Effect of tensile pre-strain on bending and unloading of automotive steels

In recent years, advanced high strength steels (AHSS) have been used in a wide range of automotive applications; they may have property variations through the thickness and the properties may also be dependent of prior processing including pre-straining. In order to model forming processes precisely using, for example, finite element analysis, it is important that material input data should adequately reflect these effects. It is known that shape defects in roll forming are related to small strains in material that has undergone prior deformation in a different strain path. Much research has already been performed on the change in the Young’s Modulus once a steel sheet has been plastically deformed,however many of these tests have only been conducted using tensile testing, and therefore may not take into account differences in compressive and tensile unloading. This research investigates the effect of tensile pre-straining on bending behaviour for various types of material;in bending, one half of the sheet will load and unload in compression and hence experience deformation under a reversed stress. Four different materials were pre-strained in tension with 1%, 3%, 7%, 11% and 25% elongation. Using a free bending test, moment curvature diagrams were obtained for bending and unloading. The results showed that the characteristics of the moment curvature diagram depended on the degree of pre-straining; more highly strained samples showed an earlier elastic-plastic transformation and a decreased Young's Modulus during unloading. This was compared to previous literature results using only tensile tests. Our results could influence the modeling of springback in low tension sheet operations, such as roll forming.

Microstructural analysis of pre-strained TRIP steel after fatigue testing

The widespread introduction of multiphase sheet steels in the automotive industry has led to considerable interest in the fatigue properties of these materials. The different microstructural phases within matelials such as TRIP steels can influence the fatigue behaviour due to the manner in which the cyclic strain is accommodated within these phases. In this study fully reversed straincontrolled fatigue tests were perfonnrmed on a commercially-produced uncoated TRIP 780 steel both in the as-received and 20 % prestrained condition. The pre-strained TRIP steel showed significant cyclic softening at higher strain amplitudes, whereas some initial work hardening was observed at lower strain amplitudes before cyclic softening. The cyclic stabilised strength of the pre-strained TRIP steel was independent of strain amplitude, while the cyclic stabilised strength of the as-received TRIP steel increased with strain amplitude. Transmission Electron Microscopy TEM was used to examine the effect of the cyclic deformation on the microstructure of the different conditions, with the differences in fatigue behaviour explained based on the differences in the deformation structure formed within the steel (i.e. dislocation density and sub-structure and microband formation).

On the low temperature strain aging of bainite in the TRIP steel

The aging behavior of a thermomechanically processed Mo-Al-Nb transformation-induced plasticity steel with ultrafine microstructure was investigated using transmission electron microscopy and atom probe tomography (APT). Strain aging at 73 K (200 °C) for 1800 seconds led to a significant bake-hardening response (up to 222 MPa). Moreover, aging for 1800 seconds at room temperature after 4 pct pre-strain also revealed a bake-hardening response (~60 MPa). The experimental results showed the formation of carbon Cottrell atmospheres around dislocations and the formation of carbon clusters/fine carbides in the bainitic ferrite during aging. It is proposed that this is associated with the high dislocation density of bainitic ferrite with formation of a complex dislocation substructure after pre-straining and its high average carbon content (~0.35 at. pct). The segregation of carbon and substitutional elements such as Mn and Mo to the retained austenite/bainitic ferrite interface during aging was observed by APT. This segregation is likely to be the preliminary stage for Mo-C particles’ formation. The aging after pre-straining also induced the decomposition of retained austenite with formation of ferrite and carbides.

On the Bauschinger effect in dual phase steel at high levels of strain

The effect of volume fraction and hardness of martensite on the Bauschinger effect in Dual Phase (DP) steel was investigated for strain levels close to those observed in automotive stamping. Five different grades of DP steel were produced by controlled heat treatment allowing the examination of the Bauschinger effect for three different volume fractions of martensite and three levels of martensite hardness. Compression-tension and shear reversal tests were performed to examine the Bauschinger effect at high levels of forming strain. Good correlation between the shear reversal and the compression-tension test was observed suggesting that for DP steel, shear stress strain data, converted to equivalent stress-strain, may be applied directly to characterize kinematic hardening behavior for numerical simulations. Permanent softening was observed following strain reversal and increased with martensite volume fraction and pre-strain level. While the Bauschinger ratio saturates at 3% pre-strain, the Bauschinger strain increases linearly with forming strain without showing saturation. This suggests that to model material behavior accurately in forming processes involving complex loading paths and high levels of strain, test data generated at high strain is required.

Effect of initial processing on the microstructure-property relationships in bake-hardened C-Mn-Si TRIP steels

The effect of pre-straining (PS) and bake-hardening (BH) on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) steels after: (i) thermomechanically processing (TMP) and (ii) intercritical annealing. The steels were characterised before and after PS/BH by transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile tests. The main microstructural differences were the higher volume fraction of bainite and more stable retained austenite in the TMP steel. This led to a difference in the strain-hardening behavior before and after BH treatment. The higher dislocation density in ferrite and formation of microbands in the TMP steel after PS and the formation of Fe3C carbides between the bainitic ferrite laths during BH for both steels also affected the strain-hardening behavior. However, both steels after PS/BH treatment demonstrated an increase in the yield and tensile strength.

Design of a new biocompatible ti-based shape memory alloy and its superelastic deformation behaviour

Titanium-nickel (Ti-Ni) shape memory alloys have been widely used for biomedical applications in recent years. However, it is reported that Ni is allergic and possibly carcinogenic for the human body. Therefore, it is desirable to develop new Ni-free Ti-based shape memory alloys for biomedical applications. In the present study, a new Ti-18Nb-5Mo-5Sn (wt.%) alloy, containing only biocompatible alloying elements, was designed with the aid of molecular orbital method and produced by vacuum arc melting. Both β and α″ martensitic phases were found to coexist in the alloy after ice-water quenching, indicating the martensitic transformation. The phase transformation temperatures of the Ti-18Nb-5Mo-5Sn alloy were M_s = 7.3 °C, M_f = −31.0 °C, A_s = 9.9 °C, and A_f = 54.8 °C. Superelasticity was observed in the alloy at a temperature higher than the A_f temperature. A totally recovered strain of 3.5 % was achieved for the newly designed Ti-based shape memory alloy with a pre-strain of 4 %.

Impact of grain microstructure on the heterogeneity of precipitation strengthening in an Al-Li-Cu alloy

The effect of grain microstructure on the age-hardening behavior is investigated on recrystallized and un-recrystallized Al-Cu-Li alloys by combining electron-backscatter-diffraction and micro-hardness mapping. The spatial heterogeneity of micro-hardness is found to be strongly dependent on the grain microstructure. Controlled experiments are carried out to change the pre-strain before artificial ageing. These experiments lead to an evaluation of the range of local strain induced by pre-stretching as a function of the grain microstructure and results in heterogeneous formation of the hardening T1 precipitates.

The need to revise the current methods to measure and assess static recrystallization behavior

Heterogeneous deformation developed during "static recrystallization (SRX) tests" poses serious questions about the validity of the conventional methods to measure softening fraction. The challenges to measure SRX and verify a proposed kinetic model of SRX are discussed and a least square technique is utilized to quantify the error in a proposed SRX kinetic model. This technique relies on an existing computational-experimental multi-layer formulation to account for the heterogeneity during the post interruption hot torsion deformation. The kinetics of static recrystallization for a type 304 austenitic stainless steel deformed at 900 °C and strain rate of 0.01s^-1 is characterized implementing the formulation. Minimizing the error between the measured and calculated torque-twist data, the parameters of the kinetic model and the flow behavior during the second hit are evaluated and compared with those obtained based on a conventional technique. Typical static recrystallization distributions in the test sample will be presented. It has been found that the major differences between the conventional and the presented technique results are due to the heterogeneous recrystallization in the cylindrical core of the specimen where the material is still partially recrystallized at the onset of the second hit deformation. For the investigated experimental conditions, the core is confined in the first two-thirds of the gauge radius, when the holding time is shorter than 50 s and the maximum pre-strain is about 0.5.

Computational inverse analysis of static recrystallization kinetics

© 2015 Published by Elsevier Ltd. All rights reserved. Accurate static recrystallization (SRX) models are necessary to improve the properties of austenitic steels by thermo-mechanical operations. This relies heavily on a careful and accurate analysis of "the interrupted test data" and conversion of the heterogeneous deformation data to the flow stress. A "computational-experimental inverse method" was presented and implemented here to analyze the SRX test data, which takes into account the heterogeneous softening of the post-interruption test sample. Conventional and "inverse" methods were used to identify the SRX kinetics for a model austenitic steel deformed at 1273 K (with a strain rate of 1 s-1) using the hot torsion test assess the merits of each method. Typical "static recrystallization distribution maps" in the test sample indicated that, at the onset of the second pass deformation with less than a critical holding time and a given pre-strain, a "partially-recrystallized zone" existed in the cylindrical core of the specimen near its center line. For the investigated scenario, the core was confined in the first half of the gauge radius when the holding time and the maximum pre strain were below 29 s and 0.5, respectively. For maximum pre strains smaller than 0.2, the specimen did not fully recrystallize, even at the gauge surface after holding for 50 s. Under such conditions, the conventional methods produced significant error.

Evolution of elastic modulus in roll forming

Roll forming is a continuous process in which a flat strip is incrementally bent to a desired profile. This process is increasingly used in automotive industry to form High Strength Steel (HSS) and Advanced High Strength Steel (AHSS) for structural components. Because of the large variety of applications of roll forming in the industry, Finite Element Analysis (FEA) is increasingly employed for roll forming process design. Formability and springback are two major concerns in the roll forming AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to investigate the effect of a change in elastic modulus during forming on springback in roll forming. FEA has been applied for the roll forming simulation of a V-section using material data determined by experimental loading-unloading tests performed on mild, XF400, and DP780 steel. The results show that the reduction of the elastic modulus with pre-strain significantly influences springback in the roll forming of high strength steel while its effect is less when a softer steel is formed.

Prediction of failure in bending of an aluminium sheet alloy

This work is dedicated to numerical prediction of the bending of thin aluminium alloy sheets, with a focus on the material parameter identification and the prediction of rupture with or without pre-strains in tension prior to bending. The experimental database consists of i) mechanical tests at room temperature, such as tension and simple shear, performed at several orientations to the rolling direction and biaxial tension ii) air bending tests of rectangular samples after (or not) pre-straining in tension. The mechanical model is composed of the Yld2004-18p anisotropic yield criterion (Barlat et al. [3]) associated with a mixed hardening rule. The material parameters (altogether 21) are optimized with an inverse approach, in order to minimize the gap between experimental data and model predictions. Then, the Hosford-Coulomb rupture criterion is used in an uncoupled way, and the parameters are determined from tensile tests, both uniaxial and biaxial, with data up to rupture. In a second step, numerical simulations of the bending tests are performed, either on material in its original state or after pre-straining in tension, with pre-strain magnitudes increasing from 0.19 up to 0.3. The comparisons are performed on different outputs: load evolution, strain field and prediction of the rupture. A very good correlation is obtained over all the tests, in the identification step as well as in the validation one. Moreover, the fracture criterion proves to be successful whatever the amount of pre-strain may be. A convincing representation of the mechanical behavior at room temperature for an aluminium alloy is thus obtained.

Influence of microstructure on strain distribution in Mg–3Al–1Zn

Inspection of pre-polished surfaces of Mg–3Al–1Zn hot-rolled plate following 5% uniaxial compression revealed a distinctive heterogeneous deformation pattern. The pattern differed depending on the face examined. The greater share of the strain was born by regions characterized by grains considerably finer than the average. These regions displayed a favourable alignment for basal slip and were probably formed by shear banding during previous rolling. It is clear that local orientation softening leads to inhomogeneous deformation despite local grain size-hardening and twin activation.

Effect of cold drawing on subsequent surface strain path and ductility during upsetting

Cold drawing is a process that sizes and smooths the surface of steel before it is cold headed to produce bolts. The effect of the changes in the mechanical properties due to cold drawing on the surface strain and ductility during the upsetting process was analysed showing that the stress and strain state can be more readily altered by changes in the process conditions (friction and height-to-diameter ratio) to cause greater increase in the failure strains than can be achieved by pre-drawing.

Yeast Rnt1p is required for cleavage of the pre-ribosomal RNA in the 3' ETS but not the 5' ETS

We have reexamined the role of yeast RNase III (Rnt1p) in ribosome synthesis. Analysis of pre-rRNA processing in a strain carrying a complete deletion of the RNT1 gene demonstrated that the absence of Rnt1p does not block cleavage at site A0 in the 5' external transcribed spacers (ETS), although the early pre-rRNA cleavages at sites A0, A1, and A2 are kinetically delayed. In contrast, cleavage in the 3' ETS is completely inhibited in the absence of Rnt1p, leading to the synthesis of a reduced level of a 3' extended form of the 25S rRNA. The 3' extended forms of the pre-rRNAs are consistent with the major termination at site T2 (+210). We conclude that Rnt1p is required for cleavage in the 3' ETS but not for cleavage at site A0. The sites of in vivo cleavage in the 3' ETS were mapped by primer extension. Two sites of Rnt1p-dependent cleavage were identified that lie on opposite sides of a predicted stem loop structure, at +14 and +49. These are in good agreement with the consensus Rnt1p cleavage site. Processing of the 3' end of the mature 25S rRNA sequence in wild-type cells was found to occur concomitantly with processing of the 5' end of the 5.8S rRNA, supporting previous proposals that processing in ITS1 and the 3' ETS is coupled.