atom model in accurately revealing the structural features of dislocation cores. While the meta-atom scheme can provide a representative averaged-out value of the core width, it fails to show the statistical variations along the dislocation line. Similarly, the model misses the spatial fluctuation of critical negative Escaig shear stress, whereas its positive counterpart is reasonably represented.

High-speed tensile tests on high-manganese steel at low temperatures

Abstract Particularly in the automotive industry and the sector of liquid–gas storage, austenitic steels with a high energy absorption capacity are used. The light metals frequently used do not meet all requirements. Therefore, especially for components involved in crash situations, the cost-effective alternative with outstanding properties is used. In this respect, the high-manganese TWIP steels are of great interest, specifically due to their strength and ductility. Also in other research areas, the high-strength steels are moving into the focus of attention. For example, the properties of welded high-manganese steels at low temperature applications, such as those encountered during storage and transport of liquefied gases, are to be investigated. For this purpose, high-speed tensile tests are used to experimentally determine how the material behaves at different low temperatures and high strain rates. The investigations carried out provide results that can be used to draw conclusions about the strain rate and temperature dependence of the mechanical characteristics. These dependencies are shown as well.

Analysis of defects and non-metallic inclusions distribution in high-strength TWIP steel Fe-25Mn-12Al-1.5C after electroslag remelting

TWIP steels belong to the list of the most innovative materials of our time due to the combination of a high mechanical characteristics level and low density. The most high-strength alloys usually contain about 25-30 wt. % manganese and about 10 wt. % aluminum. Production of such steels is complicated by the peculiarities of their chemical composition. Due to the high content of manganese and aluminum, they are prone to components liquation by density, have a greater number of shrinkage defects and an increased number of sulfides, nitrides and oxides non-metallic impurities. This determines the use of effective refining methods, which include electroslag remelting (ESR). The paper shows a comparison of Fe-25Mn-12Al-1.5C alloy structure, type and amount of non-metallic inclusions after induction melting and after refining electroslag remelting. Electron microscopy of the samples and local chemical analysis of the phases showed a large number of non-metallic inclusions — sulfides, phosphides, and oxynitrides. After refining process, it was shown that electroslag remelting contributes to a noticeable decrease of nitrogen and sulfur content, and as the result — it lowers the number of related of non-metallic inclusions. However, it seems to be an insufficiently effective method of refining materials like TWIP-steels. Relatively large size of the non-metallic inclusions, low phosphides refining ability and the crystallization conditions under which a directionally crystallized structure forms, may be noticed among the disadvantages of the ESR method. It was also established that in crystallizer zones, close to the bottom and walls, metal refines worse than its central volumes. Upper part of the ingot has shrinkage and sub-shrinkage zones enriched with gas-shrinkage defects, so it can be called a problem zone. In general, it is shown that the ESR method is not capable to solve a problem of refining high-manganese TWIP steels with a high aluminum content to the required extent.

The Hot Ductility of TWIP and TRIP Steels—An Alternative Interpretation

There is significant evidence from light metals that turbulence in casting leads to bifilm defects; enfolded, doubled-over oxide films which act like cracks in the liquid, and are inherited as cracks by the solid. This population of introduced cracks is now known to significantly influence the tensile failure behaviour of light alloys. There is evidence that analogous defects exist in steels. This paper examines the possibility that bifilms may control the hot ductility of TWIP and TRIP steels, and therefore the problems of straightening during continuous casting. Techniques for overcoming these problems are indicated.

Fatigue-resistant design in low-cycle regime by regulating the micro-structural gradient in a TWIP steel: Modelling and experiment

It remains an intricate problem how to enhance low-cycle fatigue (LCF) performance, in the premise of guaranteeing a relatively high strength (or cyclic stress) which is vital to long-time service, for engineering components. The underlying cause of this puzzle is the lack of practical theory on cracking evolution/life prediction for LCF. In this study, a two-stage constitutive LCF crack growth model was proposed for a homogeneous solid and later developed for graded structures. Based on the model, a material design strategy emphasizing the strength gradient architecture (informally, hard core and soft shell structure) is proposed and analyzed to improve the LCF properties of engineering structural materials, e.g., steels. Experimentally, sharp and gradual strength gradients were carefully introduced into a TWIP steel by a series of well-designed process combinations regarding various (heterogeneous) deformation and annealing treatments. The influences of strength/microstructure and their spatial distributions on the LCF behaviors were systematically investigated. The excellent LCF properties of the TWIP steel with a negative strength gradient are originated from the repeated strengthening and weakening of strain gradient during cyclic loading, which suppresses the fatigue crack initiation and growth. The crack initiation/propagation model developed in this study is conducive and applicable to the LCF life prediction. The theory and process of constructing microstructure gradient, and therefore mechanical property gradient, may provide new and important implications for engineering metals on anti-LCF design.

High Manganese TWIP Steel with Increased Corrosion Resistance

The paper describes the development of austenitic steel with the TWIP effect, which is alloyed with chromium to increase corrosion resistance. The experimental heat of this steel was cast in an experimental melting furnace and subsequently subjected to hot and cold rolling. After cold rolling, the appropriate recrystallization annealing temperature was applied to obtain the optimal austenitic grain size. X-ray diffraction proved that the steel contains a fully austenitic structure. After recrystallization annealing, the sheets achieved a TS of more than 950 MPa with an elongation of 40%. The corrosion resistance of this steel is increased with the addition of chromium.

Interstitial strengthening in f.c.c. metals and alloys

In this short review, we highlight instances where interstitials have been shown to substantially increase the yield strength and work-hardening rate (WHR) of f.c.c. alloys, particularly high entropy alloys, medium entropy alloys, TWIP steels and stainless steels. However, the common practice of describing interstitial strengthening in f.c.c. alloys using models that are used to explain substitutional strengthening appears to be neither appropriate nor accurate. Here we suggest, based on the literature, that the yield strength increase due to interstitials in f.c.c. alloys is more appropriately described by a linear dependence on the concentration: due to a paucity of experimental studies, the dependence of the yield strength and WHR on misfit parameters is currently unclear. Thus, the source of the strengthening remains unclear. A feature that has been observed in several f.c.c. alloys is that interstitial additions lead to a change from wavy to planar slip although the origin of this change, which may be related to changes in stacking fault energy as well as other factors, remains unclear. The paper concludes by outlining areas of future research, including the need to develop a new model for interstitial strengthening in f.c.c. alloys.

Interface Strengthening and Toughening Mechanism of Hot Rolled Multilayer TWIP/40Si2CrMo Steels

Layered metal composites play an increasingly important role in aerospace, automotive, and nuclear energy. Compared with a single metal or alloy, the layered metal composite exhibits an excellent strong-plastic matching effect. In this paper, multilayer TWIP/40Si2CrMo steels with different hot rolling reductions were successfully fabricated by the vacuum hot rolling. The results show that the multilayer steels can improve the lower yield strength of TWIP steel and lower the fracture elongation of 40Si2CrMo steel. In addition, with the increase of the hot rolling reduction, the mechanical properties and interfacial bonding strength of multilayer steels were improved, while the size and number of interfacial oxides decrease, and the fracture mode was also changed. This shows that a higher hot rolling reduction will promote the breakage of the interface oxides and make them appear dispersed, thereby improving the bonding strength of the interface, effectively suppressing the delamination and local necking of the multilayer steel, and making the multilayer steel show a higher ability of uniform plastic deformation. At the same time, under the dual action of layer thickness scale and interface strengthening effect, the brittle layer of multilayer steel presents a multiple tunnel crack mode. It was beneficial to alleviate the stress concentration and further improve the strengthening and toughening effect of multilayer steel.

Controlling Mechanical Behavior of TWIP Steels by Tuning Texture and Stacking Faults

Controlling Mechanical Behavior of TWIP Steels by Tuning Texture and Stacking Faults

Deformation behaviour of high-manganese steel with addition of niobium under quasi-static tensile loading

Abstract In this paper, the heat generated during deformation under the static testing of high-manganese TWIP steel with addition of niobium was determined. The research combined the interaction of heat generated during deformation, mechanical properties, hardness and microstructure. Temperature and strain were measured simultaneously using infrared (IR) thermography and digital image correlation (DIC) method. The average temperature measured at the necked region equals 42°C at the strain rate of 0.001 s−1 and exceeds 100°C at 0.5 s−1. Therefore at large strains, a reduction in stress was observed. The course of the hardness change coincides very well with the strain changes, however, at the strain rate of 0.5 s−1 near to the necking area the hardness equals to 360 HV2, whereas at the lower strain rates it equals to 370 HV2. These changes are connected mainly with increase in temperature to >100°C

Analysis of strain hardening behavior of a high-Mn TWIP steel using electron microscopy and cyclic stress relaxation

Electron microscopy and cyclic stress relaxation experiments were employed to study the dislocation-obstacle interactions and activated hardening mechanisms, during tensile straining of a high-Mn TWIP steel specimen, and to estimate the contributions of hardening mechanisms to the strain hardening behavior. Electron contrast channeling and high-resolution electron backscattered diffraction images showed that deformation twins and their twin boundaries affect the dislocation evolutions inside both the matrix and twin lamellae. High-resolution transmission electron microscopy suggested that twin boundaries (TBs) can be passed by dislocations through the dislocation multiplication mechanism resulting in the nucleation of sessile dislocations and alternated stacking faults at TBs. Consequently, high dislocation density was developed behind the TBs resulting in enhancement of the strain hardening rate through the composite hardening effect. Various thermally activated parameters determined through cyclic stress relaxation tests indicated that dislocation evolution, developed by TBs in terms of the composite effect, was the prevalent rate-controlling mechanism, imparting higher strengthening than TBs in terms of the dynamic Hall–Petch effect. Finally, the high nucleation tendency of Shockley partial dislocations at TBs was interpreted from the variation of the stress exponent and independent plastic strain rate, and also from the general stacking fault energy point of view.

Effect of Intercritical Rolling on the Microstructure, Texture and Mechanical Properties of Dual Phase TWIP Steel

Effect of Intercritical Rolling on the Microstructure, Texture and Mechanical Properties of Dual Phase TWIP Steel

Microstructure evolution and fracture behaviour of TWIP steel under dynamic loading

Exploring the dynamic mechanical behaviours of twin-induced plasticity steels supports the safety design of which some critical parts and structures are made. In this work, the microstructure evolution and fracture behaviours of TWIP steel at 700–3000 s−1 were investigated by split Hopkinson tension bar and SEM-EBSD characterization. The results reveal that the positive strain rate sensitivity at the early stage of strain is attributed to the promotion of twinning and dislocation multiplication. With the increasing strain rate, the adiabatic temperature rise (∼110 K) increases the stacking fault energy to more than 50 mJ m−2, so that the critical shear stress for twinning is higher than the critical shear stress for slip at fracture, resulting in the suppression of twinning and the rapid decrease of the strain hardening rate. The accumulation of dislocations near grain boundaries or twin boundaries facilitates the strain localization to further form the high dislocation density structures and the interlaced twin structures which may be the onset sources for nucleating microcracks. The transgranular cracking was shown from the side of the specimen by optical microscopy. The frontal observation of the fracture morphologies by SEM indicates that the transformation of typical ductile fractures with dimples into quasi-cleavage fractures with river patterns may be caused by the enhanced local stress. The present work elucidates the deformation mechanisms and fracture behaviours at high strain rates, providing a new perspective for further understanding the competition between the twinning and dislocation slip and the intrinsic mechanism of ductility loss under dynamic conditions of TWIP steels.

Emergence and annihilation of Kirkendall holes during magnetron sputtering aluminum coating to twinning induced plasticity steel

It is important to design a strong interface, i.e., with the inner-diffusion of elements and without fragile compounds/large holes, between a coating and its substrate. In this work, a hot-wire enhanced plasma magnetron sputtering technique was used to deposit Al coatings on the surface of TWIP at various deposition times (1 h, 2 h, and 4 h) at a controlled temperature of 400 °C. The diffusion behavior of the Al coatings/TWIP substrate under the same temperature was investigated. The X-ray diffractometer (XRD) results showed a typical Al recrystallized morphology with the preferred orientation (1 1 1). Combining the results of transmission electron microscope (TEM) morphology and glow discharge optical emission spectroscopy (Pulsed RF GD-OES), the Kirkendall holes were seen at the interface between the TWIP substrate and Al coating deposited for 1 h, which were generated by the diffusion of Al atoms through the interface and into the surface of TWIP substrate. Extending the deposition time to 4 h, the holes were annihilated by the delayed diffusion of iron (Fe) and manganese (Mn) atoms from TWIP steel to the Al coating. It was confirmed by the calculation of GD-OES that the diffusion rate of Al atoms (0.12 µm2/h) was slower than that of Fe (0.34 µm2/h) and Mn atoms (0.37 µm2/h). This investigation of the inner-diffusion behavior and the finding of the emergence/annihilation of Kirkendall holes will be helpful to guide the setting of deposition parameters to establish a strong interface between the Al coating/TWIP substrate.

Numerical Modeling and Simulations of Twinning-Induced Plasticity Using Crystal Plasticity Finite Element Method

In the current work, a fully implicit numerical integration scheme is developed for modeling twinning-induced plasticity using a crystal plasticity framework. Firstly, the constitutive formulation of a twin-based micromechanical model is presented to estimate the deformation behavior of steels with low stacking fault energy. Secondly, a numerical integration scheme is developed for discretizing constitutive equations through a fully implicit time integration scheme using the backward Euler method. A time sub-stepping algorithm and the two-norm convergence criterion are used to regulate time step size and stopping criterion. Afterward, a numerical scheme is implemented in finite element software ABAQUS as a user-defined material subroutine. Finally, finite element simulations are executed for observing the validity, performance, and limitations of the numerical scheme. It is observed that the simulation results are in good agreement with the experimental observations with a maximum error of 16% in the case of equivalent stress and strain. It is also found that the developed model is able to estimate well the deformation behavior, magnitude of slip and twin shear strains, and twin volume fraction of three different TWIP steels where the material point is subjected to tension and compression.

An Energy-Based Method for Lifetime Assessment on High-Strength-Steel Welded Joints under Different Pre-Strain Levels.

Pre-loading on engineering materials or structures may produce pre-strain, especially plastic strain, which would change the fatigue failure mechanism during their service time. In this paper, an energy-based method for fatigue life prediction on high-strength-steel welded joints under different pre-strain levels was presented. Tensile pre-strain at three pre-strain levels of 0.2%, 0.35% and 0.5% was performed on the specimens of the material Q345, and the cyclic stress and strain responses with pre-loading were compared with those without pre-loading at the same strain level. The experimental work showed that the plastic strain energy density of pre-strained welded joints was enlarged, while the elastic strain energy density of pre-strained welded joints was reduced. Then, based on the strain energy density method, a fatigue life estimation model of the high-strength-steel welded joints in consideration of pre-straining was proposed. The predicted results agreed well with the test data. Finally, the validity of the developed model was verified by the experimental data from TWIP steel Fe-18 Mn and complex-phase steel CP800.