Papers by Keyword: Bake-Hardening

Abstract: The influence of pre-straining and bake-hardening on the mechanical properties of thermomechanically processed 0.2C-1.5Si-1.5Mn-0.2Mo-0.004Nb (wt%) steel was analysed using tensile test, transmission electron microscopy (TEM) and atom probe tomography (APT). This steel after processing had high strength (~1200MPa) and good ductility (~20%) due to the formation of fully bainitic microstructure with nanolayers of bainitic ferrite and retained austenite. The bake hardening (BH) of pre-strained (PS) samples increased the yield strength of steel up to 690MPa and showed the bake-hardening response of 220MPa due to the operation of several strengthening mechanisms such as transformation induced plasticity during pre-straining and pinning the dislocations by carbon during bake-hardening treatment. The carbon content of the bainitic ferrite and retained austenite before and after bake-hardening treatment, the solute distribution between these phases and the local composition of fine Fe-C clusters and particles formed during bake-hardening treatment was calculated using APT. The bainitic ferrite and retained austenite microstructural characteristics such as thickness of the layers and their dislocation density before and after bake-hardening treatment were studied using TEM.

Abstract: 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.

Abstract: Arcelor produces « Bake-Hardening » steels for automotive outer panels, which present the advantages of a remarkable drawability combined with a significant hardening after stamping and paint baking by the car maker. This hardening enables to increase the dent resistance of those automotive parts. In order to give easy design criterion and support the development for new “bake-hardening” steels, a physically-based model for Bake-Hardening steels has been developed. It is suitable to predict: -the physical phenomenon of strain ageing based on Cottrell atmospheres formation. A detailed description of the strain ageing kinetics is given based on a generalized form of the Harper model taking into account the diffusion of carbon atoms in the stress field of a dislocation, progressive carbon depletion in the matrix and saturation of the available dislocation sites. -the plastic instabilities propagation during tensile testing according to Piobert–Lüders phenomenon using the finite element method. A local mechanical behaviour is introduced whose shape schematically describes the local dislocation behaviour. The effect of the grain size on the velocity of the Lüders’ band front is especially enlightened. -the effect of ageing process on dent resistance. To do so, a physical extension of the former approach to more complex loading paths is proposed.

Abstract: Heat-treatable Al-Mg-Si aluminum alloys for automotive body panel applications are shipped and formed in T4 temper while still formable and are subsequently given a bake cycle to increase strength by age hardening while the paint is cured. However, the hardening response during the rather short industrial paint-bake is impaired upon natural ageing after the solution treatment. It is thus essential to counteract the adverse effect of natural ageing on bake hardening. While this is often accomplished by an additional ageing cycle before the paint-bake treatment, reversion treatments ought to be used once substantial natural ageing has first taken place to improve the bake hardening response of a twin-roll cast 6016 sheet. The present work was undertaken to improve the paint bake response of a twin-roll cast 6016 sheet by employing reversion treatments before the paint-bake cycle. Reversion treatments between 200°C and 250°C was effective in improving the bake hardening response of the twin-roll cast 6016 sheet which, when processed without reversion, failed to meet in-service strength requirements with a rather poor bake hardening response of 35 MPa.

Abstract: In this study, asymmetric warm rolling (AWR) has been applied to improve the formability of bake-hardenable Al-3mass%Mg-1mass%Cu-(Ag) alloy sheets. The T4 sheets by the AWR are superior to those by conventional rolling (CR) in Lankford value and drawability. This improvement is caused by a change in the recrystallization texture to have a predominant orientation close to {111}//ND. The unusual texture for recrystallized aluminum alloy sheets is due to the shear deformation introduced during AWR. The Al-Mg-Cu alloy sheets by AWR have been confirmed to possess combined bake-hardenability and the improved formability. However, the bake-hardening response of the Al-Mg-Cu alloy by AWR is slightly inferior to that of the identical alloy by CR.

Abstract: In this work, the bake-hardening (BH) response of an Al-3.0Mg-1.0Cu (in mass%) alloy has been improved by the small addition of Ag as a good example of our proposed Nanocluster Assist Processing (NCAP) technique. From the detailed observation through high resolution transmission electron microscopy (HRTEM), it is found that the origin of the increased hardness in the Ag-added alloy is attributed to the densely and uniformly formed Z phase at the expense of Guinier-Preston- Bagaryatsky (GPB) zones and the S’ phase. It is new findings that the Z phase is formed even in the ternary alloy although the chemical composition lies in the (α+S+T) phase field. Based on the threedimensional atom probe (3DAP) results, furthermore, it is suggested that nanoclusters of Mg, Ag and/or Cu provide effective nucleation sites for the Z phase, whereas nanoclusters of Mg and Cu do less. Such unique characteristics of Ag are clearly seen in the newly constructed interaction energy map (IE map).

Abstract: The behavior of bake-hardening of the laser weldment was investigated. The bake-hardening steel(BH steel) was welded with Nd:YAG laser followed by plastic deformation and subsequent heat-treatment. Then the influence of laser welding on the behavior of bake-hardening was investigated. The hardness of the laser weld metal significantly increased after welding. After the plastic deformation, both the base metal and weld metal became harder by work-hardening. The heat treatment resulted in more increment of hardness in both the base metal and weld metal by bake-hardening. The amount of bake-hardening reached a maximum value at the plastic strain of 5% or more. We modified a kinetic equation proposed for predicting the strength of a low-carbon bake-hardening steel and applied to the estimation of hardness of the base metal and weld metal. The calculated hardness values agree with the experimental data. The calculated activation energy for bake-hardenig was that for diffusion of carbon and nitrogen atoms in α-Fe. Thus the hardening is thought to be governed by diffusion of these solute atoms.