Influencing the Wettability of HSS-Steels by Addition of Alloying Elements to the Zinc Bath

Transcription

1 Influencng the wettng angle of znc on steel by alloyng elements Influencng the Wettablty of HSS-Steels by Addton of Alloyng Elements to the Znc Bath Guzh Zeng, Bernd Fredrch IME Process metallurgy and metal recyclng department and char of RWTH Aachen Unversty, Intzestraße 3, Aachen, Germany Abstract The contanng of hgh amounts of alloyng elements such as S, P, Mn reduces dramatcally the wettablty of Hgh Strength Steels (HSS) and therefore, deterorates ther coatng qualty from contnuous hot-dp-galvanzng n conventonal Al alloyed Znc baths. In ths paper, the wettng behavor of dfferent Zn alloys (Ca, Mg, Cu, N, T, V, Zr, Y, Ce, La) on three specal types of steel substrates have been nvestgated. Frstly, the surface tensons of these Znc alloys were calculated based on thermochemcal databases and modellng. Then the wettng angels of these Zn alloys on the steel substrates are measured va the sessle drop method. By comparng the fnal shape of ther droplets formed on the steel substrates, a qualtatve seres of the nfluence of the alloyng elements was obtaned. The wettablty of the same znc alloy wth the hgh alloyed DP500 steel s worse than wth normal M3A13 steel and low alloy 31AXV steel. Under a reductve hydrogen atmosphere, ther wettng behavor can be strongly ncreased n compare to that of under a 95% argon-5% hydrogen atmosphere. Among the nvestgated Zn contanng alloys, Vanadum and Zrconum as alloyng elements show the most effectve mprovement of the wettablty on hgh strength steel substrates. Keywords: Wettablty, Wettng Angle, Znc Alloy, Hgh Strength Steel, Hot-dp Galvanzng. Proceedngs of EMC

2 Guzh Zeng, Bernd Fredrch 1 Introducton The growng demands of weght reducton and passenger safety n automoble, promote the applcaton of hgh strength steels (HSS) sheets n the constructon of the car bodes steadly. The ULSAB- AVC program (Ultra Lght Steel Auto Body - Advanced Vehcle Concept [1]) expects that 80% of steels used n automoble ndustry should be ether DP (dual phase) or TRIP (transformaton nduced plastcty) steels. Comparng to conventonal unalloyed and low alloyed steels, HSS are nowadays usually alloyed wth a hgh amounts of S, Mn, Cr together wth other elements, n order to acheve both hgher tensle strength and better formablty [1]. In order to protect the steel sheets aganst corroson, a layer of znc or znc alloy s always coated on the surface of the steel sheets, mostly acheved by hot dp galvanzng (HDG) the strp pror to pressng. However, the ncreased amounts of alloyng elements lke P, Mn or S n the hgh strength steels causes problems n the znc galvanzng process. Pror to mmerson nto the HDG bath, the Fe-oxde and Fe-hydroxde layer created durng the cold rollng process s completely removed n an annealng furnace. Durng ths treatment the man alloyng elements of HSS (e.g. Mn, S, Al, Cr, V and T), whch have strong affnty towards oxygen, wll mgrate preferentally through easy dffuson paths, such as gran and sub gran boundares, to the substrate surface and are oxdzed there [2]. The oxdaton products do not form a contnuous layer on the surface but are present as dspersed slands, whch wll strongly deterorate the wettablty of the steel strp surface and result n a poor coatng qualty. Elements lke S and P wll also nfluence the knetcs and velocty of the reactons between znc and ron durng the galvanzng process [3]. Dependng on the S and P content a strong rregular layer grows wth an nhomogeneous surface constructon, whch agan wll damage the adheson and wettng behavor between Zn melt and steel strps. To reduce the deteroraton of the alloyng elements and mprove the adheson of the znc coatng, on one hand the steel substrate should be pre-treated to change the chemcal composton and to prevent the external oxdaton of the alloyng elements at the surface. Ths can be acheved by annealng n reactve atmosphere, where reducng, ntrdng or carburzng occurs [4]. On the other hand, reactve alloyng elements could be added nto the znc bath, n order to ncrease the reactvty between znc and the steel surface, such mprovng the wettablty and the adheson of the znc coatng. Alumnum s one of the most effectve alloyng elements to reduce the nfluence of S n steel durng Znc galvanzng. Alumnum n the range of wt% s added to the contnuous znc galvanzng bath to suppress the growth of brttle Fe-Zn ntermetallc phases at the steel coatng nterface by formng of a Fe 2 Al 5 nhbton layer [6]. Zn-Al baths wth a reactve thrd alloy element were commercally produced for strp HDG, such as Galfan (95% Zn, 5% Al, traces of Mschmetal contanng Cerum and Lanthanum), Lavagal (35% Al-Zn-Na-Mg) and Galvalume (55% Al, 43.3% Zn, 1.6% S). The addton of Mschmetal mproves the wettablty and fludty of the molten znc bath and such avod the formaton of the brttle ntermetallc phase between the steel substrate 2 Proceedngs of EMC 2009

3 Influencng the wettng angle of znc on steel by alloyng elements and the coatng. Ths fnally leads to a sgnfcant ncrease n formablty and corroson lfe of the steel strp [7]. It s also reported that the presence of N ( %) n Znc HDG baths, known as Techngalva, reduces substantally the thckness and corroson rate of the galvanzed coatng by suppressng the reactvty of steel contanng up to 0.25% S [7]. Katfors and Papadmtrou have studed the nfluence of copper, cadmum and tn on the consttuton and the thckness of the coatngs [5]. They have shown that copper forms a ternary Fe-Zn-Cu ntermetallc compound, promotes the δ1 phase formaton, hnders the ζ phase growth and results n a worse adherence of the coatng. Cadmum wth a concentraton above 1 wt.-% promotes the ζ phase and Γ phase and hnders the growth of the δ1 phase. By addton of up to 3 wt.-% tn no sgnfcant changes n morphology and thckness of the obtaned galvanzed coatngs occur. Reumont et. al. have reported that the Sandeln-effect can be prevented f >0.025% T was contaned n the znc bath [8]. Ths paper presents the results of a German DFG research project, where varous elements (Ca, Mg, Cu, N, T, V, Zr, Y, Ce, La), whch have a partcular bg affnty to oxygen, were alloyed wth Zn to nvestgate ther nfluences on the wettng behavor and reacton knetc between Znc and specal steels. The surface tenson of these znc alloys was frstly calculated on the bass of thermochemcal data of ts pure elements. Ther wettng behavors on dfferent steel sheets (normal M3A13 steel, low alloyed 31AXV steel and hgh alloyed DP500 steel) have been studed by means of the sessle drop method. SEM and WDX methods were used to analyze the mcrostructure and component dstrbuton of the coatng layers. 2 Thermochemcal evaluaton of surface propertes 2.1 Wettng angle and surface tenson The wettablty of a lqud s quantfed by the wettng angle between the nterface of the lqud phase and surface of the sold phase, when a lqud droplet s n thermal equlbrum wth a horzontal sold surface, as shown n Fgure1. Generally, when the wettng angle θ ranges between 0 and 90, the surface s strongly or partally wettng. The lqud does not wet the surface sgnfcantly f the wettng angle θ ranges between 90 and 180. If the wettng angle s equal to 0, the lqud s perfect wettng and t spreads spontaneously over the sold surface, the deal stuaton for a HDG process. For wettng angle equalng to 180, the lqud s called perfectly non-wettng. Proceedngs of EMC

4 Guzh Zeng, Bernd Fredrch Fgure 1: Contact angle and balance of nterfacal forces The value of wettng angle θ s determned at the force equlbrum bult up by the nterfacal tensons of sold-gas σ s,g, lqud-gas σ l,g and sold-lqud σ s,l n relaton of the Young s equaton [9]: σ l, g cosθ = σ s, g σ s, l (1) Grfalco and Good have derved the followng expresson for the nterfacal tenson σ s,l between the lqud and sold phase [10]: σ s, l = σ s, g + σ l, g 2Φ σ s, gσ l, g (2) where Ф s the nteracton parameter, whch depends on the type of atomc/molecular bondng between the lqud and sold phases. Combnng equatons (1) and (2), the wettng angle θ can be expressed by: σ s, g cosθ = 2Φ 1 (3) σ l, g Accordng to equaton (3), t can be seen that f the surface tenson of the lqud σ l,g s kept as small as possble, the value of cos θ ncreases, whch means that the wettng angle θ decreases and so that the wettablty between the lqud droplet and sold surface ncreases. 2.2 Theory for thermochemcal modellng of surface tenson Accordng to Gbbs adsorpton theory, the surface phase and the bulk phase of alloys are consdered to be n thermochemcal equlbrum, the chemcal potental of each alloy component n the monolayer surface s equal to that of the correspondng component n the bulk phase. Therefore, the surface tenson σ of alloy can be expressed as: os ob S B μ μ RT a σ = + ln (4) S S a os ob (σ s the surface tenson,, are the standard chemcal potental of component n the μ μ S B surface phase and the bulk phase,, are the actvty of the component n the surface a a phase and bulk phase, R s the unversal gas constant, T s the absolute temperature, partal molar surface area of component ) S s the Assumng that no volume effects n the surface occur by mxng, so that the partal molar surface area of one of component S s equal to the molar surface area to derve the Butler s equaton relatve to one of the components [11]: S of pure component, t s possble 4 Proceedngs of EMC 2009

5 Influencng the wettng angle of znc on steel by alloyng elements S S S o RT a o RT X RT γ σ = σ + ln = σ + ln + ln B B B S a S X S γ (5) wth σ o ( os ob = μ μ ) S as the surface tenson of the pure component ; X s and X s denote the mole fractons and γ s and γ b are the actvty coeffcents of the component n surface phase and n the bulk phase. Such the surface tenson of a bnary alloy system can be expressed based on the surface tenson of ther pure elements as follows: RT X RT γ RT X RT γ σ = σ + + = σ + + (6) S S S S o 1 1 o ln ln 2 ln ln B B B B S1 X1 S1 γ1 S2 X2 S2 γ 2 In assumpton of a perfect surface soluton model wth deal behavor of the bulk and surface phases, the actvty coeffcents of the component n surface phase equals to that of n the bulk S B S S phase: γ = γ. On the bass of X + X =, the followng equaton can be obtaned: [11] o o { σ σ } { σ σ } X exp ( ) S RT + X exp ( ) S RT = 1 (7) By usng the expanson expϕ = 1+ ϕ, the surface tenson of the bnary alloy can be calculated relatve to the surface tenson of pure component 1 and 2: 0 0 X1σ 1 S1 + X 21σ 2 S2 σ = (8) X1S1 + X 2S2 For ternary alloys, the calculaton of surface tenson can be derved n the smlar way as n the bnary systems: X1σ 1 S1 + X 21σ 2 S2 + X 3σ 3 S3 σ = (9) X 1S1 + X 2S2 + X 3S3 The surface areas S of the pure elements at the relevant temperatures can be calculated from the denstes of corresponded elements by the Brandes equaton [11]: 1/3 2/3 S = kn V where k s a geometrc factor wth a value of for a closed packed lattce, N s the Avogardo s number wth a value of 6.02*1023 and V s the molar volume of the component determned from ts densty. It was found that the molar volume V and surface tenson σ of pure elements n alloys are lnearly dependng on the temperature, based on ther values V m, and σ m, at the meltng temperature, T m, : [12] Proceedngs of EMC

6 Guzh Zeng, Bernd Fredrch V { + ( T T )} = V α (11) m, 1 m, dσ σ = σ m, + ( T Tm, ) (12) dt 2.3 Surface tenson calculaton of znc alloys Accordng to the descrbed theory, the surface tenson σ of bnary znc alloys ZnX and ternary ZnAl0.2X was calculated at the assumpton of a perfect surface soluton model by combnng the equatons (8) to (12). The molar volume V and surface tenson σ of pure alloyng elements n znc alloys at dfferent temperature were calculated by usng standard thermochemcal data, leadng to the values shown n Table 1. Table 1: Molar volume and surface tensons of pure elements [12] Element T m, (K) { + ( T T )} dσ V = Vm, 1 α m, σ = σ m, + ( T Tm, ) dt V m, (10-6 m 3.mol -1 ) α (10-4 K -1 ) σ m, (mn.m -1 ) dσ /dt (mn.m -1.K -1 ) Zn Al Ca Mg La Ce Y * Cu Note: * reference from [13]. Zr T N V Fgure 2 left shows the calculated surface tensons of ZnX bnary alloys wth a molar fracton of X from 0 to 0.2% and Fgure 2 rght those of ZnAl0.2X ternary alloys whch contan 0.2% of Al havng the same molar fracton of X. The temperature used for calculaton s set to 500 C, whch s a typcal temperature for znc galvanzng. Comparng Fgure 2, t can be seen that n both alloy systems each element X shows smlar nfluences to the total surface tenson. Wth the addton of Ca and Mg, whch has a low surface tenson at ts meltng temperature σ m,, the surface tenson of ther 6 Proceedngs of EMC 2009

7 Influencng the wettng angle of znc on steel by alloyng elements Zn alloys decreases as well as the composton of the alloyng element ncreases. There are no bg varatons n the surface tenson of Zn alloys contanng Ce, La and Y. By addng Cu, Zr, T, N and V the surface tenson of the ZnX and ZnAl0.2X ncreased. Surface tenson of ZnX alloy σ [mn/m] ZnCa ZnMg ZnLa ZnCe ZnY ZnAl ZnCu ZnZr ZnT ZnN ZnV Surface tenson of Zn0.2AlX alloy σ[mn/m] Zn0.2AlCa Zn0.2AlMg Zn0.2AlLa Zn0.2AlCe Zn0.2AlY Zn0.2AlCu Zn0.2AlZr Zn0.2AlT Zn0.2AlN Zn0.2AlV Composton of X n ZnX alloy [%] Composton of X n Zn0.2Al-X alloy [%] Fgure 2: The surface tenson of bnary ZnX alloys (left) and ternary ZnAl0.2X alloys (rght) wth composton of X from 0 to 0.2% at a temperature of 500 C 3 Expermental Dfferent alloyng elements Ca, Ce, Cu, La, Mg, N, T, V, Y and Zr, were chosen to form alloys wth Znc for an expermental wettablty study. These elements have relatve bg affnty to oxygen n order to reduce the formaton of the troublesome metal oxde (SO 2, MnO, P 2 O 5 ), whch dsturb the good adheson between Zn on steel. The meltng temperatures of all these alloyng elements are consderably hgher then the meltng pont of the bass metal znc ( C), some of them are over 1000 C, as shown n Table 1. In order to avod massve evaporaton of znc, the maxmum alloyng temperature were set below 700 C. Among these alloyng elements, only Cu and Mg are completely soluble n znc wth alloy composton of ZnX0.05, ZnX0.2 and ZnAl0.2X0.2. Most of them are not soluble n sold znc but buld ntermetallc phases wth znc. These alloy elements were at frst pre-alloyed wth pure znc (> % Zn) to obtan relatve hgher alloy composton or as hgh as possble. Then the prealloy s melted wth a respectvely calculated mass of pure znc to manufacture the fnal alloys wth accurate alloy compostons. Both the pre-alloy and the fnal alloy process were conducted under Argon atmosphere n a sealed nducton furnace. The castng mould was placed near the crucble nsde the sealed furnace, whch s shown n Fgure 3, so that the melted alloy can be casted under nert atmosphere as well. Proceedngs of EMC

8 Guzh Zeng, Bernd Fredrch 1. ceramc crucble 2. smelter 3. thermocouple 4. pourng spout 5. castng mould 6. castng funnel Fgure 3: The work room of the nducton furnace. A self desgned castng mould, as shown at the left sde n Fgure 4, was used for castng. The meltng znc alloys was poured through the centre column of the mould and then the lqud flows to the bottom of the mould separately at both sde. As the bottom column was flled up, the excess lquds flow from bottom upon at the rest four parallel columns wth rod length of 12 mm and dameter of 5mm at the same tme. The flow drecton and the obtaned rod ngot are shown at the rght sde of Fgure 4. By usng ths bottom castng technology, the mpurtes and slag rest formed n the smeltng can be excluded n the centre column and alloy wth more accurate composton and more homogeneous composton dstrbuton can be ganed from the four sde columns. The manufactured znc alloy ngot was then cut nto small cylnders wth a length of 3 mm for further wettablty tests. Fgure 4: The opened castng mould (left) and the casted ngot (rght) The wettng angles of these znc alloy droplets on the flat steel substrates were measured by means of the sessle drop method. The schematc llustraton of the whole setup for the measurement of wettng angle s shown n Fgure 5. The experments were conducted n a self desgned electrcal 8 Proceedngs of EMC 2009

9 Influencng the wettng angle of znc on steel by alloyng elements resstance furnace wth a tghtly sealed quartz glass tube (Fgure 6). The temperature nsde the furnace was controlled by a PID electrcal heater. The pcture of the formed droplet was taken by a CCD camera OCA and the wettng angle between droplet of Zn alloy and the steel substrate can be measured by the equpment SCA20 from Dataphyscs Instrument GmbH. The wettablttes of Zn alloy to three specal types of steel substrates (hgh strength steel DP500, low alloyed steel 31AXV and normal steel M3A13) wth chemcal compostons shown n Table 2 were compared. Table 2: Chemcal composton of tested steels n wt.- % C S Mn P S Cr Mo N Al T DP < AXV M3A Before each experment, the steel substrate samples were mechancally polshed by emery paper and then degreased wth alcohol and pckled n a 10% dluted HCl for about 10 seconds at 60 C to remove the ol and oxde flm from the steel surfaces. The cleaned steel substrate were then rnsed wth dstlled water and mmersed nto an ascorbc acd bath for around 5 seconds to mnmze further oxdaton of the steel surface and then placed above a graphte table wth nclned hole for thermocouple to measure the real temperature of the steel substrate. furnace camera Fgure 5: Illustraton of the whole expermental setup Proceedngs of EMC

10 Guzh Zeng, Bernd Fredrch Fgure 6: Detaled schematc of the set up of droplet formng system The Znc alloy samples were flled at the front end of a graphte tube fxed by a metal holder on the slca tube as shown n Fgure 6. A groove was desgned at the front end of the graphte tube for held thermocouple to measure the real temperature of the znc alloy samples. As the samples were prepared and the furnace was sealed, the sealed slca tube was frstly evacuated for 10 mnutes by a mechancal vacuum pump and then backflled wth nert gas (pure hydrogen or pure hydrogen) wth a flow rate of about 20ml/mn. The electrcal heater s turned on to heat up the znc samples nsde the furnace to the expermental temperature 500 C, where t was held for 30 mnutes to allow the znc alloy meltng completely. Meanwhle, the metal holder was cooled by runnng water through the coolng ppe at the end of the slca tube to protect the gummy sealng rng, as shown n Fgure 6. When the znc alloy samples were completely smelt, the molten znc alloy nsde the graphte tube was pushed out by the stanless steel screw and an alloy droplet fell down onto the steel substrate placed below. The shape change of the formed droplets wth tme can be recorded by a hgh speed dgtal camera OCA from Dataphyscs Instrument GmbH through the glass wndow of the furnace. The wettng angle between and Zn alloy droplet and the steel substrates was then calculated by the graphcal analyss program SCA20also from Dataphyscs Instrument GmbH. The baselne and the outlne of the droplet were defned automatcally or manually when necessary. Wth the defnton of both 10 Proceedngs of EMC 2009

11 Influencng the wettng angle of znc on steel by alloyng elements lnes an teratve process s ntated to get the three phase pont, the cross pont of the outlne and the baselne. The contact angle of the droplet s the angle between the baselne and the tangents of the outlne. The value of the contact angle s then read by the scale around as shown n Fgure 8. Fgure 8: The prncple of the contact angle measurement After experments, the surface appearances of the steel substrates samples adhered wth znc alloy droplets were recorded by normal dgtal camera ntally. The cross sectons of some samples are embedded n resn and then mechancal polshed and carbon coated for scannng electron mcroscope (SEM) analyss of the morphology of the formed droplet by ZEISS DSM 962 SEM analyss machne. Oxford ISIS electron - dspersve spectroscopy EDS and wavelength - dspersve spectroscopy (WDS) were attached and used to chemcally analyze the dstrbuton and dffuson behavors of Zn, Fe and other alloyng elements. 4 Results and dscusson 4.1 Dynamc change of the wettng angle As the lqud droplet of znc alloy s contactng wth the steel substrate, the shape of the droplet decreases dramatcally. It takes normally only a few seconds tll t completely soldfes and reaches equlbrum. Fgure 7 shows the shape changes of a ZnCe0.2 droplet from a half ball (non wettng) to a flattened lens (good wettng) wthn 4 seconds after contactng the steel surface. Ths dynamc change of the wettng angle between the lqud Zn alloy droplet and Fgure 7: Dynamc varaton of the shape of a ZnCe0.2 droplet after contactng a steel substrate Proceedngs of EMC

12 Guzh Zeng, Bernd Fredrch the steel substrate s nfluenced by multple factors. The man factors are dffuson and reacton nteractons between lqud znc and ts alloyng elements and ron wth ts alloyng elements. The Znalloy droplet begns to be soldfed after the very frst contact wth the substrate. At the same tme, znc s evaporated from the surface of the droplet, whose effect can be also observed as a shadow n Fgure 7. Furthermore, the alloyng elements exsted n Zn alloy wth a relatve hgh oxygen affnty react wth the resdual oxygen remaned n the furnace atmosphere and affect the wettng behavor of the droplet as well. Fnally, the gravty force of the droplet tself nfluences the force equlbrum and the fnal shape of the droplet. Therefore, t s necessary to keep the droplet n a comparable mass and to measure the contact angle after the same contact tme, n order to compare the contact angle of the droplets of dfferent Zn alloys. 4.2 The nfluence of alloyng elements on wettablty Frst of all, the wettablty was able to be qualtatvely compared by the fnal shape of formed droplet after the soldfcaton. Such, the fnal shape of the droplet after soldfcaton was dvded nto four types: a sphere, a flattened sphere, a cap, and a lens, whch are defned accordng to ther H/D value, the rato of the heght of the droplet (H) and the dameter of the contactng area between the droplet and the substrate (D), as shown n Fgure 9. The wettablty between the lqud droplets and the steel substrate ncreases as the H/D value decreases. Typcal droplet shapes and the derved wettablltes from sessle droplet test of ZnX0.2 alloys wth (X= Ca, Mg, Cu, N, T, Y, Ce, La, V, Zr) on a 31AXV steel substrate under Vargon (95% argon, 5% hydrogen) are lsted exemplary n Table 3. (a) sphere ( H/D>0.7) (b) flattened sphere (0.7>H/D>0.4) (c) cap (0.4>H/D>0.1) (d) lens (0.1>H/D>0) Fgure 9: Four types of droplet shapes occurrng after contactng of the steel substrate Obvously the wettablty of these ZnX0.2 alloys dd not reach the targeted value of a contnuous layer and the tendency of the nfluences of the alloyng element s dfferent from that of the fndng resulted from the calculaton n chapter 2.3. Ths dfference s due to the facts that the wettablty of the tested steels s strongly affected by the nteracton of the gnoble alloyng elements n the Zn alloy as well as n the steel wth the atmosphere. Unfortunately the testng devce could not fully exclude oxygen access. Snce Ca or Mg are well known oxygen affnty elements, they can be easly oxdzed and form an oxde flm over the droplet surface and then deterorate the wettablty of ther Zn alloy, even n atmosphere wth very low oxygen partal pressure. 12 Proceedngs of EMC 2009

13 Influencng the wettng angle of znc on steel by alloyng elements Table 3: Droplet shapes and qualtatve wettablltes of ZnX0.2 (X= Ca, Mg, Cu, N, T, Y, Ce, La V, Zr) on 31AXV steel substrate under Vargon (95% argon, 5% hydrogen) atmosphere Droplet shape sphere flattened sphere cap lens ZnX alloy ( X=) Ca, Mg, Cu, N, Y Cu, La, N, Y Ce, La, T, V, Zr Ce, La, V, Zr wettablty very bad bad mprovng moderate ele- alloyng ment Ca, Mg Cu, N, Y La, T Ce, V, Zr calculated effect T, V Cu, N, Zr, La,Ce,Y Mg, Ca By usng reductve hydrogen as protectng atmosphere, the adheson and wettablty of the Znalloys could be mproved a lot compared to Vargon atmosphere, as shown n Fgure 10 (c) and (d) exemplary for ZnCe0.2. It can be seen that under Vargon atmosphere, the wettablty of a DP500 steel substrate s very bad. When hydrogen s used as protectng atmosphere, the droplet rapdly spreads out flatly when t contacts all three types of steel substrates, as shown n Fgure 10 (a) (b) (c). The radus of the coated area reaches about 15 mm at the M3A13 and 31AXV steel substrate, whle for the DP500 steel substrate t s smaller. Ths s certanly caused by surface oxdaton of the contaned alloyng elements (e.g. Mn, S, Al, Cr, V and T) n the DP500 steel, whch lmt the wettablty of the steel surface. (a) M3A13, H 2 (b) 31AXV, H 2 (c) DP500, H 2 (d) DP500, Vargon Fgure 10: Fnal form of ZnCe0.2 droplets after contactng dfferent steel substrates under Vargon (95% argon, 5% hydrogen) and pure H 2 From Fgure 10 (a), (b), (c), several dffuson layers can be seen at the fnal coatng, after the lqud ZnCe0.2 alloy droplet spread out over the steel substrate. Ths may be caused by the nteracton dffuson and reacton between Zn and Fe. Drect above the steel substrate, mght be a compact layer of ntermetallc gamma (Γ) wth a composton of Fe 3 Zn 10 or Fe 5 Zn 21.The mddle layer mght Proceedngs of EMC

14 Guzh Zeng, Bernd Fredrch contan the delta (δ) phase layer and zeta (ζ) phase layer. The upper droplet layer soldfyng before dffusng away, should be the znc rch eta (η) phase layer. The dynamc change of the shapes of the Zn alloy droplets were recorded by the hgh speed camera and ther wettng angles on steel substrates were measured by the graphcal analyss program SCA20. Due to the quck dynamc varaton of the droplet shape durng the measurements, the pctures of the Zn alloy droplets at the frst moment of contactng the steel substrate were taken for comparson. However, as the vsblty nsde the furnace was low due to the closed nstallaton of the furnace, the comparable low radaton due to moderate temperatures and also due to Znevaporaton, the sharpness and the contrast of obtaned pctures of the droplets were relatve low. The analyss program was not able to defne the outlne and base lne of the droplets very exact and t was hardly to obtan an exact value of wettng angles between the droplets and the steel substrates. Therefore, the measured wettng angel values of pure Zn respectvely the bass alloy ZnAl0.2 were used as reference values (100%) to compare the measured wettng angles of dfferent ZnX0.05, ZnX0.2, and ZnAl0.2X0.2 alloys, whch were obtaned at the same condtons as the references. A relatve wettng angle value Θ of these alloys can be obtaned by the followng calculaton: θ Θ = *100 (13) θ r where Θ s the relatve value of wettng angle; θ s the measured value of wettng angle; θ r s the measured value of the wettng angle of reference metals or alloy. The obtaned wettng angles of the nvestgated alloys obtaned at dfferent condtons are summarzed n Fgure 11. Fgure 11 (a) shows the relatve values of the wettng angles of the ZnX0.2 alloys on 31AXV low alloyed steel substrates under Vargon (95% argon, 5% hydrogen) atmosphere, usng the wettng angle of pure znc as reference. The correspondng values for DP500 wth ZnAl0.2 alloy as reference value are shown n Fgure 11 (b), (c), (d), where (b) presents the results of ZnX0.2 alloy, (c) those of ZnX0.05 and (d) those ZnAl0.2X0.2 alloys. Accordng to ther relatve value of the wettng angles, the nfluences of the alloyng elements to the wettablty of the steel substrate can be classfed nto three levels: postve, natural and negatve nfluence. Comparng the results of the 0,2% alloyed Zn-/ZnAl-matrx t can bee seen, that the unnoble elements T, Ce, V, La, Mg, Y and especally Zrconum show a postve effect. Ths s most probably due to nterphase reactons lke oxde destructon, whch have cleanng effect for the hgh alloyed steel surface. Despte the careful precondtonng of the steel substrates the remanng oxygen and ntrogen molecules n the flushng gas wll be absorbed by the steel alloyng elements lke Mn and S formng thn ceramc layers. The presence of very gnoble elements n the Znc-droplet wll lead to crackng of these layers whch has a postve nfluence on the wettng behavors. In case of the 0.05% alloyed Zn-/ZnAl-matrx ths effect s not that sgnfcant, most probably due to the fact that the gnoble elements are oxdzed and lost by the remanng reactve gas components leadng to decreased reducton actvtes of the Znc-droplet. 14 Proceedngs of EMC 2009

15 Influencng the wettng angle of znc on steel by alloyng elements Ca Cu 110 relatve wettng angle N Y Mg La T ZnX0.2/ 31AXV/ Vargon Zr Ce V relatve wettng angle N Cu La ZnX0.2/ DP500/H 2 Mg Zr (a) (b) relatve wettng angle Zr N T Ca Ce La Y Mg V relatve wettng angle Ce Cu T V N Y Zr ZnX0.05/ DP500/H 2 ZnAl0.2X0.2/ DP500/H 2 (c) (d) Fgure 11: Relatve wettng angles of dfferent ZnX0.05, ZnX0.2, and ZnAl0.2X0.2 alloys: (a) 31AXV steel substrates under Vargon (95% argon, 5% hydrogen) atmosphere; (b) (c) (d) on DP500 steel substrates under pure hydrogen atmosphere 4.2 Mcroscopc analyss of the nterface The ZnV alloy representng a postve nfluence as well as the ZnCu alloy representng a negatve nfluence on the wettablty of Zn, are both here further dscussed. The cross sectons of the coated DP500 steel substrates were mcroscopc analyzed under SEM, as shown exemplary n Fgure 12. The dstrbuton profles of the weght percent concentratons of the respondng elements over the nterface are presented n Fgure 13. Proceedngs of EMC

16 Guzh Zeng, Bernd Fredrch ZnV alloy coat Interface Scannng drecton ZnCu alloy coat Interface Scannng drecton Steel substrate Steel substrate (a) (b) Fgure 12: Metallographc cross sectons of the coated DP500 steel substrates (a) ZnV alloy (b) ZnCu alloy ZnV_DP500 steel ZnCu_DP500 steel weght percent /% Zn Fe weght percent /% Zn Fe dstance from the nterface /um dstance from the nterface /um weght percent /% 1,4 1,2 1 0,8 0,6 0,4 ZnV_DP500 steel Mn Cr V weght percent /% 1,4 1,2 1 0,8 0,6 0,4 ZnCu_DP500 steel Mn Cr Cu 0,2 0, dstance from the nterface /um dstance from the nterface /um Fgure 13: Element concentraton profle at the cross secton of the ZnV and ZnCu coated DP500 steel substrate The resulted ZnCu layer (245μm) happens to be much thcker than the ZnV alloy layer (85μm). In both cases the nteracton reacton zone between Zn and Fe ranges of about 10μm. The dffusve mpregnaton depth of Fe nto the Znc-layer s about 85 μm and very large for both alloys. For the ZnV alloy, a hgh content of the V element s detected close the surface of the droplet, whle dffu- 16 Proceedngs of EMC 2009

17 Influencng the wettng angle of znc on steel by alloyng elements son of Cr and Mn contanng n DP500 steel substrate nto the Zn droplet seems to be suppressed. Ths could be caused by a stronger oxygen affnty of V, whch drves ths element to dffuse, such nhbtng the oxdaton of Cr and Mn. Ths may be the reason for an mproved wettablty of the steel substrate. On the other hand for the ZnCu alloy t can be seen that Cu dffuses towards the steel substrate, and due to mssng oxygen affne elements a hgh content of Cr and Mn dffuses towards the surface of the droplet formng an oxde flm by getterng the remanng oxygen molecules n the furnace. The next nvestgatons wll be focused on the nano-szed nterface for further explanaton the mechansms whch lead to a better nteracton between the sold and the lqud phase. 5 Conclusons The nfluence of alloyng elements, such as Ca, Mg, Cu, N, T, V, Zr, Y, Ce, La n a Zn bath on the wettng behavor of dfferent types of steel substrates has been nvestgated. Based on the assumpton of deal solutons, thermochemcal modelng has been performed and the surface tensons of many Znc alloys were calculated. It shows that especally the addton of Ca and Mg decreases the surface tenson of the alloy, whle V, N, T, Zr and Cu may ncrease the surface tenson. Nevertheless ths vew does not nclude nter phase reactons lke oxde destructon, whch can have a much stronger nfluence on wettng. By usng the sessle drop method, the wettng angle between the Zn alloy droplet and the steel substrates were measured. The wettng angles vary dynamcally n a very short tme after the droplet contacts the steel substrate and a sutable assessment method has been developed. The hgh alloyed DP500 steel shows a much worse wettng behavor than that of the standard M3A13 and the low alloyed 31AXV steel. Under a reductve hydrogen atmosphere, ther wettng behavor can be strongly ncreased n comparson wth that of under a 95% argon/5% hydrogen atmosphere. Among the nvestgated Zn alloys, V, Zr as an alloyng element show the most effectve mprovement on the wettng behavor wth HSS steel sheets. 6 Acknowledgement The fnancal support from the German Research Foundaton (DFG - Deutsche Forschungsgemenschaft) from s gratefully acknowledged. The authors would lke to thank Dr. Wnkler, Dr. Schmtz, Mr. Bünck, and Ms. Jang for ther contrbutons n ths project. Dr. Rchter from the nsttute GFE/ RWTH-Aachen Unversty s also apprecated for carryng out the SEM analyss. Proceedngs of EMC

18 Guzh Zeng, Bernd Fredrch 7 Reference [1] Hot dppng galvansng of hgh strength steels, steel tmes Internatonal, Dec 2002/ Jan 2003, Vol.27, Issue 1, pp [2] Olefjord I., Lejon W., Jelvestam U.: Selectve surface oxdaton durng annealng of steel sheets n H2/N2, Appled Surface Scence, 1980, Vol.6. pp [3] Masahko, H., Tosho, N., Norak,U.: Effect of S n steel on Hot Dp Galvanzng and Galvanealng, Galvatech 98, 1998, pp [4] Ahrens, M., Bleck, W. and Staudte, J.: Surface condton by reactve gases durng contnuous annealng of sheet steel, Journal of materals processng Technology, 2001, Vol. 117, pp [5] Katfors, N., Papadmtrou, G.: Influence of Cu, Cd and Sn addtons n the galvanzng bath on the structure, thckness and crackng behavour of the galvanzed coatng, Surface and Coatng Technology, 1996, Vol. 78, pp [6] Culcas J.D., Sere P.R., Elsner C.I..: Control of the growth of znc ron phases n the hot-dp galvanzng process, Surface and Coatng Technology, 1999, Vol. 122, pp [7] Lynch R.F.: Hot-Dp Galvanzng Alloys, Journal of Metals, 1987 Aug., pp [8] Reumont G., Glorant T., Perrot P.: Expermental nfluence of the knetcs on galvanzed coatngs when saturatng a znc bath wth alloyng elements, Journal of materals scence letters, 1996, Vol. 15, pp [9] Jaycock M.J., Parftt G.D.: Chemstry of Interfaces, Wley, NY, 1981, pp [10] Grfalco L.A., Good R.J.: a theory for the estmaton of surface and nterfacal energes. I. Dervaton and applcaton to nterfacal tenson, Journal of physcal chemstry, 1957, Vol.61, pp [11] Hajra J. P., Hong-Kee-Lee, Frohberg M.G.: Calculaton of the surface tenson of lqud bnary systems from the data of pure components and the thermodynamc nfnte dluton values, Z. Metallkd., 1991, Vol. 82 (8), pp [12] Hultgren R., Desa P.D., Hawkns D.T., Gleser M. Kelly K.K.: Selected values of thermodynamc propertes of bnary alloys, ASM, Metals Park, Oho, [13] Parads P-F., Ishkawa T.: Thermophyscal propertes of molten Yttrum measured by noncontact technques, Mcrogravty scence technology, 2009, Vol. 21, pp Proceedngs of EMC 2009