Transformation Induced Plasticity Steel Sheets Research Articles

Analysis of temperature and stress-strain fields during laser beam welding of a TRIP steel

Transformation Induced Plasticity (TRIP) steels belong to the group of Advanced High Strength Steels (AHSS) that are characterized by good strength-strain combination and formability required for special applications in the automotive industry. The excellent combination of strength, ductility and formability of TRIP steels is achieved by a careful control of microstructure development in the process of their production. The microstructure of multiphase TRIP steels typically consists of ferrite, carbide-free bainite and metastable retained austenite, which can be transformed into martensite by plastic deformation. Upon crash, this feature allows the TRIP steels to absorb more energy, ensuring greater passenger safety. In the automotive industry, TRIP steels are mainly joined by resistance spot welding, laser or electron beam welding. Generally, the thermal cycle of a fusion welding process destroys the sophistically designed microstructure of these steels in fusion and heat-affected zones resulting in deterioration of mechanical properties of the weld joint. Negative consequences of the welding process can be eliminated using proper welding parameters. The paper deals with numerical simulation and analysis of the temperature and stress-strain fields developed during the laser beam welding of a CMnSiNb TRIP steel sheets with the thickness of 2 mm. Simulation model takes into account non-linear temperature and phase dependent material properties. The heat input during the laser beam welding is modelled using the conical volumetric heat source. The optimal welding parameters for production of butt joints of CMnSiNb TRIP steel sheets using the TruDisk 4002 disc laser with the maximum power of 2 kW are designed.

Microstructural development and its effects on tensile properties of a high Ni TRIP steel produced by repetitive corrugation and straightening via rolling (RCSR)

Microstructural development and its effect on tensile properties of a Fe- 24Ni-0.3C Transformation Induced Plasticity (TRIP) steel subjected to repetitive corrugation and straightening via rolling (RCSR) process was investigated in this study. Microstructure developed during the application of one to 45 cycles of RCSR on TRIP steel sheets were analyzed by optical microscope and scanning electron microscope (SEM) equipped with electron backscatter diffraction detector and X-ray diffraction techniques. Results indicated that the volume fraction of martensite formed due to the deformation-induced by amplification of applied strain during RCSR process was about 75% accompanied with the formation of a large fraction of low angle boundaries. To study, tensile and hardness tests were carried out on several deformed samples their mechanical properties. The results also showed that after 40 cycles of the RCSR process, yield strength increased from 150 MPa in the starting material to 1157 MPa in the deformed state. As well, the uniform elongation reduced from 170% to 30% and the hardness value increased from 130 to 340 HV. This amount of improvement in tensile strength for this type of alloy by the application of RCSR has not been reported to date. Therefor the technique used in this research for the improvement of tensile properties can be very helpful for an alloy designer when very high tensile properties for a particular application is required.

Effect of bending temperatures on the microstructure and springback of a TRIP steel sheet

Transformation-induced plasticity (TRIP) steel possesses high strength and formability, enabling the use of a thinner gauge material and allowing for the fabrication of complex shapes. In this research, we measured the effect of bending temperatures on the microstructure and air-bending springback angle of TRIP steel at temperatures from 25 to 600 °C. Real-time in situ X-ray diffraction and scanning electron microscopy were used for pre- and postbending analysis. As the prebending temperature increased from 25 °C to 600 °C, the retained austenite (RA) volume fraction decreased, and the RA transformed to bainite at temperatures above 400 °C. The springback angle was positively correlated with the prebending RA volume fraction, with the smallest springback angle achieved at 400 °C. Additionally, the springback angle was positively correlated with the bending angle, because the RA transformation ratio contributed to increased strain hardening. Further microstructure analysis revealed that the RA became elongated in the tension direction as the bending temperatures increased.

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.

Study of Solidification Cracking Susceptibility during Laser Welding in an Advanced High Strength Automotive Steel

Susceptibility to weld solidification cracking in transformation-induced plasticity steel sheets was studied using a modified standard hot cracking test used in the automotive industry. To vary the amount of self-restraint, bead-on-plate laser welding was carried out on a single-sided clamped specimen at increasing distances from the free edge. Solidification cracking was observed when welding was carried out close to the free edge. With increasing amount of restraint, the crack length showed a decreasing trend, and at a certain distance, no cracking was observed. With the aid of a finite element-based model, dynamic thermal and mechanical conditions that prevail along the transverse direction of the mushy zone are used to explain the cracking susceptibility obtained experimentally. The results indicate that the transverse strain close to the fusion boundary can be used as a criterion to predict the cracking behavior. The outcome of the study shows that optimum processing parameters can be used to weld steels closer to the free edge without solidification cracking.

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.

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.

Finite element investigation on the effect of FSSW parameters on the size of welding subdivided zones in TRIP steels

In this study, the effect of friction stir spot welding (FSSW) parameters, including rotational speed, dwell time, and plunge depth on size of the subdivided zones, which are formed by heat and strain imposed by welding process during joining of transformation-induced plasticity (TRIP) steels, was determined using finite element modeling and response surface methodology (RSM). At first, the finite element results were validated with experimental data and then the RSM was applied for developing a mathematical model which can predict the main effects of the above parameters and their impacts on the size of stir zone, thermomechanically affected zone, and heat-affected zone in 1.2-mm-thick TRIP steel sheets using data which obtained by finite element simulations. The analysis of variance was performed to check the adequacy of the developed model. A comparison was also made between the predicted and actual results. Of the three investigated parameters, dwell time was found to have great influence on the size of welding zones followed by tool rotation speed and plunge depth, respectively. Also, it was found that the maximum stir zone (SZ) + thermomechanically affected zone (TMAZ) can be obtained when selecting a proper combination of the rotational speed at 1400–1500 rpm and plunge depth at 1.94–2.06 mm.

Texture developed during deformation of Transformation Induced Plasticity (TRIP) steels

Automotive industry is currently focusing on using advanced high strength steels (AHSS) due to its high strength and formability for closure applications. Transformation Induced Plasticity (TRIP) steel is promising material for this application among other AHSS. The present work is focused on the microstructure development during deformation of TRIP steel sheets. To mimic complex strain path condition during forming of automotive body, Limit Dome Height (LDH) tests were conducted and samples were deformed in servo hydraulic press to find the different strain path. FEM Simulations were done to predict different strain path diagrams and compared with experimental results. There is a significant difference between experimental and simulation results as the existing material models are not applicable for TRIP steels. Micro texture studies were performed on the samples using EBSD and X-RD techniques. It was observed that austenite is transformed to martensite and texture developed during deformation had strong impact on limit strain and strain path.

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.

Study on the Forming Accuracy of TRIP Steel Products in ISF

Incremental Sheet Forming (ISF) is a flexible technology for forming customized productions in small batch, and has received wide attention, especially in the automobile manufacturing industry, where TRIP(Transformation Induced Plasticity) steel is widely used for its great ductility and high strength. But the springback of TRIP steel products is more serious than that of common steel ones, which induce the low dimensional accuracy and block the industrial application. This work would illustrate the experimental research on the dimensional accuracy of TRIP steel products in ISF. To make a reasonable accuracy evaluation, square cone-shaped samples of 1060Al and 08F steel were formed by ISF as the reference. By comparing the outline unconformity and the dimensional deviation, it comes to the conclusion that dimensional accuracy of TRIP steel sheet products is much lower than that of other materials.

Deformation-induced austenite grain rotation and transformation in TRIP-assisted steel

Uniaxial straining experiments were performed on a rolled and annealed Si-alloyed TRIP (transformation-induced plasticity) steel sheet in order to assess the role of its microstructure on the mechanical stability of austenite grains with respect to martensitic transformation. The transformation behavior of individual metastable austenite grains was studied both at the surface and inside the bulk of the material using electron back-scattered diffraction (EBSD) and X-ray diffraction (XRD) by deforming the samples to different strain levels up to about 20%. A comparison of the XRD and EBSD results revealed that the retained austenite grains at the surface have a stronger tendency to transform than the austenite grains in the bulk of the material. The deformation-induced changes of individual austenite grains before and after straining were monitored with EBSD. Three different types of austenite grains can be distinguished that have different transformation behaviors: austenite grains at the grain boundaries between ferrite grains, twinned austenite grains, and embedded austenite grains that are completely surrounded by a single ferrite grain. It was found that twinned austenite grains and the austenite grains present at the grain boundaries between larger ferrite grains typically transform first, i.e. are less stable, in contrast to austenite grains that are completely embedded in a larger ferrite grain. In the latter case, straining leads to rotations of the harder austenite grain within the softer ferrite matrix before the austenite transforms into martensite. The analysis suggests that austenite grain rotation behavior is also a significant factor contributing to enhancement of the ductility.

Effect of retained austenite phase on springback of cold-rolled TRIP steel sheets

Abstract We examined the effects of retained austenite phase on the springback of cold-rolled transformation-induced plasticity (TRIP) steel sheets through a series of V-bending tests. Prior to testing, the intercritically annealed TRIP steel sheets were heat-treated at different austempering temperatures (300, 350 and 400 °C) to obtain three specimens with different retained austenite phase fractions. Results show that the amount of springback in TRIP steel decreased as the fraction of retained austenite phase increased. The amount of springback was reduced by about 30% when the fraction of retained austenite phase increased by 5%. When the area fractions of the retained austenite phase of TRIP steel were less than 3%, we observed micro-cracks in the near-surface points of a V-shaped specimen where compressive and tensile stresses were concentrated. The dependency of material's direction upon springback was very weak in the V-bending test where no constraint such as a blank holder exists during testing.

Thermal Stability of Retained Austenite in TRIP-aided Cold-rolled Steel

In order to study the thermal stability of retained austenite in the cold-rolled transformation induced plasticity(TRIP) steel,thermal decomposition of retained austenite in a cold-rolled TRIP steel sheet with the composition of 0.25C-1.78Mn-1.23Al-0.54Cu-0.33Ni is investigated by means of differential scanning calorimetry(DSC).The decomposition temperature and the activation energy of the retained austenite in TRIP steel are measured by the Kissinger method,and the decomposition temperature is higher than those of quenched steels.The kinetic parameters n and A are obtained by JMA equation,which are used to predict thermal decomposition kinetic of retained austenite in TRIP steel during isothermal transformation.The results show that the retained austenite has a high stability when bainite isothermal transformation temperature is 420 ℃,there is still more than 90% of the retained austenite without decomposing even if the bainite isothermal transformation time is 600 s.The results also show that the amount of retained austenite predicted by JMA equation agrees well with the actual value measured by X ray diffraction(XRD) when bainite isothermal transformation temperature is 420 ℃.

Investigation on the TRIP Steel Sheet Forming Processes with Respect to Punch Speed Loading-Paths

Loading path is one of importance factors that influence the formability of sheet forming process. In this study, the effect of punching speed loading paths (PSLP) on forming and spring-back processes of TRIP (transformation induced plasticity) steel has been investigated. Four kinds of loading paths with three punch speeds are introduced to verify the cup drawing and U-channel spring-back processes based on a constitutive model accompanying the strain-induced martensite transformation. The results show that higher punch speed results in the thickness uniformity of drawing cup and the spring-back angle of stamping U-channel increased with the same loading path. Furthermore, a given loading path (C4) not only increases the minimum thickness of cup but also decreases the spring-back angle of U-channel.

Prediction of martensitic transformation and deformation behavior in the TRIP steel sheet forming

This paper presents a methodology for constitutive model construction of the multiphase steel coupling with TRIP (transformation-induced plasticity) effect and anisotropy characters. The TRIP effect is described by the evolution of phase transformation which is dependent on stress-state and temperature. The phase transformation kinetic model considering the dependence of the transformation rate on the multiaxial stress-state is established by means of simple-shear, uniaxial tension, plain strain and equibiaxial stretching tests at room temperature. It is shown that the stress triaxiality is decisive for the phase transformation rate. The higher the stress triaxiality, the faster the martensitic transformation, i.e. the more unstable the retained austenite is transformed. The proposed constitutive model is implemented into a commercial FE code to obtain a stress–strain curve of TRIP steel and predict the martensitic transformation at different regions of the rectangular deep drawing parts, which corresponds well with the experiment results. The variation of the elastic modulus with the martensitic transformation during stamping is also considered to improve the accuracy of springback simulation for TRIP steel. The results above present a foundation for the formability control technology.

Failure behaviour of laser spot welds of TRIP800 steel sheets under coach–peel loading

This study is carried out to investigate the laser spot welds (LSWs) of advanced high strength steel sheets of 1·0 mm thickness for application to the automotive industry. Mechanical properties and failure behaviour of LSWs for transformation induced plasticity steel sheets (TRIP800) subjected to monotonic coach–peel loading are investigated by experimental and finite element (FE) simulation methods. Microstucture of LSW and fracture surfaces of the welded specimens were examined by scanning electron microscopy (SEM) to describe the microstructural features and to clarify the crack initiation mechanism respectively. Based on simulation solutions equivalent plastic strain was obtained to describe local deformation of LSW joints. Experimental results revealed a 'plug type' of failure mode of the circular LSW joints under 'peel–coach' loading condition. This type of ductile failure is the most common failure mode for spot welds used in the automotive industry. Numerical simulation of damage process was compared with experimental results and this revealed that fracture path was successfully predicted.

Characteristics of the warm deep drawability of a transformation-induced plasticity steel sheet

Warm deep drawability in a square cup drawing was investigated using a newly developed high-strength steel sheet with retained austenite that was transformed into martensite during formation. For this investigation, six different temperatures between room temperature and 250°C, and five different drawing ratios ranging from 2.2 to 2.6 were considered. The results showed that the maximum drawing force and the drawing depth were affected by the change in temperature, and a more stable thickness strain distribution was observed at elevated temperatures. However, blue shortness occurred at over 200°C. FEM analysis using the LS-DYNA code was used to compare the experimental results with the numerical results for the thickness strain distribution.