Improvement of Galvanizability of Silicon-bearing Steel by Electrodeposited Iron Coating Containing Oxygen

, pp. 115 120 Improvement of Galvanizability of Silicon-bearing Steel by Electrodeposited Iron Coating Containing Oxygen Yoichi TOBIYAMA, 1) Sakae FUJITA 1) and Toshio MARUYAMA 2) 1) Steel Research Laboratory, JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima, 721-8510 Japan. E-mail: y- tobiyama@jfe-steel.co.jp, s-fujita@jfe-steel.co.jp 2) Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8552 Japan. E-mail: maruyama@ mtl.titech.ac.jp (Received on March 14, 2011; accepted on September 20, 2011) Iron coatings were prepared by electrodeposition in a bath containing citrate to improve the galvanizability of silicon-bearing steel. The iron coating electrodeposited from the bath containing citrate contains more than 4 mass% oxygen. This oxygen originates from the incorporation of the hydroxide of Fe 3+ during electroplating. The oxygen-containing coating improves the galvanizability of the silicon-bearing steel by suppressing the surface segregation of silicon during annealing. This is because silicon in the steel reacts with oxygen in the oxygen-containing coating to form internal oxides during annealing. KEY WORDS: galvanizing; electrodeposited iron coating; oxygen; silicon; internal oxidation; surface segregation. 1. Introduction In automobile industries, demands to reduce weight and to secure the crashworthiness of cars have been increasing in recent years. A variety of high-strength steels with excellent press formability have been developed in order to satisfy these requirements. On the other hand, from the viewpoint of preventing rust in automobile bodies, various types of zinc- and zinc alloy-coated steel sheets were developed and adopted in practical applications. Among these, galvannealed steel sheets (GA) are more frequently used because of high corrosion resistance at low cost. The addition of alloying elements to the base steel in order to obtain the desired mechanical properties gives rise to various problems in the manufacture of GA. It has been reported that the surface segregation of alloying elements such as silicon and manganese during recrystallization annealing causes poor galvanizability in the continuous galvanizing line (CGL). 1 12) It was also reported that alloying elements such as silicon and phosphorus retard the alloying reaction between Zn and Fe during galvannealing on the CGL. 3,4,13 16) The electroplating technique to form an iron coating on steels may give a solution to these problems. 17 21) However, the mechanism responsible for improving the galvanizability of silicon bearing-steel with an electrodeposited iron coating has not been systematically clarified. The authors have demonstrated that the oxygen incorporated in an iron coating plays the major role for improving the galvanizability of silicon-bearing steel. 21) This paper proposed the mechanism for improvement of galvanizability in silicon-bearing steel by an iron coating with incorporated oxygen. 2. Experimental 2.1. Materials Cold rolled high-strength steel with a thickness of 0.70 mm was used as the specimen. Table 1 shows the chemical composition of the steel. Strips of 100 mm wide and 200 mm long were cut from the cold rolled steel. The strip was degreased, followed by pickling in 5 mass% HCl and iron electroplating. Table 2 shows the conditions of the electroplating of iron. The concentration of sodium citrate in the bath was changed in the range of 0 to 40 g/dm 3. Furthermore, iron sulfate (III) of 0 g/dm 3 to 35.8 g/dm 3 was put into the bath with sodium citrate of 10 g/dm 3 in order to add Fe 3+ Table 1. Chemical Compositions of steel (mass%). C Si Mn P S 0.0040 0.80 0.29 0.097 0.008 Table 2. Conditions of iron electroplating. Bath compositions FeSO 4 7H 2O 300 g/dm 3 Bath temperature 60 C ph 1.8 Current density 100 A/dm 2 Flow rate Na 2SO 4 50 g/dm 3 Na 3C 6H 5O 7 0 40 g/dm 3 Fe 2(SO 4) 3 0 35.8 g/dm 3 (as Fe 3+ 0 10 g/dm 3 ) 60 m/min. 115 2012 ISIJ

to the bath in the concentration of 0 to 10 g/dm 3. Electroplating of iron was performed to prepare a coating of which weight is ranged from 3 to 10 g/m 2. For comparison of the galvanizability, two types of steel were prepared by electroplating in baths with 10 g/dm 3 of sodium citrate and without sodium citrate under the bath condition of no additional iron sulfate (III). 2.2. Characterization of Steels Electroplated with Iron The oxygen content in the electrodeposited iron coating was measured using the instrument of LECO, TC-136. The specimen is melted in a carbon crucible and the oxygen in the specimen is extracted as carbon monoxide. Carbon monoxide is oxidized to carbon dioxide, which is measured by infrared absorption. The oxygen contents in steels with and without an iron coating were measured and the amount of oxygen in the coating was calculated from those in the two steels. The concentration of oxygen in the coating was determined as the ratio of weights of oxygen to that of the coating. The carbon content in the coating was measured using the instrument of LECO, CS-444. The specimen is melted in a ceramic crucible under oxygen atmosphere and the carbon in the specimen is extracted as carbon dioxide, which is measured by infrared absorption. The weight of carbon in the coating was also obtained by subtracting the weight of carbon in steel without the coating from that with the coating. 2.3. Galvanizing Galvanizing of steel was performed using a hot-dip galvanizing simulator as shown in Fig. 1. Strips of 70 mm wide and 180 mm long were cut from the iron-electroplated steel sheets. The steel was annealed in an infrared furnace at 850 C for 1 s to 90 s in flowing N 2-5vol% H 2 gas with a dew point of 20 C. The steel was then cooled to 470 C in the above gas and dipped in a Zn bath of 0.145 mass% Al for 1 s. After galvanizing, the coated Zn was wiped to approximately 50 g/m 2 by N 2 gas ejected from wipers installed above the bath. Annealed steel for characterization was cooled to room temperature in flowing N 2 gas after annealing at 850 C without dipping in the bath. Galvanizability was evaluated by a rating of 5 degree as shown in Table 3. The rating is based on the area ratio of bare-spot defects on a galvanized steel surface. The rating of 5 indicates no bare spot. 2.4. Characterization of Annealed Steels Steels were annealed at 850 C for 40 s. Concentration depth profiles of the annealed steels with the iron coatings were analyzed by glow discharge spectroscopy (GDS). The sputtering ratio of 2 nms 1 was adopted for analysis on the surface with selectively oxidized SiO 2 and that of 10 nms 1 for analysis on the internally oxidized zone. Surface oxides which segregated during annealing were identified by FT-IR at an angle of incidence of 70. 3. Results 3.1. Effect of Concentration of Sodium Citrate in the Electroplating Bath on Oxygen and Carbon Contents in Electrodeposited Iron Coating Figure 2 shows the oxygen and carbon contents in the electrodeposited coating as a function of the concentration of sodium citrate in the electroplating bath. The bath contains no iron sulfate (III). The iron coating electrodeposited in the bath without sodium citrate contained 0.2 mass% oxygen. The oxygen content in the coating increased rapidly as the concentration of sodium citrate increases in the bath. The bath with sodium citrate of 10 g/dm 3 gave about 4 mass% oxygen in the coating. The oxygen content gradually increased when the sodium citrate content exceeded Table 3. Rating of bare-spot defects. Index Bare-spot area ratio (%) 5 0 4 0 0.02 3 0.02 0.05 2 0.05 5 1 more than 5 Fig. 1. Layout of the galvanizing simulator. Fig. 2. Effect of concentration of sodium citrate on oxygen and carbon contents in electrodeposited iron coatings. 2012 ISIJ 116

10 g/dm 3. On the other hand, the carbon content in the coating was not affected by sodium citrate in the bath and the concentration was less than 1 mass% when the sodium citrate content was 40 g/dm 3 in the bath. Figure 3 shows the relationship between the concentration of Fe 3+ in the bath and the oxygen content in the iron coating. The iron coating prepared in the bath without Fe 3+ contained oxygen of approximately 4 mass% and the oxygen content increased to 7 mass% in the coating prepared in the bath with Fe 3+ of 3 g/dm 3. The oxygen in the coating gradually increased when the Fe 3+ content was greater than 3 g/dm 3. with oxygen-containing coating of 3 g/m 2 was in the rating of 4 and the bare-spot defect was completely suppressed at the weight of coating more than 5 g/m 2. These facts indicate that the oxygen incorporated in the iron coating plays an important role in improvement of galvanizability. Figure 5 shows the effect of annealing time on the gal- 3.2. Effect of Thickness and Oxygen Content of Electrodeposited Iron Coating on Galvanizability of Silicon-bearing Steel The galvanizability of the steel with the 4 mass% oxygencontaining coating electrodeposited from a bath with 10 g/dm 3 of sodium citrate was investigated in comparison with that of the steel with the oxygen-less coating electrodeposited from a bath without sodium citrate. Figure 4 shows the effect of the weight of the iron coating on the galvanizability of silicon-bearing steel. Galvanizability was evaluated by the ratings shown in Table 3 and was shown as the average of the ratings of four galvanized sheets. The galvanizability of the steel with the oxygen-less iron coating improved as the weight of coating increased. However, complete suppression of bare-spot defects required a coating weight of 10 g/m 2. On the other hand, the galvanizability of the steel Fig. 4. Effect of coating weight and oxygen content in the iron coating on index of bare-spot defects. Fig. 3. Effect of concentration of Fe 3+ on oxygen content in electrodeposited iron coatings. Fig. 5. Effect of annealing time on index of bare-spot defects. Fig. 6. GDS depth profiles of annealed silicon-bearing steel sheets, with the oxygen-less coating and the oxygen-containing coating (annealed for 40 s). 117 2012 ISIJ

vanizability of iron-coated steels in comparison with noncoated steel. The non-coated steel annealed for 1 s showed poor galvanizability, and annealing for more than 40 s deteriorated the galvanizability. Although the galvanizability of the steel with oxygen-less iron coating annealed for 1 s was in the rating of 5, further annealing deteriorated the galvanizability. On the other hand, the steel with oxygen-containing coating showed good galvanizability up to the annealing time of 90 s. 3.3. Surface Segregation on the Steel with the Iron Coating during Annealing Figure 6 shows GDS depth profiles of the annealed steel with the oxygen-less (A) and 4 mass% oxygen-containing iron coatings (B). In the oxygen-less coating, surface segregation of silicon and oxygen was observed in the range of 10 nm beneath the surface. On the other hand, surface segregation was not observed in the oxygen-containing coating. Figure 7 shows FT-IR spectra of steels with and without iron coatings. Absorption peaks indicating SiO 2 were observed in steels without the coating and with the oxygenless coating. However, no peak was observed in the steel Fig. 7. FT-IR spectra of steel sheets with and without the iron coating, annealed for 40 s. with 4 mass% oxygen-containing coating. Figure 8 shows the concentration-profiles of the interface between the steel and the iron coatings by GDS. In the 4 mass% oxygen-containing coating, peaks of silicon, manganese and oxygen were detected at a depth of about 0.6 μm, which corresponds to the interface between the steel and the coating. In contrast, such peaks were not observed at the interface in the oxygen-less iron coating. 4. Discussion 4.1. Incorporation of Oxygen in Electrodeposited Iron Coatings The iron coating electrodeposited from a bath with 10 g/dm 3 of sodium citrate (oxygen-containing coating) includes approximately 4 mass% oxygen, whereas the iron coating electrodeposited from a bath without sodium citrate (oxygenless coating) contains 0.2 mass% oxygen. Addition of sodium citrate to the bath does not increase the content of carbon in the iron coating. These results exclude the possibility of incorporation of sodium citrate in the coating. It is known that citrate acts as a complexing agent in an iron electroplating bath. 22) Since the inclusion of oxygen in the coating requires sodium citrate in the bath and the content of oxygen in the coating increases with increase of the concentration of Fe 3+ in the bath with sodium citrate, Fe 3+ complexed by a citrate ion may play an important role in including oxygen in the iron coating. The iron coating contained approximately 4 mass% oxygen even when no Fe 3+ was added to the bath. This indicates that Fe 3+ exists in the bath because Fe 2+ in the bath is oxidized by oxygen in the air and the cathode reaction during electroplating. The critical ph for precipitation of iron (III) hydroxide is calculated to be 1.4 using the solubility product of reaction (1), 2.5 10 39 23) when the concentration of Fe 3+ in the bath is 10 g/dm 3. Fe 3+ + 3OH = Fe(OH) 3... (1) The critical ph of 1.4 is lower than that in the bath of 1.8. Nevertheless, Fe 3+ in the bath does not precipitate as hydroxide because Fe 3+ is complexed by citrate in the bath. Fig. 8. GDS depth profiles of annealed silicon-bearing steel sheets, with the oxygen-less coating and the oxygen-containing coating (annealed for 40 s). 2012 ISIJ 118

Fig. 9. Schematic illustrations showing the mechanism for preventing the surface segregation of silicon by the oxygencontaining coating. In a similar way, the critical ph for iron (II) hydroxide precipitation is calculated to be 6.7 using the solubility product of reaction (2), 2.2 10 15, 23) and the concentration of Fe 2+ in the bath, 60.3 g/dm 3. Fe 2+ + 2OH = Fe(OH) 2... (2) It has been reported that the ph near the cathode increases due to the generation of hydrogen during electroplating in sulfate baths. 24) The critical ph for precipitation of iron (III) hydroxide at the experimental condition is lower than that for precipitation of iron (II) hydroxide. Therefore, the precipitation of the hydroxide of Fe 3+ dissociated from the complex proceeds with an increase in the ph near the cathode during electroplating. On the other hand, ph near cathode does not reach to the critical ph for precipitation of iron (II) hydroxide and Fe 2+ electrodeposits as iron on the steel without forming Fe(OH) 2. Therefore oxygen is incorporated in the iron coating because the hydroxide of Fe 3+ is codeposited in the coating during iron electroplating. 4.2. Role of Oxygen Incorporated in the Iron Coating in Suppression of Surface Segregation of Silicon during Annealing Figure 9 shows a schematic illustration showing the mechanism in which the oxygen-containing coating prevents the surface segregation of silicon during annealing. (Step A) In the case of the oxygen-less coating, silicon diffuses through the coating during heating. (Step B and C) During annealing, silicon diffuses to the surface of the coating and segregates as SiO 2 on the surface of the steel due to selective oxidation. The galvanizability of the steel with the oxygen-less coating after annealing for more than 40 s deteriorates because SiO 2 forms on the surface of the steel. 1,2) (Step D) In the oxygen-containing coating, Fe(OH) 3 exists. It has been reported that Fe(OH) 3 colloid dehydrates Table 4. Oxygen pressures of two (or three) phase equilibrium at 850 C. 26) Oxides Log(P O2 /Pa) Fe 3O 4 Fe 2O 3 3.5 FeO Fe 3O 4 11.0 Fe FeO 12.6 Fe Fe 2SiO 4 SiO 2 13.8 Si SiO 2 27.9 to form hematite at 300 C. 25) During the heating process to the annealing temperature Fe(OH) 3 may dehydrate and form hematite, releasing oxygen and hydrogen in the metal phase by the following reaction: 2Fe(OH) 3 Fe 2O 3 + 3O + 6H... (3) (Step E) The hematite probably forms wüstite by reacting with iron in the iron coating at an annealing temperature of 850 C according to reaction (4), Fe 2O 3 + Fe 3FeO... (4) (Step F) Table 4 summarizes the oxygen partial pressures of two- (or three-) phase coexistences at 850 C, which are calculated using thermodynamic data. 26) At the oxygen pressure of log(po 2 /Pa)= 12.6 for the coexistence of wüstite and iron, silicon forms internal oxides, such as SiO 2 and Fe 2SiO 4 at the interface between the wüstite and iron by the following reactions. 2FeO + Si SiO 2 + Fe... (5) 4FeO + Si Fe 2SiO 4 + 2Fe... (6) These reactions capture silicon and suppress the transport of silicon to the surface. Using the diffusion coefficient of 5.15 10 15 m 2 s 1 reported by Borg et al., 27) the diffusion dis- 119 2012 ISIJ

tance of silicon is calculated to be about 0.7 μm at 850 C for 90 s. The amount of silicon in this volume beneath the surface is estimated to be 44 mg/m 2 in the case of silicon content in the steel of 0.8 mass%. The amount of oxygen required for internal oxidation of silicon is 50 mg/m 2 to form SiO 2 and 101 mg/m 2 to form Fe 2SiO 4. On the other hand, the amount of oxygen is 200 mg/m 2 in the iron coating of 5 g/m 2 in which the oxygen content is 4 mass%. This amount of oxygen is 2 to 4 times larger than that required for the complete formation of internal oxides. (Step G) GDS demonstrates that silicon forms internal oxides, resulting in segregation at the interface between the steel and the iron coating. Because oxygen already exists at the interface between the steel and the iron coating during the annealing, the formation of internal oxides is controlled by the diffusion of solute silicon through the steel, and a silicon depletion zone is formed beneath the interface. Because silicon reacts with oxygen to form internal oxides in the oxygen-containing coating, the segregation of silicon oxides at the surface is suppressed, resulting in improvement of the galvanizability. 5. Conclusions The iron coating electrodeposited from the bath containing citrate contains more than 4 mass% oxygen. This oxygen originates from the incorporation of the hydroxide of Fe 3+ during electroplating. The oxygen-containing coating improves the galvanizability of the silicon-bearing steel by suppressing the surface segregation of silicon during annealing. This is because silicon in the steel reacts with oxygen in the oxygencontaining coating to form internal oxides during annealing. REFERENCES 1) Y. Hirose, H. Togawa and J. Sumiya: Tetsu-to-Hagané, 68 (1982), 665. 2) Y. Hirose, H. Togawa and J. Sumiya: Tetsu-to-Hagané, 68 (1982), 2551. 3) A. Nishimoto, J. Inagaki and K. Nakaoka: Tetsu-to-Hagané, 68 (1982), 1404. 4) N. Fujibayashi, Y. Tobiyama and K. Kyono: CAMP-ISIJ, 10 (1997), 609. 5) Y. Tsuchiya, S. Hashimoto, Y. Ishibashi, J. Inagaki and Y. Fukuda: Tetsu-to-Hagané, 86 (2000), 396. 6) I. Hertveldt, S. Claessens and B. C. D. Cooman: Mater. Sci. Technol., 17 (2001), 1508. 7) I. Hashimoto, K. Saito, M. Nomura, T. Yamamoto and H. Takeda: Tetsu-to-Hagané, 89 (2003), 31. 8) E. D. Bruycker, B. C. D. Cooman and M. D. Meyer: Steel Res. Int., 75 (2004), 147. 9) Y. Suzuki and K. Kyono: J. Surf. Finish. Soc. Jpn., 55 (2004), 48. 10) Y. Suzuki, Y. Fushiwaki, Y. Tobiyama and C. Kato: CAMP-ISIJ, 18 (2005), 1501. 11) Y. Takada, M. Suehiro, M. Sugiyama and T. Senuma: Tetsu-to- Hagané, 92 (2006), 21. 12) X. S. Li, S. Baek, C.-S. Oh, S.-J. Kim and Y.-W. Kim: Scr. Mater., 57 (2007), 113. 13) M. Arai, Y. Adachi, T. Nakamori and N. Usuki: Tetsu-to-Hagané, 83 (1997), 713. 14) C. E. Jordan, R. Zuhr and A. R. Marder: Metall. Trans., 28A (1997), 2695. 15) T. Hashimoto, K. Tahara, E. Hamada, M. Sakurai, J. Inagaki and M. Sagiyama: Tetsu-to-Hagané, 84 (1998), 727. 16) Y. Suzuki, Y. Sugimoto and S. Fujita: J. Met. Finish. Soc. Jpn., 58 (2007), 183. 17) M. Himeno, T. Yoshiwara, S. Shijima and M. Goto: Tetsu-to- Hagané, 67 (1981), S1000. 18) M. Hori, T. Usuki and T. Nakamori: CAMP-ISIJ, 7 (1994), 602. 19) T. Ooi, A. Takase, M. Oomura and S. Shimada: CAMP-ISIJ, 7 (1994), 603. 20) T. Ooi, A. Takase, M. Oomura and S. Shimada: CAMP-ISIJ, 7 (1994), 1513. 21) S. Umino, Y. Tobiyama, C. Kato and K. Mochizuki: CAMP-ISIJ, 7 (1994), 1512. 22) T. Fukuzuka, K. Kajiwara and K. Miki: Tetsu-to-Hagané, 66 (1980), 807. 23) A. J. Bard: Chemical Equilibrium, Kagakudojin, Kyoto, (1975), 196. 24) H. Fukushima, T. Akiyama, J.-H. Lee, M. Yamaguchi and K. Higashi: J. Met. Finish. Soc. Jpn., 33 (1982), 574. 25) K. Imasaka, Y. Kanatake, J. Suehiro and M. Hara: IEEJ Trans. FM, 126 (2006), 349. 26) O. Kubaschewski, E. L. Evans and C. B. Alcock: Metallurgical Thermochemistry, Pergamon Press, Oxford, (1965), 421. 27) R. J. Borg and D. Y. F. Lai: J. Appl. Phys., 41 (1970), 5193. 2012 ISIJ 120