Twinning-induced Plasticity Steel Sheet Research Articles

Strain-induced solid-state coating of TWIP steel sheets with zinc

We propose the strain-induced solid-state coating process of twinning-induced plasticity (TWIP) steel with zinc, achieved through concurrent rolling of TWIP steel and zinc sheets. Our study explores the effects of strain, strain rate, and rolling temperature on the morphology and intermixing at the Zn-TWIP steel interface. We found that two simultaneous mechanisms are responsible for the formation of a joint interface between the sheets in the rolling temperature range of 100–380 °C, namely mechanical intermixing at the interface and accelerated diffusion along moving dislocations and other defects introduced during rolling. Since it is impossible to deconvolute these mechanisms, we introduced the concept of an ‘apparent diffusion coefficient’ to represent the diffusion-like mass transfer resulting from mechanical intermixing and diffusion. These diffusion coefficients were determined by primary electron energy variation method in scanning electron microscope and by energy-dispersive X-ray spectroscopy in transmission electron microscope. Notably, these diffusion coefficients were found to be higher than the grain boundary diffusion coefficients of Zn in γ-Fe, primarily due to the dynamic nature of interface microstructure formation during rolling.

Shear and laser cutting effects on hydrogen embrittlement of a high-Mn TWIP steel

The influence of shear and laser cutting on hydrogen embrittlement of a hot-rolled Fe-18Mn-0.4C-1.2Al-1.8Cr TWIP (twinning induced plasticity) steel sheet in 3 mm thickness was investigated by hydrogen pre-charging and subsequent slow strain rate tests. Due to the severe plastic deformation during shearing, the ductility of the shear-cut specimen was significantly reduced in tensile test compared to the laser-cut specimen. After hydrogen pre-charging, the laser-cut specimen exhibited less susceptibility to hydrogen embrittlement than the shear-cut specimen. The fractographic analysis revealed that the shear-affected zone acted as crack initiation site during tensile test, which exhibited a flat fracture zone with high stress triaxiality. In hydrogen-charged condition, the flat fracture zone transited from large primary dimples to the mixed features with dimples and quasi-cleavage features.

Laminated TRIP/TWIP Steel Composites Produced by Roll Bonding

In order to investigate the roll bonding of high-alloy transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) steel, roll-bonded sheets of the TRIP and TWIP steel were manufactured starting from hot rolling, followed by brushing and cold rolling. Both, the microstructure and mechanical properties of the roll-bonded sheets were characterized by metallographic investigations, and tensile and T-peel tests. Preliminary results, such as an occurrence of an adhesive bonding between two TWIP steel sheets and between TRIP and TWIP steel sheet after a thickness reduction of approximately 50% were obtained. Moreover, the formation of deformation-induced martensite leads to outstanding mechanical properties of the roll-bonded composite sheet. An ultra-fine grained microstructure was observed in the bonding zone after only one roll-bonding process. The obtained promising results demonstrate the possibility of the development of an accumulative roll-bonding process for TRIP/TWIP steel composites.

Determination of forming limits of TWIP steel sheet using linear and nonlinear strain paths

.The main goal of the present paper is investigation of the mechanical behaviour of high manganese content steel sheet which is well known in the literature as Twinning-Induced Plasticity (TWIP) under linear and nonlinear loading path. Contrary to Nakajima experiment, cruciform specimen under biaxial loading can provide linear and nonlinear strain path. This work aims to determine the numerical Forming Limit Curve (FLC) for TWIP sheet using cruciform specimen under biaxial loading. For this purpose, uniaxial tensile tests are performed on standard dog-bone samples which are extracted from rolling, transversal and diagonal directions to identify the flow curve as well as Lankford ratios. Data are used as input for numerical simulation of cruciform specimen. Results show the applicability of cruciform specimen for linear and nonlinear strain paths.

Microstructural evolution of liquid metal embrittlement in resistance-spot-welded galvanized TWinning-Induced Plasticity (TWIP) steel sheets

The weldability is one of the key factors governing applications of TWinning-Induced Plasticity (TWIP) steels to automotive industries demanding high economy, environmental friendliness, and high performance. During spot welding of Zn-coated Al-containing TWIP steel sheets, liquid metal embrittlement (LME) frequently occurs by Zn infiltration into grain boundaries to form cracks, but the direct observation and detailed analysis of LME cracking are quite difficult because it occurs instantaneously within a second. Here in the present study, the LME was investigated by detailed microstructural evolutions of small Zn infiltrations or cracks as well as formation behavior of various intermetallic phases. In the heat-affected zone, the applied tensile stress and spot-welding heat tore down a diffusion-inhibiting Fe2Al5 layer formed between the Zn-coated layer and the TWIP steel substrate, and formed Zn-containing ferrite (α-Fe(Zn)) particles on the steel surface which provided paths for liquid Zn infiltration. α-Fe(Zn) particles played critical roles in accelerating the LME by reducing the ductility because they were brittle due to high contents of Zn and Al. In the present Al-containing TWIP steels, the increase in welding current generally aggravates the LME.

Strain Effect on the Microstructure, Mechanical Properties and Fracture Characteristics of a TWIP Steel Sheet

Twinning-induced plasticity (TWIP) steels are a highly promising group of steels for the production of complex structural components in cold forming operations for car body manufacturing. In this work, the effect of cold rolling strain on the microstructure, mechanical properties and fracture characteristics of a TWIP steel sheet used for automobile body structure was studied by means of optical microscopy, scanning electron microscopy, electron back-scattered diffraction technique, microhardness measurement, tensile test and fractography. TWIP steel sheets were cold rolled with reductions of 0, 15 and 30%. An increase of the cold rolling strain led to an increase of deformation twinning activity in certain favourably oriented grains and resulted in significant increase in ultimate tensile strength and hardness of TWIP steel. However, the ductility of TWIP steel significantly decreased with increasing degree of cold rolling strain. The increase in the ultimate tensile strength was almost linear with the increase in cold rolling strain. After cold rolling reduction of 30%, the ultimate tensile strength increased by approximately 50%, whereas the elongation decreased by approximately 85%. The size and depth of the dimples in the fracture surface decreased with the increase of the twin boundaries at 30% cold rolling strain, leading to highly limited plasticity through the tensile testing.

Effect of Weld Current on the Microstructure and Mechanical Properties of a Resistance Spot-Welded TWIP Steel Sheet

In this study the effect of the weld current on the microstructure and mechanical properties of a resistance spot-welded twinning-induced plasticity (TWIP) steel sheet was investigated using optical microscopy, scanning electron microscopy–electron back-scattered diffraction (SEM–EBSD), microhardness measurements, a tensile shear test and fractography. Higher weld currents promoted the formation of a macro expulsion cavity in the fusion zone. Additionally, higher weld currents led to a higher indentation depth, a wider heat-affected zone (HAZ), coarser grain structure and thicker annealing twins in the HAZ, and a relatively equiaxed dendritic structure in the centre of the fusion zone. The hardness values in the weld zone were lower than that of the base metal. The lowest hardness values were observed in the HAZ. No strong relationship was observed between the hardness values in the weld zone and the weld current. A higher joint strength, tensile deformation and failure energy absorption capacity were obtained with a weld current of 12 kA, a welding time of 300 ms and an electrode force of 3 kN. A complex fracture surface with both brittle and limited ductile manner was observed in the joints, while the base metal exhibited a ductile fracture. Joints with a higher tensile shear load (TSL) commonly exhibited more brittle fracture characteristics.

Joining TWIP to TWIP and TWIP to aluminium: A comparative study between joining processes, joint properties and mechanical performance

In the present study, Resistance Spot Welding (RSW), Self-Piercing Riveting (SPR), Kerb Konus Riveting (KKR) and EJOWELD techniques were investigated for joining of TWIP steel sheets with one another and with Al. This is the first time the KKR and EJOWELD methods were compared to conventional joining methods The main focus was on the mechanical properties of similar joints produced between Zn-coated Fe–12Mn–1.5Al–1Si–0.5C Twinning Induced Plasticity (TWIP) steel and dissimilar joints between Zn-coated TWIP and Al 5754 or Al 6111 aluminium alloy sheet. The nugget solidification after fusion welding was evaluated through characterization with SEM-EBSD. In the case of RSW produced joints it was found that the joints with smaller grain size and larger orientation diversity exhibited larger maximum load capability. Regarding the joints produced with the EJOWELD method, grain size did not vary much compared with the base metal. Mechanical properties and the fracture behaviour of these joints were identified through the application of the lap shear method and failure modes. In the case of TWIP/TWIP combination, the results demonstrated that RSW samples exhibited the highest maximum load and corresponding elongation, while in dissimilar joining, SPR exhibited the largest maximum load. The EJOWELD method for the dissimilar joints showed similar mechanical properties to the SPR, in terms of maximum load, while KKR poorer properties. By comparing the mechanical properties of all the joints, it can be concluded that the primary factor of joint strength in RSW were the joining parameters (preweld and number of weld currents, welding time), while in the case of the other techniques it was the sheet strength.

Misorientation-Dependent Twinning Induced Hardening and Texture Evolution of TWIP Steel Sheet in Plastic Deformation Process

The quantitative contribution of twinning to hardening behavior and its effect on crystal orientation need to be explored in greater depth for design and forming of twinning-induced-plasticity (TWIP) steel products. To address this issue, the characteristics of twinning formation in the plastic deformation of Fe-30Mn-3Si-2Al TWIP steel are investigated in terms of intergranular misorientation distribution using electron back-scattering diffraction (EBSD), which reveals that most deformation twins adhere to the high-angle grain boundaries (HAGBs) of the face-center-cube (FCC) type TWIP steel. Texture measurements are conducted to show a stable volume fraction of major components including Goss, S and A orientations, while Copper shifts towards Brass orientation. A crystal plasticity finite element (CPFE) model based on virtual polycrystalline microstructure adopting representative volume element (RVE) is employed to simulate the deformation to reveal the correlation between misorientation-dependent twinning and hardening behavior of TWIP steel. The results demonstrate that the proportion of twinning hardening to overall hardening is larger than slip hardening. The stability of texture evolution is simulated to predict the anisotropy of TWIP steel. This research substantiates the twinning induced hardening and texture evolution in deformation of TWIP steel and thus is essential for accurate prediction of the mechanical behaviors.

MIG-Welding of Dissimilar Advanced High Strength Steel Sheets

The need for steel materials with increasing strength is constantly growing. The main application of such advanced high strength steels (AHSS) is the automobile industry, therefore the welding process of different types of AHSSs in dissimilar welding joint was investigated. To simulate the mass production of thin steel sheet constructions (such as car bodies) automated metal inert gas (MIG) welding process was used to weld the TWIP (twinning induced plasticity) and TRIP (transformation induced plasticity) steel sheets together. The welding parameters were successfully optimized for butt welded joints. The joints were investigated by visual examination, tensile testing, quantitative metallography and hardness measurements. The TRIP steel side of the joints showed increased microhardness up to (450-500 HV0.1) through increased fraction of bainite and martensite. Macroscopically the tensile specimen showed ductile behaviour, they broke in the austenitic weld material.

Deep drawing behavior of twinning-induced plasticity-cored three-layer steel sheet

Herein, we report the results of our experimental and computational investigation of the deep drawing behavior of twinning-induced plasticity (TWIP)-cored three-layer steel sheet. Various directional tensile tests for monolayer sheets of the TWIP and low-carbon (LC) steels were performed for evaluation of flow curves and anisotropic coefficients, which are used as input data of the finite element analyses for deep drawing of the monolayer and three-layer sheets. The experimental deep drawing behaviors of the layered sheets of three thickness ratios, and of the monolayer sheets, are computationally analyzed in terms of earring and variations of local thickness along the positions in the drawn cups. It was found that the three-layer sheets are more planar-isotropic in the sense of earring than the monolayer sheets, even more than the TWIP steel sheet. The thickness uniformity along the initial radial direction in the layered sheets is in between the heterogeneous TWIP steel and the relatively homogeneous LC steel. The present results shed light on the future direction of development of steel sheets with planar-isotropic and homogeneous deep-drawing characteristics.

On the potential of magnetron sputtering for manufacturing of thin high Mn TWIP steel foils

Some high manganese austenitic steels are mechanically metastable. The addition of some alloying elements in the steel may result in a decrease of the materials staking fault energy, inducing a modification of the plastic deformation mechanism from classical dislocation glide to twinning or martensitic transformation. Theses steels are designated as TWIP (Twinning Induced Plasticity) and TRIP (Transformation Induced Plasticity) steels and show at different extents a very high strength and an excellent ductility. These materials are thus particularly appropriate for the forming industry.However, thin foils of these steels are not available due to the difficulty of rolling down TWIP and TRIP steel sheets to the desired thickness (15–30μm), which implies to regularly recover the original austenitic structure through several heat treatments. Moreover, most of the technical literature on the topic deals about samples with thicknesses around 1mm. Hence, very little is known about the behavior of TWIP and TRIP steel foils at the micrometer scale.In this study, magnetron sputtering was used to manufacture 30μm 25Mn-3Si-3Al steel foils and the influence of the process characteristics, in particular the target power, the roughness of the substrate and several annealing treatments on the tensile properties and the microstructure of theses foils was investigated. Scanning electron microscopy was employed to reveal the changes of morphology while the evolution of the microstructure and dislocation density were followed by x-ray diffraction.

Prediction of earing in TWIP steel sheets based on coupled twinning crystal plasticity model

By combining the nonlinear finite element analysis techniques and crystal plasticity theory, the macroscopic mechanical behaviour of crystalline material, the texture evolution and earing type characteristics are simulated accurately. In this work, a crystal plasticity model exhibiting deformation twinning is introduced based on the crystal plasticity theory and saturation-type hardening laws for FCC metal Fe-22Mn-0.6C twinning-induced plasticity (TWIP) steel. Based on the crystal plasticity finite element (CPFE) model and parameters which have been determined for TWIP steel, a simplified finite element model for deep drawing process is promoted by using crystal plasticity constitutive model. The earing characteristics in typical deep drawing process are well simulated. Further, the earing profile and drawing forces are calculated and compared with the experimental results from the references. Meanwhile, the impacts of drawing coefficient and initial texture on the earing characteristics are investigated for controlling the earing. According to the result of the simulation, earing rate manifests a downward trend with the drawing coefficient increasing, and two different earing types of 6-0°/60° and 4-0°/90° occur with the brass, cube and gauss type of initial texture.

An investigation into the fracture mechanisms of twinning-induced-plasticity steel sheets under various strain paths

The formability of the automotive twinning-induced-plasticity (TWIP) steel sheets is investigated through the hemispherical-punch stretching test. The forming limit diagram (FLD) at fracture is constructed both experimentally and numerically. Numerical studies are also performed to assess the applicability of various uncoupled fracture criteria to predicting the FLD. The results illustrate that the FLD may not be properly predicted using only a ductile or a shear fracture criterion. The principal strains at fracture can be predicted with good accuracy by taking both types of fracture mechanism simultaneously into consideration (e.g. models by Han-Kim (2003) and Lou (2012)). This can be justified by describing the damage process of TWIP steel sheets as the nucleation, growth, and shear coalescence of voids.

Cut quality assessment of CO2 laser cutting of twinning-induced plasticity steel sheets

In this study, the effects of the main process parameters involved in CO2 laser cutting of 1.5-mm twinning-induced plasticity steel sheets have been investigated by means of experimental tests. The quality of the cut edges of the sheets was evaluated by analyzing the kerf width, kerf deviation, roughness of the cut surfaces, and dross attachment. The process parameters used were the laser power, cutting speed, oxygen pressure, and pulse frequency of the CO2 laser beam. The optimal settings of the process parameters were predicted using the Taguchi parameter design approach with an L27(313) orthogonal array and calculating the average effect of both the laser parameters and the signal-to-noise ratios. Analysis of variance was performed to evaluate the statistical significance and the contribution of the laser process parameters to the examined kerf features and roughness. On the basis of the Taguchi approach, the predicted optimal settings of the process parameters were verified through confirmation tests. The results show that the selected process parameters, as well as their interactions, can have a remarkable effect on the cutting quality of twinning-induced plasticity steel sheets. The laser power is the main process parameter that affects the kerf width on both sheet sides, while the pulse frequency and its interaction with the laser power are the main factors that influence the kerf deviation, followed by the cutting speed. The roughness of the cut surfaces is mainly affected by the oxygen pressure and the interaction between laser power and cutting speed. Dross attachment was primarily observed in the specimens cut with high laser power and low cutting speed.

Localized amorphism after high-strain-rate deformation in TWIP steel

The microstructural features of shear localization, generated by a high-strain-rate deformation (∼105s−1), of a twinning-induced plasticity (TWIP) steel containing about 17.5wt.% Mn were well characterized by means of optical microscopy, transmission electron microscopy and electron backscatter diffraction. The high deformation rate was obtained by a ballistic impact penetration test on the TWIP steel sheet. In addition to the deformation twins observed as the main microstructural characterization in the matrix, some shear bands consisting of complex microstructures were also evidenced in the highly deformed area. Inside the shear band, there exist a large region of amorphous phase and a smooth transition zone that also contains nanocrystalline phases. The grain size decreases gradually in the transition zone, changing from a coarse scale (>100nm) to a fine scale (<10nm) adjacent to the amorphous region. The coexistence of the amorphous state and the fine-scaled nanocrystalline phase clearly suggests that melting inside the shear bands occurred, which is corroborated by calculations showing a very high rise in temperature due to localized plastic deformation and extremely rapid cooling by heat dissipation into the specimen.

Microstructure and mechanical properties of laser welded austenitic high manganese steels

The microstructure of laser welded austenitic twinning induced plasticity steel sheets joints was investigated by means of optical microscopy, SEM and electron backscattering diffraction in order to differentiate the fusion zone, heat affected zone and base material, as well as to establish present phases, grain size distribution and grain misorientation distribution caused by the welding process. Measurements of EDX were taken into account to evaluate the effect of Mn segregations. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joints. In addition, the twinning phenomena progress was assessed by investigating the texture evolution in the base material and fusion zone of samples plastically deformed by 5, 10 and 15%. Grain refinement was found in the fusion zone affecting substantially the mechanical properties of the welding, being the most resistant and harder region.

Strain Rate Sensitivity of 31Mn-3Al-3Si TWIP Steel with Partially Recrystallized Fine Grained Structure

Strain rate sensitivity of the strength of TWIP (Twinning Induced Plasticity) steel with the mixture of recrystallized fine grains and rolling-deformation microstructures was studied. The 31mass%Mn-3%Al-3%Si TWIP steel sheet was severely cold-rolled to a reduction of 92% and subsequently annealed at various temperatures ranging from 600oC to 700oC in order to obtain the partial recrystallized microstructure with various fraction of recrystallized microstructure. The 600oC annealed specimen keeps similar morphologies as observed in the as-rolled structure consisting of both the fine lamellar dislocation cell structure and the twin/matrix lamellar structure; whereas, in the specimen annealed at 625oC or 675oC , the partially recrystallized fine grains (d~1µm) with a few dislocations evolve. The volume fraction of recrystallized fine grains increases with increasing of the annealing temperature while the mean diameter of the recrystallized grains is not changed largely. The tensile deformation behaviors were measured at various strain rates ranging from 10-3sec-1 to 102sec-1. The strength and elongation become smaller and larger, respectively, with increasing the fraction of the recrystallized microstructure. The activation volume of dislocations becomes larger with increasing the fraction of recrystallized microstructure.