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昌河铃木利亚纳三厢尾灯的曲面造型与工装设计.rar

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    铃木 利亚纳三厢 尾灯 曲面 造型 工装 设计
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    Supper strong nanostructured TWIP steels for automotive applicationsG Yuana,b ,M Huanga,b,na Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen, China b Department of Mechanical Engineering, The University of Hong Kong, Pok fulam Road, Hong Kong, China Received 26 September 2013; accepted 4 December 2013 Available online 12 March 2014AbstractA ductile and super strong nanostructured twinning-induced plasticity (TWIP) steels were fabricated by cold rolling and recovery treatment. This strong and ductile nanostructured alloy can be used for the anti-intrusion part of body-in-white. Cold rolling was used to produce intensive nano-twins so that the microstructure was greatly refined. Recovery is employed to anneal dislocations for improving the ductility. A physical model is proposed to describe the relationship between the yield stress and the twin density. Furthermore, the present work also found that the activation energy for recovery is 160 J/mol, which implies that the recovery mechanism is governed by dislocation core diffusion. TWIP steel; GND; Recovery1.IntroductionThe high manganese austenitic twinning-induced plasticity (TWIP) steel is well known for their exceptional combination of high strength and large ductility during plastic deformation at ambient temperature. Their outstanding mechanical properties induced by the high strain-hardening rate provide remarkable potential for automotive applications. However, the yield stresses (YS) of coarse-grained TWIP steels with different grades are lower than 400 MPa. This limits the immediate application of TWIP steels in automotive industry, especially in the antiintrusion part of body-in-white . Several approaches have been tried to improve the YS. The methods include reducing grain size down to a few micrometers , the V, Ti, Nb carbides precipitation , cold rolling followed by annealing treatment and partial recrystallization . Among them, the conventional technique, pre-straining and subsequent recovery treatment, has proved to be an effective way to increase the YS while maintain relatively high ductility. Observed by Bouaziz et al. , the mechanical nano-twins induced by cold rolling are stable during recovery treatments. Therefore, the high YS can be obtained by introducing intensive nano-twins while the ductility is improved due to the decrease of dislocation density. In this study, mechanical properties and microstructure evolution of cold rolled TWIP steel during recovery were measured in order to investigate the recovery kinetics of the cold rolled TWIP steel.2. ExperimentsThe TWIP steel with a composition of Fe–22Mn–0.6C (wt %) was chosen in this work. The received hot rolled sheets were rolled at room temperature to 30% reduction in thickness and consequently recovered at 500 1C for 180 s, 1800 s and 3600 s respectively. The tensile tests in the rolling direction of all the samples were performed at room temperature with a strain rate of 2 104 s1 on a hydraulic tensile test machine. The flat specimens had 10 mm gage length, 10 mm gage width and 2.9 mm gage thickness. A Vickers hardness tester was used to carry out the hardness measurement, with a load of 5.0 kg-force (HV5). For each sample, the hardness of at least nine points was measured.Microstructure of the recovered specimens was characterized by electron back-scattered diffraction (EBSD) in a field emission gun scanning electron microscope (SEM). The EBSD test were performed on the normal direction (ND) plane parallel to both the transverse direction (TD) and rolling direction (RD). The EBSD samples were mechanically polished and further electropolished for 30 s at a temperature of 15 1C. The electrolyte solution contains 5 vol% of perchloric acid, 15 vol% of acetic acid and 80 vol% ethanol.Fig. 1. Engineering stress–strain curves of Fe–22Mn–0.6C TWIP steel with different treatmentsTable 1 YS and uniform elongation (UEl) of each sample.3.Results and discussions3.1. Tensile tests and work hardening ratesFig. 1 shows the tensile curves of the Fe–22Mn–0.6C TWIP steel after cold rolling and recovery treatment. The engineering stress–strain curves of as-received fully recrystallized (with average grain size of 20 mm) material and fine-grained (with average grain size of 2.7 mm) sample are also plotted in Fig. 1. The YS and uniform elongation (elongation at necking) for all the samples are listed in Table 1 for comparison.The grain refinement of Fe–22Mn–0.6C TWIP steel from 20 mm down to 2.7 mm provides an increment of 125 MPa to the YS. According to a fitted Hall–Petch equation of Fe– 22Mn–0.6C , the YS could be higher than 600 MPa with submicron grain size. Nevertheless, due to the limitation of manufacturing technique, the minimum grain size achieved in current industrial process is around 2.5 mm for the Fe–22Mn– 0.6C TWIP steel. The 30% of thickness reduction by cold rolling leads to the YS of 1436 MPa combined with little ductility. After heated at 500 1C for only 180 s, the cold rolled TWIP steel exhibits obvious increase of uniform elongation (from 2.8% to 13.7%) with YS of 1330 MPa. The ductility shows further increase with extension of recovery treatment duration. This indicates that the metallurgical process, cold rolling follow by recovery treatment, is an efficient method to acquire excellent combination of high YS and ductility.Fig. 2 presents the work hardening rate as a function of true stress. The work hardening rate of 30% cold rolled sample decreases rapidly, while the recovery-treated samples demonstrate a clearly better combination of stress and work hardening rate.3.2. Evolution of microstructure and GND densityFig. 3, the EBSD Kikuchi pattern quality image, shows the microstructural evolution of 30% cold rolled Fe–22Mn–0.6C TWIP steel with recovery duration. No obvious decrease in density of deformation twins was observed. This suggests that the decrease of YS from 1436 MPa (as rolled specimen) to 1195 MPa (cold rolled specimen with annealed at 500 1C for 3600 s) is attributed to the annihilation of dislocations due to recovery treatment. In order to investigate the dislocation density evolution during annealing, the density of geometrically necessary dislocations (GND) was introduced in this study. The transmission electron microscopy (TEM) approach is an accurate method to determine the dislocation density owing to its high resolution to distinguish dislocations. Nevertheless, to measure the average dislocation density by TEM is time consuming and the detected area is small. Obtaining GND density from EBSD has been proved to be an effective way to estimate the dislocation density over a relatively large sample area . Fig. 4 shows the GND density distributions of the cold rolled and annealed specimens. The calculated GND densities and corresponding YS are listed in Table 2. It is obvious that the GND density decreases with increasing recovery duration.The GND density of as rolled specimen is , which 21509.mhas the same order of magnitude as other TWIP steel (Fe–31Mn–3Al–3Si steel with average grain size of 18 mm, at 0.4 true strain, the dislocation density was measured to be about . 2-157.This suggests that the method of calculating GND density from EBSD is feasible for TWIP steel.3.3. Modeling of the yield stressAs the dislocation density does not increase during elastic deformation, the YS sy can be expressed as.where is due to solid solution, is a constant, M is the Taylor factor, tybM00is the shear modulus, and is the dislocation density of the specimen before tensile loading. In tpresent work, was chosen to represent . According to Eq. , if the value of is GNDtGNDdecreased by 10%, the reduction of value should be less than 10%. However, the data listed in yTable 2 reveals an opposite trend. For instance, a comparison of and YS of as rolled specimen GNDand 500 ℃–3600 s annealed specimen shows that decreases by 4.9%, while the reduction of GNDYS is 16.8%. This reverse trend indicates that some other form of stress also contributes to the YS。Fig. 2. Work hardening rates as a function of true stresses. The straight line represents the Considère0 s criterion for necking.Fig. 3. The EBSD Kikuchi pattern quality images of the Fe–22Mn–0.6C TWIP steel 30% cold rolled (a) and subsequently recovered at 500 1C for 180 s (b), 1800 s (c) and 3600 s (d).Fig. 4. GND density distribution maps of the Fe–22Mn–0.6C TWIP steel 30% cold rolled (a) and subsequently recovered at 500 1C for 180 s (b), 1800 s (c) and 3600 s (d).Table 2 GND densities with corresponding YS.It is observed that the back-stress in Fe–22Mn–0.6C TWIP steels, arises from dislocations stopped at twin and grain boundaries, causes strong Bauschinger effect . The cold rolling introduces intensive nano-twins so that there will be dislocations piled up at the deformation twin boundaries. Therefore the back-stress sb is included in Eq. asandbGNDyM0Fedbnb12where n is the number of dislocations which are stopped at boundaries, d is the average grain size, e and F are twin mean thickness and twin volume fraction, respectively. The modeling results are listed in Table 3. The good agreement between model predictions and experimental results implies that the back-stress contributes to the YS and the recovery treatment will remove dislocations both in the bulk and at the boundaries (twin and grain). The parameters used in present model are summarized in Table 4. Except for n and (shown in Table 2). It is worth noting that the same e (mean GNDthickness of twins) and F (volume fraction of twins) are used for all the predictions, which indicate that recovery at 500 1C for 3600 s will only remove dislocations but not the twins. This consists with the EBSD observations.3.4. Recovery mechanismFig. 5 shows the decrease of the Vickers hardness (HV5) with increasing annealing duration. During recovery treatment, the dislocation density is evolving with annealing time as GNDtm1lnTable 3 Modeling resultsTable 4 Parameters used in the modelFig. 5. Vickers hardness (HV5) vs time of recovery treated at 450 and 500 1C.For the 30% cold rolled Fe–22Mn–0.6C TWIP steel, (3600 S, 723 K)and (600 S, 773 K) treatments produce the same hardness, then according to Eq. , the activation energy of recovery process is . molkjeVQrec/1673.The recovery is believed to be governed by dislocation diffusion. Therefore the underlying mechanism can be determined by comparing recovery activation energy obtained from experiment and the activation energies of self and core diffusion. Owing to the lack of activation energy data of the present alloy, the present study utilizes the activation energy data of γ–Fe . It is noted that the experimental derived recovery activation energy is much smaller than the activation energy of self diffusion in γ– Fe ( ), but very close to the activation energy of core diffusion molkjeV/2708.in γ –Fe ( ). Consequently, it is suggested that the recovery process of 30% lj/1596.cold rolled Fe–22Mn–0.6C is controlled by diffusion from the dislocation cores.4.ConclusionsThe mechanical properties and recovery kinetics of the 30% cold rolled Fe–22Mn–0.6C TWIP steel was experimentally characterized. The main conclusions of this work are drawn as follows:(1) An excellent combination of high yield stress and uniform elongation can be realized by cold rolling with subsequent recovery for a suitable duration. (2) For the investigated cold rolled material, the recovery treatment at a temperature of 500 1C for 3600 s will increase the ductility by reducing dislocation density. At the meantime, the deformation twins which were induced by cold rolling remain and contribute to the high yield stress.(3) A model was proposed to describe the yield stress by taking back-stress into account. The good predictions suggest that recovery will eliminate dislocations both in the bulk and at the twin boundaries and grain boundaries.(4)The recovery activation energy of the 30% cold rolled Fe– 22Mn–0.6C TWIP steel was experimentally determined to be , close to the dislocation core molkjevOrc/1673.diffusion activation energy of γ–Fe. This implies that the recovery of the investigated TWIP steel is controlled by diffusion from dislocation cores.Acknowledgments This work was partially supported by a NSFC Grant (51301148) and a Grant from the Research Grants Council of the Hong Kong Special Administration Region, China (HKU 719712E)References[1] C. Scott, S. Allain, M. Faral, N. Guelton, Revue de Metallurgie, Cah. Inf. Tech. 103 (2006) 293–302.[2] O. Grässel, L. Krüger, G. Frommeyer, L.W. Meyer, Int. J. Plasticity 16 (2000) 1391–1409.[3] R. Xiong, R. Fu, Y. Su, Q. Li, X. Wei, L. Li, J. Iron Steel Res. Int. 16 (2009) 81. [4] O. Bouaziz, N. Guelton, Mater. Sci. 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