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Behaviour of coated steel sheets in forming processes
ELSEVIER Journal of Materials Processing Technology 53 (1995) 167-176
Journal of Materials Processing Technology
B e h a v i o u r o f c o a t e d steel sheets in f o r m i n g p r o c e s s e s J.Z.Gronostajski Institute of Mechanical Engineering and Automation, Technical University of Wroctaw, ul. Lukasiewicza 3/5, 50-371 Wroclaw, Poland
1. INTRODUCTION The demand for good corrosion protection in wide variety of finished products has resulted in the increasing use of various kinds of coated steel sheets [l-4] manufactured according to three processes: - electrogalvanized coating - hot dip galvanized coating - roller coating. Usually, the coatings are made of the following materials: pure zinc, alloyed zinc and plastic. A surface of coatings is essential not only for the protection, but also for the decoration, of a very wide range of products. The forming processes of coated steel sheets are limited by the similar phenomena as arise during the deformation of uncoated steel sheets, namely plastic instability, fracture, wrinkling caused by inappropriate mechanical properties, and additionally by the other phenomena like as: formation of cracks within the coating, lack of adherence between the base steel and the coating and powdering and abrasion of coating. That is the reason, that special attention has to be paid to the influence of the deformation conditions on the performance of the coating, as regards protection against corrosion. The adhesion of the coatings - either metallic or organic must remain good, and the surface should not be damaged during forming. When the coating begins to crack, the main property of coated steel sheets, i.e., their corrosion resistance, starts to deteriorate due to the exposure of the base steel to the atmosphere. Generally the application of coated steel sheets in press forming raises three main questions: the formability of the coated steel sheets, the adherence of the coating during the forming operations, and the effect of the forming on the behaviour of the coating. The paper gives the answer to above questions.
2. EXPERIMENTAL PROCEDURE The results discribed in the paper were obtained on the base of investigation of following types of coated steel sheets [5-7]: zinc electrogalvanized steel sheet (ZEGS), zinc hot-dip galvanized steel sheet (ZHDGS), polyethylene coated zinc hot-dip galvanized steel sheet (PCZHDGS), acryl coated zinc hot-dip galvanized steel sheet (ACZHDGS) and PVC coated zinc hot-dip galvanized steel sheet (PVCCZHDGS). Elsevier Science S.A. 0924-0136(95)01973-I
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The thickness of the coatings and of the sheets is given in Table 1 and the chemical composition of the steels in Table 2. TABLE 1 The thickness of the coatings and the total thickness of the coated steel sheets Thickness of Material Sheet Zinc Organic Intermetallic layer coating layer (mm) (~tm) (jam) (pm) ZEGS 1.03 10 ZHDGS 0.90 28 3.1 0.95 26 60 3.5 PCZI-IDGS 1.05 23 400 3.1 ACZHI)GS 0.93 20 200 3.0 PVCCZHDGS TABLE 2 Chemical composition of the steels investi ~ated Material C [Mn I Si ] P [s weight % ZEGS 0.08 0.27 0.03 0.025 0.05 ZHDGS 0.08 0.35 0.01 0.025 0.05 PCZHDGS 0.09 0.37 0.01 0.028 0.03 0.08 0.36 0.02 0.035 0.02 ACZHDGS PVCCZHDGS 0.09 0.44 0.01 0.035 0.02
[~d 0.006 0.007 0.008 0.004 0.006
INi ICu 0.07 0.08 0.05 0.10 0.10
0.07 0.08 0.05 0.10 0.10
The forming limit diagrams were made by using specimens of varying width, to secure all the types of strain paths occurring in practice during a pressing operations. The FLDs were determined for simple strain paths (SSPs) and complex strain paths (CSPs), the latter consisting of two different linear stages of deformation. In the first stage the specimens were tensioned uniaxially to approximately e 1 = -0.06, or deformed equibiaxially to about e 1 = e 2 = 0.06, whilst in the second stage the strain paths were changed from uniaxial to nearly equibiaxial deformation. During the first stage of deformation in-plane conditions existed and in the second stage of deformation a hemispherical punch, 1 I0 mm diameter, was used. The methods used for strain analysis and plotting of the FLDs have been described elsewhere [5]. The organic coated sheet steels were investigated in three various conditions: first, with the organic coatings, nex't with a zinc layer after the removal of the organic coatings, and finally, after the organic and zinc coatings had been removed, so that the base steels were in contact with the punch. The friction coefficients between various surface layers and the steel tool were measured by using equipment similar to that one described elsewhere [8]. The study was performed without additional lubricant, but the soft zinc coating and the organic layers can be assumed to act as solid lubricant. The methods of determination of cracking limit curves, flaking limit curves and corrosion resistance of investigated steel sheets were described in papers [6, 9 and 10].
169
J.Z. Gronostajski I Journal of Materials Processing Technology 53 (1995) 167-176
3. RESULTS AND DISCUSSION Comparison o f the FLDs of the zinc hot-dip galvartized steel sheet and of the zinc electrogalvanized steel sheet for linear and complex strain paths can be seen in Fig. 1. Formability of zinc electrogalvanized steel sheet for all strain paths used in the study is much better than formability of zinc hot-dip galvanized steel sheets. The differences are mainly caused by presence of a brittle iron-zinc alloy layer between the zinc coating and the steel. The effect of the zinc coating obtained by hot-dip method and the organic coatings on the FLDs related to the FLDs of base steel sheets is presented in Fig.2 and in paper [6]. Curves were determined for each of the above mentioned three conditions. Generally, the presence of coatings improves the formability of the sheets, but the improvement depends on the type of coating material. It can be noted that the PVC layer exerts a more distinct effect on the position of FLC than do polyethylene coatings, .whilst the smallest influence is exerted by acryl. For the complex strain paths the differences in the FLCs between coated steel and bare steel are more distinct when the final stage involves biaxial stretching than when it involves uniaxial tension: this is b~cause the area of contact between the punch and the deformed sheet, as well as the contact pressure, are greater during bia_xial than uniaxial deformation. As a consequence, the effect of friction due to different surface conditions becomes more significant.
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I 0,0E1
I
/ -
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[
-O,~5-O,~O-O,OS 0 0,05 0,~0 ~2 O,:2O
Figure 1. FLDs of the ZHDGS and ZEGS Figure 2. FLDs of the PCZHDGS for simple for simple and complex strain paths and complex strain paths Compared to steel, the coatings are soft and, consequently, their shear strength is low. Hence, they can be assumed to act as solid lubricants in the forming processes, with a large thickness as compared to the surface roughness of the bare metal The analysis of the FLDs
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common with the results of fi-iction tests (Table 3) establish that the position of the FLCs increases when the friction between the punch and the formed sheet decreases, which is in agreement with the results reported by Weidemann [11], Blanchard [12] and Gerdeen [13]. Weidemann investigated the drawability of coil coated steels and found that some of the coated steels, e.g., coated with PVC, can be drawn to a greater drawing ratio than uncoated steels, due to the reduction of fi'iction between the tool and the sheet surface. TABLE 3 The coefficients of friction between the sheet and the tool Coefficient of friction Material Plastic coating Zinc-tool Steel-tool -tool ZEGS 0.18 0.20 ZHDGS 0.16 0.20 PCZHDGS 0. l 1 0.16 0.19 ACZHDGS 0.13 0.17 0.20 0.09 0.19 PVCCZHDGS 0.15 Blanchard [12] concluded that the reduction in sheet-tool friction caused by different types of zinc coating made a noticeable contribution to the increase in the performance of the metal in press forming. Veerman [14] also reported different FLCs for different lubricants, showing that the FLC increasing friction. The opinion is advanced in some reports that modification of the friction conditions between the tool and the sheet metal changes only the strain paths, and has no direct effect on the FLCs. In such analysis one additional factor must be taken into account, which is that the modification of the friction conditions changes not only the strain path but also the strain gradient in the necking area. It can be concluded that the one of the reasons for variation of formability with the type of surface conditions between the tool and the formed sheet is associated with the change of the strain gradient. The effect of the complex strain paths of the coated steel sheets on the FLDs of electrogalvanized and hot-dip galvanized steel sheets is illustrated in Fig.2 and that for the other sheets are shown in papers [5, 6]. The FLDs for the unix,dally prestrained specimens are displaced in the direction of negative minor strain and higher major strain, whilst those for equibiaxially prestrained specimens are displaced in the direction of positive minor strain and lower major strain.. This is of great importance to the press-forming industry, as it allows the formability of the material to be increased by controlling the strain paths. The effect of the CSPs on the FLCs of coated steel sheets is in agreement with the data reported in the literature [15,16] for uncoated steel sheets. The effective work-hardening rates of the steel sheet under biaxial loading are greater than those under uniaxial loading: the strain softening which occurs due to the change in strain path from biaxial to uniaxial, therefore produces a rapid loss in the ability of the bia,'dally prestrained steel to undergo uniform deformation. The effect of changing of the strain path from uniaxial to equibiaxial on the limit strain can be explained inversely. The cracking (CLCs) and flaking limit curves (FkLCs) in comparison with the FLCs for zinc electrogalvanized and zinc hot-dip galvanized steel sheets are shown in Figs 3 and 4, and those for other sheets are given in papers [10].
J.Z. Gronostajski / Journal of Materials Processing Technology 53 (1995) 167-176
171
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•
"-,
CLC,
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o,o5 .-q~s-o,~o-qos o qos o,lo e2 o~
Fig.3. CLC, FkLC and FLC of the ZEGS for Figure 4. CLC, FkLC and FLC of the simple and complex strain paths ZHDGS for simple and complex strain paths The highest value of cracking strains has the electrogalvanized zinc coating. The electrogalvanized coating has a constant chemical composition over their whole thickness nearly the constant thickness and small grain size, what is a reason, which in negative minor strain region makes coating follows deformation of the steel core without cracking. In the positive minor strain region the above mentioned properties of electrogalvanized coating are not sufficient to obtain the same results and the cracking limit curves of electrogalvanized zinc coating lie below the FLCs but much higher than CLCs of zinc hot-dip galvanized coating. The cracking of the zinc layer depends on the strain ratio. For both linear and complex strain paths, the higher is the value ofe2/e I ratio the lower is the value of the major cracking strain of zinc hot-dip galvanized steel sheets and the larger are discrepancies between FLCs and CLCs of zinc electrogalvanized steel sheets. This shows that bia.'dal stretching is an unfavorable forming mode for hot-dip galvanized and electrogalvanized steel sheets and that uniaxial tension seems to be recommendable. It have been concluded that for hot-dip galvanized steel sheets the greater is thickness of zinc layer in the range of 20 - 28 l.tm the higher is the position of the cracking limit cu~'es at the same strain paths. Cracking of the zinc coating and the growth of the cracks during deformation were analysed in terms of the formability of the coating. The area fraction of the cracks (fc) as a function of the thickness strain (es) for linear-and complex-strain paths is shown in Figs 5 and 6 respectively. The lines in each of these figures have been plotted using a simple model suggested by Ma.kimattila and Ranta-Eskola [ 17]. In the model it is assumed that if the coating is rigid the area fraction of the cracks is determined by the area increase due to deformation (solid lines). The formability index of zinc layer (k) was defined by the equation below.
J.Z. Gronostajsla" I Journal of Materials Processing Technology 53 (1995) 167-176
172
.fc
= ] -
exp
(l)
[(l -/Oej]
A value of k = 0 means that the coating does not deform plastically, whilst for k = 1 the coating deforms in the same manner as does the base steel (i.e., without cracking). The values of k at simple and complex strain paths for zinc electrogalvanized steel sheet are much higher than those for all kinds of hot-dip galvanized steel sheets [18]. The k values are greater for uniaxially deformed or predeformed specimens than that for equibiaxially deformed or predeformed specimens. This means that the formability of the zinc coating for equibiaxial deformation is lower than that for uniaxial deformation, which is in agreement with the conclusions drawn from the cracking limit curves presented in Figs 3 and 4. Schedin et al [19] for zinc hot-dip galvanized steel sheet obtained the similar results o f k = 0,8 as an average value for uniaxial tension, plane strain and equibiaxial stretching.
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Figure 5. Area fraction of the cracks of the Figure 6. Area fraction of the cracks of the zinc zinc layer as a function of the thickness strain layer as a function of the thickness strain of the PCZHDGS for complex strain paths of the PCZHDGS for simple strain paths The cracking usually is initiated within a brittle intermetallic layer, from where it propagates mostly to the surface. Early in the forming stage only intercrystalline cracks were formed for all forming strain paths, and for larger equibiaxial and plane deformation transcrystalline cracks were created also. According to Engberg at all [20] intercrystalline cracking in hot-dip galvanized zinc coating is associated to accumulation of oxygen due to grain boundary diffusion, but lead appearing as a thin film in interdendritic areas due to segregation during solidification is responsible for transcrystalline cracking. The differences of the intermetallic layer thickness (Table 1) of various coated steel sheets are very small, and probably can be neglected in the analysis of the effect of the layer thickness on the cracking phenomena of zinc coating.
J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
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x
~ ~
°
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Figure 7. Cracks in the zinc coating of an Figure 8. Cracks in the zinc coating of an ACZHDGS specimen uniaxially deformed to ACZHDGS specimen equibi&xially deformed ee=0.42 to ee=0.42 When the major strain was much higher than the minor strain, i.e., for uniaxial strain, the cracks were oriented mainly perpendicularly to the major strain (Fig7), but for nearly the same values of major and minor strain as those of equibiaxial stretching, the orientation of cracks is random (Fig.8). When uniaxially predeformed specimens are subjected to uniaxial or plane strain, the effect of prestrain dominates and the majority of the cracks are perpendicular to the major strain direction. The dominating effect of prestrain was not visible when in the second step the specimens were equibiaxially deformed. When specimens were equibiaxially prestrained, the predeformation, regardless of the second strain paths, has a dominating effect on the orientation of the cracks, which are usually randomly oriented. However, in the case of specimens uniaxially and plane strained in the second stage, a somewhat greater number of cracks oriented perpendicularly to the major strain could be seen. The differences in the mechanical properties between the coating and the base steel are mainly responsible for the shear stress at the contact surface. When the shear stress being higher than the force of adhesion between the two layers the coating will be subjected to flaking at the interface with the steel. Flaking of the zinc coating occurs at almost the same strain level as does the limit strain of the base steels (Fig3 and 4), the small discrepancies between them growing significantly with increase in the value of the e2/e 1 ratio, over the whole range of linear strain paths and in the second stage of deformation for complex strain paths. Good agreement of the FLCs and FkLCs means that the adhesive forces between the zinc layer and the steel are high. The behaviour of zinc electrogalvanized steel sheet in the alternate immersion corrosion test after 720 h, for linear strain paths is illustrated in Fig.9 and for complex strain paths in Fig. 10. The corrosion resistance of the other coated steel sheets described in Tables 1 and 2 is presented in paper [9, 18].
J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
174
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iA,r ,. numZ~" ,
ll
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I ~
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I
0,5 F
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0,1
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Figure 9. The effect of simple strain path on Fig. 10.The effect of complex strain path on the corrosion resistance of the ZEGS the corrosion resistance of the ZEGS In the figures, the forming limit curves of the bare steel sheets and the flaking-limit curves and cracking-limit curves of the coatings are shown also. The percentage of red rust varies, depending on the level of deformation and on the strain paths. The specimens deformed by unia.xial tension are characterized by a higher corrosion resistance as compared with the specimens strained by equibiaxial stretching (Fig.9). However, for plane strain the corrosion resistance is intermediate between that for uniaxial and that for equibiaxial deformation. Moreover, over the whole range of deformation, the amount of red rust increases with increase in the major strain. For the complex strain paths the corrosion resistance is lower in equibiaxially prestrained than in uniaxially prestrained specimens (Fig. I0). Different results were obtained by Monford and Bragard [21] during investigation of zinc hot-dip coated and electrogalvanized steel sheets. They stated, that slight straining of hot-dip coated steels decreases the corrosion resistance, and further straining - up to the forming limit curve - has no additional effect, but for electrogalvanized zinc coated steels the straining has no significant effect on the corrosion resistance. It can be noted that the corrosion resistance depends on the type of material involved. Thus, the amount of red rust determined for the PVC coated zinc hot-dip galvanized steel is greater than that for the acryl coated zinc hot dip galvanized steel, that for the latter being greater than that for polyethylene coated zinc hot-dip galvanized steel [9]. This phenomenon should be attributed primarily to the slightly different thickness of the
J.Z. Gronostajski / Journal of Materials Processing Technology 53 (1995) 167-176
175
zinc layers (Table 1) and is due to the greater heterogeneity of the zinc layers (observed by SEM) in the PVC coated steel than that in the acryl and polyethylene coated steel sheets [22]. Analysis of the results obtained has shown that the corrosion resistance is influenced not only by different thickness and heterogeneity of the zinc layer but also by cracking and flaking of the zinc layer. When cracks are small steel sheets are protected by the cathode protection of zinc. 4. CONCLUSIONS The main conclusions of the investigations can be summerised as follows: 1. The type of the coating influences the position and shape of the FLCs obtained for linear and complex strain paths. The greatest limit strain was obtained for zinc electrogalvanized steel sheet, and for such coating of hot-dip galvanized steel sheets that had the lowest coefficient of friction against the punch. 2. Deformation along complex strain paths with uniaxial tensile prestrain causes the growth of the FLCs of coated steel sheets whilst equibiaxial prestrain brings decrease of the FLCs, as compared with those obtained for linear strain paths. The effect of the complex strain path on the FLCs of coated steel sheets is in agreement with that for uncoated steel sheets. 3. The cracking limit curves of the zinc coating are influenced by the strain paths. For linear strain paths, the discrepancies between the CLCs and the FLCs reduce as the strain ratio e2/e 1 decreases. For complex strain paths, uniaxially prestrained specimens have a higher cracking limit than those which were equibiaxially prestrained. 4. The flaking limit curves of the zinc coating are almost the same as the FLCs when the e2/el ratio is changed from -1/2 to 0, in the range from 0 to 1 the discrepancies between the FkLC and the FLC reduce as the strain ratio 62/81 decreases. 5. The cracks in the zinc layer are nucleated mainly as intercrystalline and sometimes as transcrystalline. When the major principal strain is much greater than the minor principal strain, the cracks were oriented usually perpendicularly to the major strain direction. 6. For linear and complex strain paths the corrosion resistance decreases when the strain ratio e2/e 1 increases. Additionaly the corrosion resistance is effected by thickness of the zinc layer and of its heterogeneity, and is correlated with the cracking of zinc layer described by formability coefficient. REFERENCES 1. G.Arrigoni and M.Sarracino, Proc. Int. Deep Drawing Res. Group Meeting. Amsterdam, 1985, paper 12. 2. DPerry, A.Jussiaume and Y. Deleon, Proc. 15th Biennial Congress of Int. Deep Drawing Res. Group, Dearbon, 1988, ! 3. F.J.Flossdorf, Proc. Int. Deep Drawing Res. Group Meeting, Toronto, 1988, (Rapport of German Group). 4. Y.Hishida and M.Yoshida, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange, 1990, 173. 5. JZ.Gronostajski, W.J.AIi and M.S.Ghattas, Advanced Technology of Plasticity, Springer Verlag, I (1987), 423. 6. JZ.Gronostajski, W.JAli and M.S.Ghattas, J. Mater. Process. Technol., 22 (1990), 137. 7 JZ.Gronostajski, ZJ.Gronostajski and ZZimniak, Obr6bka Plastyczna, 2 (1991), 5. 8. J.Z.Gronostajski and M.Sulonen. Zesz. Nauk. AGH. Mechanika. 9, (1986), 269. 9. JZ.Gronostajski, W.JAli and M.S.Ghattas, J. Mater. Process. Technol., 23 (1990), 21.
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10. J.Z.Gronostajski, W.J.Ali and M.S.Ghattas, J. Mater. Process. Technol.,23 (1990), 321. 11. Ch.Weidemann, Mem. Sci. Revue Metall, 77 (1980), 343. 12. G.Blanchard, Proc. Int. Deep Drawing Res. Group Meeting, Amsterdam, 1985, p. 18 13. J.C.Gerdeen and B.A.EI-Jan, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange, 1990, 131. 14. C.C.Veerman, Sheet Metal Ind., 48 (1972), 351. 15. A.Melander, E. Schedin and L.Gustavsson, Report Swed. Inst. Met. Res.1983, 1 16. J.Z.Gronostajski, A.Dolny and T.Sobis, Proc.12th Biennial Congress of Int. Deep Drawing Res. Group, S.Margherita, 1982, 39. 17. S.Makimattila and A.J.Ranta-Eskola, Proc. 13th Biennial Congress Int. Deep Drawing Res. Group, Melbourne 1984, 293. 18. J.Z.Gronostajski: Formability, damage and corrosion resistance of coated steel sheets. In Materials Processing Defects Ed. by SK.Ghosh and MPredeteanu 19 E.Schedin, S.Karlsson and A.Melander, Proc. 14th Biennial Congress of Int. Deep Drawing Res. Group., KOln 1986, 460. 20 G.Engberg, A.Haglund and S.EHomstrom , Proc. 16th Biennial Congress Int. Deep Drawing Res. Group, Borlange, 1990, 131. 21. G.Monford and A.Bragard, Proc. Int. Deep Drawing Pres. Group Meeting, Amsterdam 1985, paperl3. 22. JZ.Gronostajski, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange 1990, 233.
Journal of Materials Processing Technology
B e h a v i o u r o f c o a t e d steel sheets in f o r m i n g p r o c e s s e s J.Z.Gronostajski Institute of Mechanical Engineering and Automation, Technical University of Wroctaw, ul. Lukasiewicza 3/5, 50-371 Wroclaw, Poland
1. INTRODUCTION The demand for good corrosion protection in wide variety of finished products has resulted in the increasing use of various kinds of coated steel sheets [l-4] manufactured according to three processes: - electrogalvanized coating - hot dip galvanized coating - roller coating. Usually, the coatings are made of the following materials: pure zinc, alloyed zinc and plastic. A surface of coatings is essential not only for the protection, but also for the decoration, of a very wide range of products. The forming processes of coated steel sheets are limited by the similar phenomena as arise during the deformation of uncoated steel sheets, namely plastic instability, fracture, wrinkling caused by inappropriate mechanical properties, and additionally by the other phenomena like as: formation of cracks within the coating, lack of adherence between the base steel and the coating and powdering and abrasion of coating. That is the reason, that special attention has to be paid to the influence of the deformation conditions on the performance of the coating, as regards protection against corrosion. The adhesion of the coatings - either metallic or organic must remain good, and the surface should not be damaged during forming. When the coating begins to crack, the main property of coated steel sheets, i.e., their corrosion resistance, starts to deteriorate due to the exposure of the base steel to the atmosphere. Generally the application of coated steel sheets in press forming raises three main questions: the formability of the coated steel sheets, the adherence of the coating during the forming operations, and the effect of the forming on the behaviour of the coating. The paper gives the answer to above questions.
2. EXPERIMENTAL PROCEDURE The results discribed in the paper were obtained on the base of investigation of following types of coated steel sheets [5-7]: zinc electrogalvanized steel sheet (ZEGS), zinc hot-dip galvanized steel sheet (ZHDGS), polyethylene coated zinc hot-dip galvanized steel sheet (PCZHDGS), acryl coated zinc hot-dip galvanized steel sheet (ACZHDGS) and PVC coated zinc hot-dip galvanized steel sheet (PVCCZHDGS). Elsevier Science S.A. 0924-0136(95)01973-I
SSDI
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J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
The thickness of the coatings and of the sheets is given in Table 1 and the chemical composition of the steels in Table 2. TABLE 1 The thickness of the coatings and the total thickness of the coated steel sheets Thickness of Material Sheet Zinc Organic Intermetallic layer coating layer (mm) (~tm) (jam) (pm) ZEGS 1.03 10 ZHDGS 0.90 28 3.1 0.95 26 60 3.5 PCZI-IDGS 1.05 23 400 3.1 ACZHI)GS 0.93 20 200 3.0 PVCCZHDGS TABLE 2 Chemical composition of the steels investi ~ated Material C [Mn I Si ] P [s weight % ZEGS 0.08 0.27 0.03 0.025 0.05 ZHDGS 0.08 0.35 0.01 0.025 0.05 PCZHDGS 0.09 0.37 0.01 0.028 0.03 0.08 0.36 0.02 0.035 0.02 ACZHDGS PVCCZHDGS 0.09 0.44 0.01 0.035 0.02
[~d 0.006 0.007 0.008 0.004 0.006
INi ICu 0.07 0.08 0.05 0.10 0.10
0.07 0.08 0.05 0.10 0.10
The forming limit diagrams were made by using specimens of varying width, to secure all the types of strain paths occurring in practice during a pressing operations. The FLDs were determined for simple strain paths (SSPs) and complex strain paths (CSPs), the latter consisting of two different linear stages of deformation. In the first stage the specimens were tensioned uniaxially to approximately e 1 = -0.06, or deformed equibiaxially to about e 1 = e 2 = 0.06, whilst in the second stage the strain paths were changed from uniaxial to nearly equibiaxial deformation. During the first stage of deformation in-plane conditions existed and in the second stage of deformation a hemispherical punch, 1 I0 mm diameter, was used. The methods used for strain analysis and plotting of the FLDs have been described elsewhere [5]. The organic coated sheet steels were investigated in three various conditions: first, with the organic coatings, nex't with a zinc layer after the removal of the organic coatings, and finally, after the organic and zinc coatings had been removed, so that the base steels were in contact with the punch. The friction coefficients between various surface layers and the steel tool were measured by using equipment similar to that one described elsewhere [8]. The study was performed without additional lubricant, but the soft zinc coating and the organic layers can be assumed to act as solid lubricant. The methods of determination of cracking limit curves, flaking limit curves and corrosion resistance of investigated steel sheets were described in papers [6, 9 and 10].
169
J.Z. Gronostajski I Journal of Materials Processing Technology 53 (1995) 167-176
3. RESULTS AND DISCUSSION Comparison o f the FLDs of the zinc hot-dip galvartized steel sheet and of the zinc electrogalvanized steel sheet for linear and complex strain paths can be seen in Fig. 1. Formability of zinc electrogalvanized steel sheet for all strain paths used in the study is much better than formability of zinc hot-dip galvanized steel sheets. The differences are mainly caused by presence of a brittle iron-zinc alloy layer between the zinc coating and the steel. The effect of the zinc coating obtained by hot-dip method and the organic coatings on the FLDs related to the FLDs of base steel sheets is presented in Fig.2 and in paper [6]. Curves were determined for each of the above mentioned three conditions. Generally, the presence of coatings improves the formability of the sheets, but the improvement depends on the type of coating material. It can be noted that the PVC layer exerts a more distinct effect on the position of FLC than do polyethylene coatings, .whilst the smallest influence is exerted by acryl. For the complex strain paths the differences in the FLCs between coated steel and bare steel are more distinct when the final stage involves biaxial stretching than when it involves uniaxial tension: this is b~cause the area of contact between the punch and the deformed sheet, as well as the contact pressure, are greater during bia_xial than uniaxial deformation. As a consequence, the effect of friction due to different surface conditions becomes more significant.
I
o,ss teCToQ~tv.
\X \ X!
I 0,0E1
I
/ -
~\ x, / k \. 'J t/''~7 ---zinc
, sfeel ]'05i -0,15-0:10-0,05 0 0,05 0.10 E2 0,2.0
[
-O,~5-O,~O-O,OS 0 0,05 0,~0 ~2 O,:2O
Figure 1. FLDs of the ZHDGS and ZEGS Figure 2. FLDs of the PCZHDGS for simple for simple and complex strain paths and complex strain paths Compared to steel, the coatings are soft and, consequently, their shear strength is low. Hence, they can be assumed to act as solid lubricants in the forming processes, with a large thickness as compared to the surface roughness of the bare metal The analysis of the FLDs
170
J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
common with the results of fi-iction tests (Table 3) establish that the position of the FLCs increases when the friction between the punch and the formed sheet decreases, which is in agreement with the results reported by Weidemann [11], Blanchard [12] and Gerdeen [13]. Weidemann investigated the drawability of coil coated steels and found that some of the coated steels, e.g., coated with PVC, can be drawn to a greater drawing ratio than uncoated steels, due to the reduction of fi'iction between the tool and the sheet surface. TABLE 3 The coefficients of friction between the sheet and the tool Coefficient of friction Material Plastic coating Zinc-tool Steel-tool -tool ZEGS 0.18 0.20 ZHDGS 0.16 0.20 PCZHDGS 0. l 1 0.16 0.19 ACZHDGS 0.13 0.17 0.20 0.09 0.19 PVCCZHDGS 0.15 Blanchard [12] concluded that the reduction in sheet-tool friction caused by different types of zinc coating made a noticeable contribution to the increase in the performance of the metal in press forming. Veerman [14] also reported different FLCs for different lubricants, showing that the FLC increasing friction. The opinion is advanced in some reports that modification of the friction conditions between the tool and the sheet metal changes only the strain paths, and has no direct effect on the FLCs. In such analysis one additional factor must be taken into account, which is that the modification of the friction conditions changes not only the strain path but also the strain gradient in the necking area. It can be concluded that the one of the reasons for variation of formability with the type of surface conditions between the tool and the formed sheet is associated with the change of the strain gradient. The effect of the complex strain paths of the coated steel sheets on the FLDs of electrogalvanized and hot-dip galvanized steel sheets is illustrated in Fig.2 and that for the other sheets are shown in papers [5, 6]. The FLDs for the unix,dally prestrained specimens are displaced in the direction of negative minor strain and higher major strain, whilst those for equibiaxially prestrained specimens are displaced in the direction of positive minor strain and lower major strain.. This is of great importance to the press-forming industry, as it allows the formability of the material to be increased by controlling the strain paths. The effect of the CSPs on the FLCs of coated steel sheets is in agreement with the data reported in the literature [15,16] for uncoated steel sheets. The effective work-hardening rates of the steel sheet under biaxial loading are greater than those under uniaxial loading: the strain softening which occurs due to the change in strain path from biaxial to uniaxial, therefore produces a rapid loss in the ability of the bia,'dally prestrained steel to undergo uniform deformation. The effect of changing of the strain path from uniaxial to equibiaxial on the limit strain can be explained inversely. The cracking (CLCs) and flaking limit curves (FkLCs) in comparison with the FLCs for zinc electrogalvanized and zinc hot-dip galvanized steel sheets are shown in Figs 3 and 4, and those for other sheets are given in papers [10].
J.Z. Gronostajski / Journal of Materials Processing Technology 53 (1995) 167-176
171
!El
•
"-,
CLC,
II°, ~s o,lo o#5 o qos eOo Ez 02o
o,o5 .-q~s-o,~o-qos o qos o,lo e2 o~
Fig.3. CLC, FkLC and FLC of the ZEGS for Figure 4. CLC, FkLC and FLC of the simple and complex strain paths ZHDGS for simple and complex strain paths The highest value of cracking strains has the electrogalvanized zinc coating. The electrogalvanized coating has a constant chemical composition over their whole thickness nearly the constant thickness and small grain size, what is a reason, which in negative minor strain region makes coating follows deformation of the steel core without cracking. In the positive minor strain region the above mentioned properties of electrogalvanized coating are not sufficient to obtain the same results and the cracking limit curves of electrogalvanized zinc coating lie below the FLCs but much higher than CLCs of zinc hot-dip galvanized coating. The cracking of the zinc layer depends on the strain ratio. For both linear and complex strain paths, the higher is the value ofe2/e I ratio the lower is the value of the major cracking strain of zinc hot-dip galvanized steel sheets and the larger are discrepancies between FLCs and CLCs of zinc electrogalvanized steel sheets. This shows that bia.'dal stretching is an unfavorable forming mode for hot-dip galvanized and electrogalvanized steel sheets and that uniaxial tension seems to be recommendable. It have been concluded that for hot-dip galvanized steel sheets the greater is thickness of zinc layer in the range of 20 - 28 l.tm the higher is the position of the cracking limit cu~'es at the same strain paths. Cracking of the zinc coating and the growth of the cracks during deformation were analysed in terms of the formability of the coating. The area fraction of the cracks (fc) as a function of the thickness strain (es) for linear-and complex-strain paths is shown in Figs 5 and 6 respectively. The lines in each of these figures have been plotted using a simple model suggested by Ma.kimattila and Ranta-Eskola [ 17]. In the model it is assumed that if the coating is rigid the area fraction of the cracks is determined by the area increase due to deformation (solid lines). The formability index of zinc layer (k) was defined by the equation below.
J.Z. Gronostajsla" I Journal of Materials Processing Technology 53 (1995) 167-176
172
.fc
= ] -
exp
(l)
[(l -/Oej]
A value of k = 0 means that the coating does not deform plastically, whilst for k = 1 the coating deforms in the same manner as does the base steel (i.e., without cracking). The values of k at simple and complex strain paths for zinc electrogalvanized steel sheet are much higher than those for all kinds of hot-dip galvanized steel sheets [18]. The k values are greater for uniaxially deformed or predeformed specimens than that for equibiaxially deformed or predeformed specimens. This means that the formability of the zinc coating for equibiaxial deformation is lower than that for uniaxial deformation, which is in agreement with the conclusions drawn from the cracking limit curves presented in Figs 3 and 4. Schedin et al [19] for zinc hot-dip galvanized steel sheet obtained the similar results o f k = 0,8 as an average value for uniaxial tension, plane strain and equibiaxial stretching.
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Figure 5. Area fraction of the cracks of the Figure 6. Area fraction of the cracks of the zinc zinc layer as a function of the thickness strain layer as a function of the thickness strain of the PCZHDGS for complex strain paths of the PCZHDGS for simple strain paths The cracking usually is initiated within a brittle intermetallic layer, from where it propagates mostly to the surface. Early in the forming stage only intercrystalline cracks were formed for all forming strain paths, and for larger equibiaxial and plane deformation transcrystalline cracks were created also. According to Engberg at all [20] intercrystalline cracking in hot-dip galvanized zinc coating is associated to accumulation of oxygen due to grain boundary diffusion, but lead appearing as a thin film in interdendritic areas due to segregation during solidification is responsible for transcrystalline cracking. The differences of the intermetallic layer thickness (Table 1) of various coated steel sheets are very small, and probably can be neglected in the analysis of the effect of the layer thickness on the cracking phenomena of zinc coating.
J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
'
";"
x
~ ~
°
"
'
"o
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4,
Figure 7. Cracks in the zinc coating of an Figure 8. Cracks in the zinc coating of an ACZHDGS specimen uniaxially deformed to ACZHDGS specimen equibi&xially deformed ee=0.42 to ee=0.42 When the major strain was much higher than the minor strain, i.e., for uniaxial strain, the cracks were oriented mainly perpendicularly to the major strain (Fig7), but for nearly the same values of major and minor strain as those of equibiaxial stretching, the orientation of cracks is random (Fig.8). When uniaxially predeformed specimens are subjected to uniaxial or plane strain, the effect of prestrain dominates and the majority of the cracks are perpendicular to the major strain direction. The dominating effect of prestrain was not visible when in the second step the specimens were equibiaxially deformed. When specimens were equibiaxially prestrained, the predeformation, regardless of the second strain paths, has a dominating effect on the orientation of the cracks, which are usually randomly oriented. However, in the case of specimens uniaxially and plane strained in the second stage, a somewhat greater number of cracks oriented perpendicularly to the major strain could be seen. The differences in the mechanical properties between the coating and the base steel are mainly responsible for the shear stress at the contact surface. When the shear stress being higher than the force of adhesion between the two layers the coating will be subjected to flaking at the interface with the steel. Flaking of the zinc coating occurs at almost the same strain level as does the limit strain of the base steels (Fig3 and 4), the small discrepancies between them growing significantly with increase in the value of the e2/e 1 ratio, over the whole range of linear strain paths and in the second stage of deformation for complex strain paths. Good agreement of the FLCs and FkLCs means that the adhesive forces between the zinc layer and the steel are high. The behaviour of zinc electrogalvanized steel sheet in the alternate immersion corrosion test after 720 h, for linear strain paths is illustrated in Fig.9 and for complex strain paths in Fig. 10. The corrosion resistance of the other coated steel sheets described in Tables 1 and 2 is presented in paper [9, 18].
J.Z. Gronostajski /Journal of Materials Processing Technology 53 (1995) 167-176
174
o.6 ~"1 ~
iA,r ,. numZ~" ,
ll
rust ~',
I ~
50
I
0,5 F
O,t.r-0,3 0,2
0,1
-0,2
-0,1
0
0,1 •2 0,2-0,2
-0,1
0
0,1 Ez 0,2
Figure 9. The effect of simple strain path on Fig. 10.The effect of complex strain path on the corrosion resistance of the ZEGS the corrosion resistance of the ZEGS In the figures, the forming limit curves of the bare steel sheets and the flaking-limit curves and cracking-limit curves of the coatings are shown also. The percentage of red rust varies, depending on the level of deformation and on the strain paths. The specimens deformed by unia.xial tension are characterized by a higher corrosion resistance as compared with the specimens strained by equibiaxial stretching (Fig.9). However, for plane strain the corrosion resistance is intermediate between that for uniaxial and that for equibiaxial deformation. Moreover, over the whole range of deformation, the amount of red rust increases with increase in the major strain. For the complex strain paths the corrosion resistance is lower in equibiaxially prestrained than in uniaxially prestrained specimens (Fig. I0). Different results were obtained by Monford and Bragard [21] during investigation of zinc hot-dip coated and electrogalvanized steel sheets. They stated, that slight straining of hot-dip coated steels decreases the corrosion resistance, and further straining - up to the forming limit curve - has no additional effect, but for electrogalvanized zinc coated steels the straining has no significant effect on the corrosion resistance. It can be noted that the corrosion resistance depends on the type of material involved. Thus, the amount of red rust determined for the PVC coated zinc hot-dip galvanized steel is greater than that for the acryl coated zinc hot dip galvanized steel, that for the latter being greater than that for polyethylene coated zinc hot-dip galvanized steel [9]. This phenomenon should be attributed primarily to the slightly different thickness of the
J.Z. Gronostajski / Journal of Materials Processing Technology 53 (1995) 167-176
175
zinc layers (Table 1) and is due to the greater heterogeneity of the zinc layers (observed by SEM) in the PVC coated steel than that in the acryl and polyethylene coated steel sheets [22]. Analysis of the results obtained has shown that the corrosion resistance is influenced not only by different thickness and heterogeneity of the zinc layer but also by cracking and flaking of the zinc layer. When cracks are small steel sheets are protected by the cathode protection of zinc. 4. CONCLUSIONS The main conclusions of the investigations can be summerised as follows: 1. The type of the coating influences the position and shape of the FLCs obtained for linear and complex strain paths. The greatest limit strain was obtained for zinc electrogalvanized steel sheet, and for such coating of hot-dip galvanized steel sheets that had the lowest coefficient of friction against the punch. 2. Deformation along complex strain paths with uniaxial tensile prestrain causes the growth of the FLCs of coated steel sheets whilst equibiaxial prestrain brings decrease of the FLCs, as compared with those obtained for linear strain paths. The effect of the complex strain path on the FLCs of coated steel sheets is in agreement with that for uncoated steel sheets. 3. The cracking limit curves of the zinc coating are influenced by the strain paths. For linear strain paths, the discrepancies between the CLCs and the FLCs reduce as the strain ratio e2/e 1 decreases. For complex strain paths, uniaxially prestrained specimens have a higher cracking limit than those which were equibiaxially prestrained. 4. The flaking limit curves of the zinc coating are almost the same as the FLCs when the e2/el ratio is changed from -1/2 to 0, in the range from 0 to 1 the discrepancies between the FkLC and the FLC reduce as the strain ratio 62/81 decreases. 5. The cracks in the zinc layer are nucleated mainly as intercrystalline and sometimes as transcrystalline. When the major principal strain is much greater than the minor principal strain, the cracks were oriented usually perpendicularly to the major strain direction. 6. For linear and complex strain paths the corrosion resistance decreases when the strain ratio e2/e 1 increases. Additionaly the corrosion resistance is effected by thickness of the zinc layer and of its heterogeneity, and is correlated with the cracking of zinc layer described by formability coefficient. REFERENCES 1. G.Arrigoni and M.Sarracino, Proc. Int. Deep Drawing Res. Group Meeting. Amsterdam, 1985, paper 12. 2. DPerry, A.Jussiaume and Y. Deleon, Proc. 15th Biennial Congress of Int. Deep Drawing Res. Group, Dearbon, 1988, ! 3. F.J.Flossdorf, Proc. Int. Deep Drawing Res. Group Meeting, Toronto, 1988, (Rapport of German Group). 4. Y.Hishida and M.Yoshida, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange, 1990, 173. 5. JZ.Gronostajski, W.J.AIi and M.S.Ghattas, Advanced Technology of Plasticity, Springer Verlag, I (1987), 423. 6. JZ.Gronostajski, W.JAli and M.S.Ghattas, J. Mater. Process. Technol., 22 (1990), 137. 7 JZ.Gronostajski, ZJ.Gronostajski and ZZimniak, Obr6bka Plastyczna, 2 (1991), 5. 8. J.Z.Gronostajski and M.Sulonen. Zesz. Nauk. AGH. Mechanika. 9, (1986), 269. 9. JZ.Gronostajski, W.JAli and M.S.Ghattas, J. Mater. Process. Technol., 23 (1990), 21.
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10. J.Z.Gronostajski, W.J.Ali and M.S.Ghattas, J. Mater. Process. Technol.,23 (1990), 321. 11. Ch.Weidemann, Mem. Sci. Revue Metall, 77 (1980), 343. 12. G.Blanchard, Proc. Int. Deep Drawing Res. Group Meeting, Amsterdam, 1985, p. 18 13. J.C.Gerdeen and B.A.EI-Jan, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange, 1990, 131. 14. C.C.Veerman, Sheet Metal Ind., 48 (1972), 351. 15. A.Melander, E. Schedin and L.Gustavsson, Report Swed. Inst. Met. Res.1983, 1 16. J.Z.Gronostajski, A.Dolny and T.Sobis, Proc.12th Biennial Congress of Int. Deep Drawing Res. Group, S.Margherita, 1982, 39. 17. S.Makimattila and A.J.Ranta-Eskola, Proc. 13th Biennial Congress Int. Deep Drawing Res. Group, Melbourne 1984, 293. 18. J.Z.Gronostajski: Formability, damage and corrosion resistance of coated steel sheets. In Materials Processing Defects Ed. by SK.Ghosh and MPredeteanu 19 E.Schedin, S.Karlsson and A.Melander, Proc. 14th Biennial Congress of Int. Deep Drawing Res. Group., KOln 1986, 460. 20 G.Engberg, A.Haglund and S.EHomstrom , Proc. 16th Biennial Congress Int. Deep Drawing Res. Group, Borlange, 1990, 131. 21. G.Monford and A.Bragard, Proc. Int. Deep Drawing Pres. Group Meeting, Amsterdam 1985, paperl3. 22. JZ.Gronostajski, Proc. 16th Biennial Congress of Int. Deep Drawing Res. Group, Borlange 1990, 233.