metals
Article Influence of Continuous Annealing Temperature on Mechanical Properties and Texture of Battery Shell Steel
Beijia Ning 1,2, Zhengzhi Zhao 1,2,* , Zhiying Mo 1,2, Hong Wu 3, Chong Peng 3 and Honggen Gong 3 1 Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (B.N.); [email protected] (Z.M.) 2 Beijing Laboratory for Modern Transportation Advanced Metal Materials and Processing Technology, University of Science and Technology Beijing, Beijing 100083, China 3 Xinyu Iron Steel Group Co., Ltd., Xinyu 338001, China; [email protected] (H.W.); [email protected] (C.P.); [email protected] (H.G.) * Correspondence: [email protected]; Tel.: +86-10-6233-2617
Received: 25 November 2019; Accepted: 11 December 2019; Published: 27 December 2019
Abstract: To study the influences of continuous annealing temperature on microstructure, mechanical properties and textures of battery shell steel, continuous annealing experiments were conducted at 710 ◦C, 730 ◦C, 760 ◦C and 780 ◦C respectively. The mechanical properties and normal anisotropy index (r) were measured by tensile test and the textures were investigated using the method of electron backscatter diffraction (EBSD). The results show that as annealing temperature rose, the grain size, fracture elongation and r value increased, whereas the strength and yield ratio decreased. The yield strength was 122 MPa, the tensile strength was 286 MPa, meanwhile the elongation and r value arrived at 38.8% and 2.3 when the annealing temperature rose to 780 ◦C. After annealing, the main texture in battery shell steel is {111} <112>, followed by {111} <110>. With the increase of annealing temperature, textures in {001} crystallographic plane weakened while textures in {111} plane strengthened, which is beneficial to the deep drawability of the steel.
Keywords: battery shell steel; annealing temperature; mechanical properties; crystallographic texture
1. Introduction In recent years, as the sustainable development concept of energy-saving and environment protection gets popular, new energy vehicles have been developed rapidly. The battery shell steel used to produce the shell of battery pack, which is the core component of new energy automobiles, has received extensive attention at the same time [1]. As a kind of precise cold-rolled steel sheet, the battery shell steel has high technology content and high-quality requirements. The thickness of sheet used commonly is 0.25–0.50 mm, and the surface quality is required to reach “05” level in both sides [2,3], thus bringing great difficulties to production. During the process of forming, the steel needs to go through deep drawing and ironing for more than one time. All processes go on automatically with high speed but many problems exist in them, such as blocking in the mold, fracture, earing, etc. [4]. These problems have a direct connection with deep drawability of the steel, which is influenced by many factors including strengthening mechanisms, grain size, morphology and orientation. All these micro factors are affected by the annealing process directly and significantly. Strain hardening and internal stress occur in cold rolling, making the steel become hard and brittle. To recover the ductility and increase the formability, annealing treatment is required. Normal anisotropy index (r) is an important factor for optimizing deep drawability. The value of r can be
Metals 2020, 10, 52; doi:10.3390/met10010052 www.mdpi.com/journal/metals Metals 2020, 10, 52 2 of 9 increased with a strong γ fiber (ND//<111>), and weak α (RD//<110>) and θ (ND//<100>) fibers [5,6]. Though a certain texture may have benefits on deep drawability for one particular deformation state, it may result in poor properties for another. For battery shell steel under the ideal plain-strain condition, a better deep drawability can be obtained when there are strong {111} textures. By optimizing the parameters of the annealing process, the density of {111} texture and r value can be increased, thus improving the drawability [7–9]. For battery shell steel, a lot of studies on surface defects and experiments on formability characterization have been done. However, there is not enough attention put on the annealing process, which can optimize the formability and avoid the defects primarily as mentioned. In the present study, effects of annealing temperature on microstructures, mechanical properties, component of textures were investigated based on optical microscope (OM), electron backscatter diffraction (EBSD) and tensile test. Moreover, the anisotropy of the annealed steel was paid attention to, especially the relationship between r values and typical textures. Combining all the results, the suitable annealing temperature is aimed to be put out to obtain stronger favorable textures in order to achieve a higher r value and better drawability of the annealed sheet. Since the property of battery shell steel is improved, larger competitiveness of some steel mills can be realized and more contribution can be made to the development of new energy automobiles.
2. Materials and Methods The material used for the experiment is a kind of cold-rolled steel sheet with a thickness of 0.3 mm from a commercial steel company. The grade of the experimental steel is XDCK and the chemical composition is shown in Table1.
Table 1. Chemical composition of the tested steel (mass fraction, %).
C Si Mn S P Alt Ti N 0.010 0.300 0.100–0.500 0.010 0.015 0.010–0.070 0.010–0.050 0.003 ≤ ≤ ≤ ≤ ≤
Specimens with the dimension of 70 mm 220 mm were cut from the cold-rolled sheet and × then continuous annealing tests were carried out using CCT-AY-II (made by ULVAC, Tokyo, Japan). In different annealing tests, specimens were heated to the soaking temperatures of 710 ◦C, 730 ◦C, 760 ◦C and 780 ◦C respectively with a hold of 80 s. After a short slow cooling, they were continuously cooled rapidly to about 400 ◦C for over aging treatment and finally cooled to room temperature. For tensile tests, samples with a gauge length of 50 mm and a gauge width of 12.5 mm according to standard GB/T 228.1-2010 were cut along the directions of 0◦, 45◦ and 90◦ to rolling direction respectively and directly from each annealed sheet. Then tensile tests were conducted at room temperature with a constant speed (2 mm/min). The r values in each direction were measured in accordance with standard GB/T 5027-2016 and the ∆r value was calculated by:
∆r = (r 2r + r )/2, (1) 0 − 45 90 where r0, r45 and r90 represent the r values in three directions (0◦, 45◦ and 90◦). For the same direction on the same annealed sheet, three samples were tested and the results were averaged. The microstructures of the cold rolled sheet and annealed samples were observed by OLS4100 (made by OLYMPUS, Tokyo, Japan). The samples for EBSD were cut perpendicular to the rolling direction and were electrolytically polished with a solution containing 10 vol% perchloric and 90 vol% ethanol. The polishing process was carried out using a DC power supply for 10–20 s at 15 V and 1 A. Metals 2020, 10, 52 3 of 9
3. Results and Discussion
3.1. Microstructure of the Annealed Sheet Metals 2020, 10, x FOR PEER REVIEW 3 of 9 It can be seen in Figure1a that the cold rolling morphology of the tested material is a kind of fibrous differentstructure temperatures elongated along are rolling shown direction. in Figure The 1b microstructures–e. In all four samples, of the samples only ferrite annealed phase at differentexisted. temperatures are shown in Figure1b–e. In all four samples, only ferrite phase existed. Compared ComparedMetals 2020 with, 10, xthe FOR microstructure PEER REVIEW of cold rolled sheet, recrystallization happened at all annealing3 of 9 temperatures.with the microstructure Besides, the of grains cold rolled were sheet, still elongated recrystallization along the happened rolling direction at all annealing to a certain temperatures. extent as theBesides, shapedifferent the of grainspie. temperatures With were the still riseare elongated ofshown the annealing in alongFigure the 1btemperature, rolling–e. In all direction four the samples, recrystallization to a certain only ferrite extent was phase as more the existed. shape comp of lete, pie. andWith theCompared the proportion rise of with the annealing theof equiaxedmicrostructure temperature, grains of wascold the rollefurther recrystallizationd sheet, increased. recrystallization wasUsing more the happened complete, EBSD method at and all annealing the and proportion Oxford software,of equiaxedtemperatures. hundreds grains Besides, was of grains’ further the grains sizes increased. were were still Usingcounted elongated the and EBSD along then methodthe therolling grain and direction Oxfordsize distribution to software,a certain extent hundredsat different as of annealinggrains’thesizes shape temperatures were of pie. counted With canthe and risebe thenobtained of the the annealing grain (Figure size temperature, 2). distribution When the the atannealingrecrystallization different temperature annealing was more temperatures was comp 710lete, °C canthe and the proportion of equiaxed grains was further increased. Using the EBSD method and Oxford sizebe obtained of grains (Figure with the2). Whenlargest the proportion annealing wa temperatures around 5.6 was μm 710. As ◦theC the temperature size of grains rose with to 73 the0 °C largest and software, hundreds of grains’ sizes were counted and then the grain size distribution at different proportion was around 5.6 µm. As the temperature rose to 730 ◦C and 760 ◦C the proportions of the 760 °Cannealing the proportions temperatures of the can grainsbe obtained having (Figure a diameter 2). When of the 7.9 annealing μm and temperature 11.2 μm increased. was 710 °C At the 780 °C grains having a diameter of 7.9 µm and 11.2 µm increased. At 780 C more grains reached the size of moresize grains of grain reacheds with the largestsize of proportion 11.2 μm waands around 15.9 μm 5.6 .μm The. As average the◦ temperature grain size rose can to 73 also0 °C beand given 11.2 µm and 15.9 µm. The average grain size can also be given according to the results of EBSD, which is according760 °C to the the proportions results of of EBSD, the grains which having is marked a diameter particularly of 7.9 μm in and Figure 11.2 μm 2. Combining increased. At the 780 analys °C es ofmarked Figuremore particularlys grains 2 and reached 3, inwe Figure thecould size2. Combining sayof 11.2 that μm the the and higher analyses 15.9 μmthe of. Theannealing Figures average2 and temperaturegrain3, we size could can sayis, also the that be greater thegiven higher the crystallizationthe annealingaccording temperaturetodegree the results is, and of is, EBSD,the the larger greater which the theis grainmarked crystallization size particularly is. degree in Figure is, and 2. Combining the larger thethe grainanalys sizees is. of Figures 2 and 3, we could say that the higher the annealing temperature is, the greater the crystallization degree is, and the larger the grain size is.
FigureFigureFigure 1. 1. MicrostructuresMicrostructures 1. Microstructures of of ofteste tested tested dsteel steel. steel. .( (a(aa)) C C Coldoldold rolled rolled sample sample sample and and and (b (– (beb)–– annealedee)) annealed annealed samples samples samples at 710 at at °C, 710 710 °C,◦C, 730 °C, 760 °C and 780 °C respectively. 730730 °C◦C,, 760 760 °C◦C and and 780 780 °C◦C respectively. respectively.
Figure 2. Cont.
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Figure 2. Grain size distribution at different annealing temperatures. (a) 710 °C, (b) 730 °C, (c) 760 °C FigureFigure 2. Grain 2. Grain size size distribution distribution at at di differentfferent annealingannealing temperatures. temperatures. (a) ( a710) 710 °C,◦ (C,b) (730b) 730°C, (◦cC,) 760 (c )°C 760 ◦C and (d) 780 °C. and (andd) 780 (d) ◦780C. °C.
Figure 3. Engineering stress–strain curves at different annealing temperatures perpendicular to the rolling direction. Figure 3. Engineering stress stress–strain–strain curves at different different annealing temperatures pe perpendicularrpendicular to the rolling3.2. Mechanical direction direction. Properties.
3.2. Mechanical MechanicalFigure Properties Properties3 shows the engineering stress–strain curve of samples annealed at different temperatures obtained from one of the three repetitive tensile tests. Figure 4a shows the effect of Figureannealing3 shows3 temperatureshows the engineeringthe on engineeringyield strength, stress–strain stress tensile– curve strainstrength, ofcurve samples yield ratioof annealedsamples and fracture at annealed di elongation.fferent temperaturesat Withdifferent temperaturesobtainedthe increase from obtained one of of annealing the from three repetitiveonetemperature, of the tensile three the tests. yieldrepetitive Figure strength, tensile4a shows tensile tests. the strength eFffigureect ofand 4a annealing showsthe yield the temperature ratio effect of annealingon yielddecreased strength, temperature generally, tensile on while strength, yield the strength, yieldfracture ratio tensileelongation and strength, fracture showed elongation.yi eldan upwardratio Withand trend. fracture the As increase the elongation. annealing of annealing With thetemperature, increasetemperature of the roseannealing yield from strength, 730 temperature, to 780 tensile °C, the strength the yield yield strength and strength, the decreased yield tensile ratio from decreased strength140 to 12 2 and generally,MPa, the the tensileyield while ratio the strength decreased from 300 to 286 MPa, and the elongation increased from 29.7% to 38.8%. When decreasedfracture elongation generally, showed while anthe upward fracture trend. elongation As the annealingshowed an temperature upward trend. rose fromAs the 730 annealing to 780 ◦C, annealing at lower temperature, the incomplete recrystallization degree and the residual stress of temperaturethe yield strength rose from decreased 730 to from780 °C 140, the to yield 122 MPa, strength the decreased tensile strength from 14 decreased0 to 122 M fromPa, the 300 tensile to 286 MPa,cold and rolling the elongation lead to high increased yield strength. from 29.7% When to 38.8%.the temperature When annealing rises, the atdriving lower temperature,force of strengthrecrystallization decreased fromincreases, 300 tothus 28 6causing MPa, andthe developmentthe elongation of grainincreased size. Thefrom grain 29.7% boundary to 38.8%. area When the incomplete recrystallization degree and the residual stress of cold rolling lead to high yield annealingreduces at due lower to grain temperature, coarsening the and incomplete the residual stressrecrystallization eliminates due degree to temperature and the increasement.residual stress of strength. When the temperature rises, the driving force of recrystallization increases, thus causing the cold Allrolling these factorslead tohave high something yield tostrength. do with the When decrease the of yieldtemperature and tensile rises, strength the and driving the increase force of recrystallizationdevelopmentof elongation. of grainincreases, However, size. Thethuswhen grain causingthe temperature boundary the development area reached reduces 780 of due°C grainthe to recrystallization grain size. coarsening The grain was andboundary gradually the residual area reducesstresscompleted, eliminates due to grainand due the coarsening to influence temperature of and residual the increasement. residual stress in coldstress All rolling eli theseminates was factors gradually due have to temperatureeliminated. something At to increasement.this do time, with the Alldecrease thesethe influence offactors yield have of and annealing tensilesomething strengthtemperature to do and with on the strengththe increase decrease was of weakened. of elongation. yield and However,tensile strength when and the temperaturethe increase ofreached elongation. 780Figure◦C However, 4b the reflects recrystallization thewhen effect the of temperatureannealing was gradually temperature reached completed, on780 the °C andvalues the the recrystallization of influence normal anisotropy of residual was ( rgradually) and stress in completed,cold rollingplanar and wasanisotropy the gradually influence coefficient eliminated. of residual (Δr). AtAs stress thisthe time,annealing in cold the rolling influence temperature was of gradually annealing rose, r90 eliminated. temperatureincreased, which At on this strength is time, beneficial to deep drawability of the steel. However, the Δr value also showed an increasing trend thewas influence weakened. of annealing temperature on strength was weakened. Figure 4b reflects the effect of annealing temperature on the values of normal anisotropy (r) and planar anisotropy coefficient (Δr). As the annealing temperature rose, r90 increased, which is beneficial to deep drawability of the steel. However, the Δr value also showed an increasing trend
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except for a minor decrease at 780 °C. This indicates that the difference among the three directions Metals 2020, 10, 52 5 of 9 gets larger with the increase of annealing temperature.
Figure 4.4. EEffectsffects of of annealing annealing temperature temperature on on mechanical mechanical properties. properties. (a) Strength,(a) Strength, elongation elongation and yield and ratio perpendicular to the rolling direction and (b) r and ∆r. yield ratio perpendicular to the rolling direction and90 (b) r90 and Δr. Figure4b reflects the e ffect of annealing temperature on the values of normal anisotropy (r) and 3.3. Texture Analysis by EBSD planar anisotropy coefficient (∆r). As the annealing temperature rose, r90 increased, which is beneficial to deepThe drawability texture results of the of steel.annealed However, specimens the ∆ arer value illustrated also showed in Figure an 5 increasing in the form trend of orientation except for adistribution minor decrease function at 780 (ODF)◦C. Thissections indicates with φ that2 = 45°. the diInff allerence cases, among the textures the three were directions mainly made gets larger up of withfine γ the fibers increase with of<111> annealing parallel temperature. to the normal direction. To make the results more visualized, density distributions of the textures on α(a) and γ(b) orientation lines are presented in Figure 6. It can be seen 3.3.in Figure Texture 6a Analysis that the bystrongest EBSD components belonging to α fibers were all close to {112} <110> and {111} <110>The at texturedifferent results annealing of annealed temperatur specimenses. Besides are illustrated the overall in strength Figure5 inof theα fibers form ofin orientationthe sample annealed at 710 °C was high. This is because the driving force of recrystallization is weak and part of distribution function (ODF) sections with ϕ2 = 45◦. In all cases, the textures were mainly made up of fineγ fibersγ fibers formed with in< the111 >processparallel of to cold the rolling normal remains direction. at Tolow make annealing the results temperature. more visualized, The α fibers density are distributionscharacterized of by the a <110> textures crystal on α direction(a) and γ perpendicula(b) orientationr to lines the are rolled presented surface in of Figure the steel6. Itplate, can bemaking seen inthe Figure deformation6a that theresistance strongest weak components and the deep belonging drawability to α offibers the steel were sheet all closepoor. to However, {112} <110 with> and the {111}increase<110 of> annealingat different temperature, annealing temperatures. the strength of Besides α fibers the decreased overall strength as a whole, of α fibersand the in {111} the sample <110> component gradually became the sharpest texture. Especially at 730 °C, α fibers almost disappeared annealed at 710 ◦C was high. This is because the driving force of recrystallization is weak and part of γ fibersexcept formed for those in close the process to {111} of <110>. cold rolling remains at low annealing temperature. The α fibers are characterizedFigure 6b by indicates a <110> thatcrystal the directionaverage texture perpendicular density to on the γ rolledorientation surface line of increased the steel plate, with makingthe rise theof annealing deformation temperature. resistance The weak density and the of deep{111} drawability<112> had the of largest the steel increasement, sheet poor. However, from 8.4 at with 710 the °C increaseto 13.8 at of 780 annealing °C The strength temperature, of γ fiber the strength is closely of relatedα fibers to decreased the stored as energy a whole, of cold and rolling the {111} textures.<110> The textures formed in the cold-rolled process mainly contain {111} <112>, {111} <110>, {112} <110> component gradually became the sharpest texture. Especially at 730 ◦C, α fibers almost disappeared exceptand {100} for <110> those closecomponents. to {111}
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time, the uniformity of γ fibers was better, which is conductive to reducing the earing of battery shell steel in the stamping process. Besides, as the annealing temperature rose, some {111} <110> and {111} <112> grains began to nucleate inside each other. However, the nucleation rate of {111} <110> grains was higher than that of {111} <112> grains. Though {111} <112> owned the crystal nucleus with the maximum grain boundary migration rate, the low nucleation rate still resulted in the weaker density MetalsMetals 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 99 in 760 ◦C than 730 ◦C. On the other side, in a certain range of temperatures, the interstitial atoms fixedfixedare fixed andand and thethe thesolidsolid solid solutionsolution solution ofof ofsubstitutionalsubstitutional substitutional solutesolute solute atomsatoms atoms increasesincreases increases asas as temperaturetemperature temperature rises,rises, which which hinderhinder the the dislocationdislocation movementmovementmovement and andand reduce reducereduce the thethe e ff effectiveeffectiveective deformation, deformation,deformation, thus thusthus providing providingproviding conditions conditionsconditions for forforthe thethe preferred preferredpreferred growth growthgrowth of of {111}of {111}{111}<112 <112><112>> texture. texture.texture. Therefore, Therefore,Therefore, when whenwhen thethethe annealingannealing temperature temperature increased,increased, increased, thethe differencedifference difference of of density density between between the the two two textures textures becamebecame moremore obvious.obvious.
2 FigureFigure 5.5.5.Orientation OrientationOrientation distribution distributiondistribution function(ODF) ffununctionction((ODFODF sections)) sectionssections (ϕ2 = 45 (φ (φ◦)2 at = di= ff45°45°erent)) atat annealing differentdifferent temperatures. annealingannealing temperatures.(temperatures.a) 710 ◦C, (b) ( 730(aa)) 710710◦C, °° (CCc,), ( 760(bb)) 730730◦C and°°CC,, (( (ccd)) ) 760760 780 °°C◦CC. andand ((dd)) 780780 °°C.C.
FigureFigure 6.6. 6. TTexexTexturetureture intensityintensity intensity distributionsdistributions distributions alongalong along αα ((aa)) andandα ( a γγ) (( andbb)) orientationorientationγ (b) orientation lineslines atat differentdifferent lines at annealingannealing different temperatures.temperatures.annealing temperatures.
ItIt is is knownknownknown that thatthat EBSD EBSDEBSD is isis a a usefula usefuluseful tool tooltool to observetoto observeobserve microstructural microstructuralmicrostructural textures, textures,textures, grain graingrain orientations, orientationsorientations grain,, graingrainboundaries, boundaries,boundaries, etc. Figure etc.etc. FigureFigure7 shows 77 theshowsshows distribution thethe distributiondistribution maps of mapsmaps several ofof typical severalseveral textures typicaltypical intexturestextures specimens inin specimensspecimens annealed annealedannealedat different atat temperaturesdifferentdifferent temperaturestemperatures and the volume andand thethe fraction volumevolume of fractionfraction each texture ofof eacheach is texturetexture shown isis in shownshown Table2 inin. The TableTable increase 2.2. ThThee increaseincrease ofof graingrain sizesize andand thethe decreasedecrease ooff elongationelongation alongalong thethe rollingrolling directiondirection withwith thethe riserise ofof annealingannealing temperaturetemperature cancan alsoalso bebe seenseen inin FigureFigure 7.7. Besides,Besides, FigureFigure 77 andand TableTable 22 givegive usus aa clearclear recognitionrecognition thatthat thethe mainmain texturestextures inin thethe annealedannealed sheetssheets areare {111}{111} <112><112> components,components, followedfollowed byby {1{111}11} <110><110> andand {112}{112} <110><110> components.components. TaylorTaylor factorfactor (M)(M) isis oftenoften usedused toto representrepresent thethe effecteffect ofof texturestextures onon yieldyield strength.strength. TheThe MM--valuevalue ofof rollingrolling texturestextures isis usuallyusually 15%15% higherhigher thanthan thatthat ofof annealingannealing texturestextures [13].[13]. WithWith thethe annealingannealing temperaturetemperature rising,rising, thethe volumevolume fractionsfractions ofof {111}{111} << 112>,112>, {111}{111} <110><110> andand otherother annealingannealing texturestextures increased,increased, thusthus resultingresulting inin thethe decreasedecrease ofof yieldyield strengthstrength,, whichwhich isis oneone ofof thethe resultsresults ofof thethe tensiletensile test.test.
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The effect of textures on anisotropy is realized by the influence of Taylor factor on yield locus, whichof grain means size andthat anis theotro decreasepy can of be elongation judged by alongthe volume the rolling fraction direction and M- withvalue the of the rise textures. of annealing {100} texturestemperature have can the also lowest be seen M-value in Figure at the7. Besides, same condition. Figure7 and Therefore, Table2 give plastic us a f clearlow behavior recognition is thatmore likelythe main to happen textures along in the the annealed <001> sheetsdirection are during {111} < 112the >processcomponents, of deep followed drawing, by thus {111} causing<110> andthe {112}appearance<110> ofcomponents. earing [14,15] Taylor. Researches factor (M) have is oftenshown used that tor value represent has a the linear effect relationship of textures with on yield the volumestrength. ratio The of M-value {111} textures of rolling to {100} textures textures is usually [16,17], 15% which higher is presented than that as of {111}/{100} annealing texturesin Table [2.13 It]. canWith be the seen annealing that the temperature value of {111}/{100} rising, the increased volume fractions as the annealing of {111} < 112temperature>, {111} < 110rose,> andwhich other is beneficialannealing for textures the improvement increased, thus of resultingr value and in the deep decrease drawability. of yield This strength, observation which is is one just of in the keeping results withof the the tensile rising test. trend of r values as shown in Figure 4b.
Figure 7. 7. DistributionDistribution maps maps of typical of typical textures textures at different at different annealing annealing temperatures. temperatures. (a) 710 ( a°C), 710 (b) ◦ C, 730(b) 730°C, (◦cC,) 760 (c) ° 760C and◦C ( andd) 780 (d )°C 780. Blue◦C. represents Blue represents {111} <112> {111} < texture,112> texture, red represents red represents {111} <110>, {111} yellow<110>, representsyellow represents {112} <110>, {112} purple<110> ,represents purple represents {001} <110> {001} and<110 green> and represents green represents {001} <001>. {001} <001>.
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Table 2. Volume fractions of typical textures at different annealing temperatures.
Annealing {111} <112>/% {111} <110>/% {112} <110>/% {001} <110>/% {001} <001>/% {111}/% {001}/% {111}/{001} Temperature/◦C 710 42.30 33.00 22.20 5.15 1.07 68.80 11.00 6.25 730 44.11 29.80 24.20 4.91 0.87 69.80 9.41 7.42 760 44.00 32.40 24.30 4.11 1.79 70.30 9.55 7.36 780 46.30 33.20 26.00 4.55 1.34 73.40 8.92 8.23
The effect of textures on anisotropy is realized by the influence of Taylor factor on yield locus, which means that anisotropy can be judged by the volume fraction and M-value of the textures. {100} textures have the lowest M-value at the same condition. Therefore, plastic flow behavior is more likely to happen along the <001> direction during the process of deep drawing, thus causing the appearance of earing [14,15]. Researches have shown that r value has a linear relationship with the volume ratio of {111} textures to {100} textures [16,17], which is presented as {111}/{100} in Table2. It can be seen that the value of {111}/{100} increased as the annealing temperature rose, which is beneficial for the improvement of r value and deep drawability. This observation is just in keeping with the rising trend of r values as shown in Figure4b.
4. Conclusions 1. In the continuous annealing process of battery shell steel, with the increase of annealing temperature, the recrystallization of ferrite phase was more complete and the average grain size was larger. Besides, both yield strength and tensile strength decreased, while the fracture elongation and r value increased. 2. The annealed sheets contained strong γ fibers and weak α fibers. The density of {111} <112> texture was the highest, followed by {111} <110> and {112} <110> textures. Besides the content of {001} texture was extremely low. 3. As annealing temperature rose, the volume fraction of {111} textures increased, while volume fraction of {001} textures decreased, causing a higher value of {111}/{001}, which is an important factor affecting the r value and the deep drawability.
Author Contributions: Conceptualization, B.N. and Z.Z.; methodology, B.N. and Z.M.; validation, H.W. and H.G.; formal analysis, Z.Z., B.N. and C.P.; writing—original draft preparation, B.N.; writing—review and editing, Z.M. and C.P.; supervision, H.W.; project administration, Z.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
References
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