Influence of Cr Addition on Selective Oxidation Behavior of Mn-Added High-Strength Steel Sheet

Transcription

1 ISIJ International, Vol. 58 (2018), ISIJ International, No. 9 Vol. 58 (2018), No. 9, pp Influence of Cr Addition on Selective Oxidation Behavior of Mn-Added High-Strength Steel Sheet Yusuke FUSHIWAKI,* Takashi KAWANO and Yasunobu NAGATAKI Steel Research Laboratory, JFE Steel Corporation, 1 Kawasaki-cho, Chuo-ku Chiba, Japan. (Received on January 30, 2018; accepted on March 15, 2018) In the manufacture of high tensile strength sheet steels (HSS) containing Cr and Mn, selective surface oxidation of Cr and Mn affects the wettability of the HSS with molten Zn and leads to coating defects in the hot-dip galvanizing process. Therefore, it is important to clarify the selective surface oxidation behavior of Cr- and Mn-bearing steel from the viewpoint of active utilization of galvannealed HSS. The focus of the present study is an investigation of the influence of the amount of Cr addition on selective surface oxidation, which is thought to determine the wettability of cold-rolled HSS containing mass% Cr-1.7 mass% Mn with molten Zn. The wettability was investigated by measuring the contact angle of molten Zn containing 0.14 mass% Al (Zn Al) on the HSS. Surface and cross-sectional analyses were performed using secondary and transmission electron microscopes. Selective surface oxidation behavior was investigated by glow discharge spectroscopy. The main results obtained are as follows. The contact angle of Zn Al on the HSS tended to decrease as the amount of Cr addition increased up to 0.6 mass% due to the increment of MnCr 2 O 4, which is known as Cr Mn spinel and reacts with molten Al more easily than MnO. KEY WORDS: Cr Mn selective surface oxidation; Cr Mn spinel; Cr Mn added high-strength steel sheet (HSS). 1. Introduction Recently, there has been great concern regarding reduction of the weight of the automotive body and improvement of collision safety, which has encouraged active use of high-strength steel sheets (HSS) for automobiles. 1,2) Various elements such as Si, Mn and Cr are added in order to obtain high strength and ductility 3,4) in the manufacture of HSS. The HSS has also been applied to galvannealed components to obtain excellent anti-corrosion properties. 5) The manufacturing process of the HSS typically includes recrystallization annealing in a continuous furnace controlled to an annealing temperature of more than 700 C in a 5 20 vol% H 2 N 2 gas atmosphere. In this atmosphere, both Cr and Mn should be thermodynamically oxidized as selective surface oxides, whereas Fe oxide can be reduced. It is well known that these selective surface oxides deteriorate the wettability of the HSS with molten Zn and lead to coating defects such as bare spots 6 10) on the HSS in the hot-dip galvanizing process immediately after annealing. Various authors have so far reported the selective surface oxidation behavior on the surface of the HSS containing both Mn and Cr ) However, they have not mentioned the relationship between the difference of the selective surface oxide and the wettability of the HSS with molten Zn. Furthermore, there * Corresponding author: y-fushiwaki@jfe-steel.co.jp DOI: is still little detailed information regarding the influence of the amount of Cr addition on the selective surface oxidation and wettability. For example, Kawano et al. 14) studied the wettability of only 0.6 mass% Cr-1.6 mass% Mn and 1.9 mass% Mn steel. In order to better understand the contribution of Cr addition to the wettability of the HSS with molten Zn, the present study focused on the influence of Cr addition on selective surface oxidation behavior, which is thought to determine the wettability of cold-rolled HSS containing mass% Cr-1.7 mass% Mn. The wettability was evaluated by measuring the contact angle of molten Zn containing 0.2 mass% Al (Zn Al) on the HSS. 2. Experimental The steel samples used in the present study were full hard cold-rolled Cr- and Mn-bearing HSS with a thickness of 1.0 mm. The compositions of the samples were 0.02 mass% C, 0.01 mass% Si, 0.0, 0.2, 0.4 and 0.6 mass% Cr and 1.7 mass% Mn. The samples were pre-cleaned by electric degreasing, which was performed in an alkaline solution of 3.4 mass% NaOH at a current density of 500 A m 2 for 10 s. The samples were then pickled in an acidic solution of 5 mass% HCl at a temperature of 60 C for 6 s, followed by annealing for recrystallization. As shown in Fig. 1(a), this annealing was conducted in an atmosphere of 5 vol% H 2-95 vol% N 2 at a dew point of 35 C. The heating rate was 2018 ISIJ

2 13 C s 1 up to 650 C and 2 C s 1 from 650 C to a maximum temperature of 800 C. After annealing, the samples were rapidly cooled to room temperature with 100 vol% N 2 with a flow rate of 200 L min 1. After cooling, the annealed samples were analyzed as follows. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy (GD-OES). The conditions utilized for these measurements were a current of 20 ma and an Ar gas flow rate of 8.3 ml s 1. The sputtering time was 30 s, and the sputtering rate was approximately μm s 1. The amount of selective surface oxidation of the annealed samples was quantified by measuring the concentration profiles of Cr and Mn at the steel surface by GD-OES. In these GD-OES profiles, the peak of the intensity in μm, which was higher than that of the matrix and was more than 0.16 μm from the surface, was defined as the segregation peak. The integrated Cr and Mn intensity (arbitrary unit) of μm was defined as the amount of selective surface oxidation of Cr and Mn. Characterizations of the surface oxides were performed using X-ray photoelectron spectroscopy (XPS) with an Al-Kα X-ray source. X-ray diffraction (XRD) was also used with Cu-Kα radiation by grazing angle incidence. Cr Mn spinel could be detected as MnCr 2 O 4 (ICDD: ). The amount of Cr Mn spinel was estimated by the integrated intensity of the MnCr 2 O 4 (220) peak with the diffraction angle 2θ of 29.9, which was normalized by the integrated intensity of the (100) peak of alpha Fe (ICDD: ) with the diffraction angle 2θ of The surface of the annealed samples was observed by scanning electron microscopy (SEM). The conditions used in this observation were an acceleration voltage of 0.5 kv and working distance of 5 mm. Cross-sectional annealed samples were prepared by focused ion beam (FIB). These samples were observed by transmission electron microscopy (TEM) to determine the morphology and distribution of the oxides in the subsurface region. Determination of the composition of the oxides was performed by energy dispersive spectroscopy (EDS). Wettability was measured by dropping molten Zn with and without 0.14 mass%al (Zn Al) onto the surface of the annealed samples. A graphite syringe with a Zn Al ingot was installed and heated using an electric heating coil to melt the ingot above the annealed sample in the main chamber. In order to avoid oxidation of the molten Zn Al, 5 vol% H 2-95 vol% Ar was introduced into the main chamber. In this study, the specimen was heated up to 460 C for 1 min, after which Zn Al droplets were dropped on the samples and their wetting behaviors at 465 C were recorded by a video camera. The droplet size was adjusted to a diameter of around 2 mm. The contact angle and wetting radius were measured from the recorded images 1 s after dropping the Zn Al droplets, as shown in Fig. 1(b). 3. Results 3.1. Influence of Cr Addition on Selective Surface Oxidation after Annealing Selective surface oxidation behavior was investigated using GD-OES. Figure 2 shows GD-OES profiles of the annealed samples of Cr-free-1.7 mass% Mn and 0.6 mass% Cr-1.7 mass% Mn, respectively. The intensity of the segregation peak of Cr increased as the amount of Cr addition increased. The width of the segregation peak of Mn was μm on the Cr-added samples. In contrast, the intensity of the segregation peak of Mn appeared to be substantially unchanged when the amount of Cr addition increased. Figure 3 shows the influence of the amount of Cr addition on the selective surface oxidation of Cr and Mn. The selective surface oxidation of Mn was almost the same even when the amount of Cr addition increased, whereas that of Cr increased monotonically with increasing Cr addition. As shown in Figs. 2 and 3, the amount of Cr addition drastically altered the selective surface oxidation behavior of the samples. Therefore, identification of the selective Fig. 2. GD-OES depth profiles of samples annealed at 800 C for 20 s with a sputtering rate of μm s 1. (a) Cr-free-1.7 mass% Mn and (b) 0.6 mass% Cr-1.7 mass% Mn. (a) (b) Fig. 1. (a) Heat pattern used in annealing of samples. (b) Schematic illustration of dropping Zn Al droplet test to measure contact angle ISIJ 1624

3 surface oxides was carried out using XPS and XRD. Figure 4 shows the influence of Cr addition on the atomic ratio of each element obtained by XPS on the very surface of the annealed samples. The atomic ratios of Fe, Mn and O were almost the same even when the amount of Cr addition increased, whereas that of Cr increased monotonically. Figure 5 shows the measured XRD patterns of the annealed Fig. 3. Influence of amount of Cr addition on selective surface oxidation of Cr and Mn. samples. In the case without Cr addition, only MnO was identified as the selective surface oxide. On the other hand, diffraction peaks of the spinel structure, which was identified as MnCr 2 O 4, were detected on the samples contained Cr. The influence of Cr addition on the amount of MnCr 2 O 4 is shown in Fig. 6. As the amount of Cr addition increased, the amount of MnCr 2 O 4 increased monotonically. SEM observation was performed in order to observe the shape of the selective surface oxide. Figure 7 shows SEM images of the surfaces of the annealed samples. In the case without Cr addition, there were many granular particles with a diameter of around 200 nm, which are considered to be selective surface oxides. With Cr addition, relatively flat selective surface oxides were observed in addition to the granular oxides Influence of Cr Addition on Wettability of Steel with Molten Zn Al Figure 8 shows the effect of Cr addition on the contact angle of the molten Zn with and without Al on the surface of the annealed samples. In the case without Al, no clear effect was observed, and the contact angle was around 130. On the other hand, in the case with Al, the contact angle decreased from 130 to 105 as the amount of Cr addition was increased up to 0.6 mass%. In order to discuss the wettability behavior shown in Fig. 4. Influence of amount of Cr addition on atomic ratio of Fe, Mn, Cr and O obtained by XPS. Fig. 6. Influence of amount of Cr addition on normalized intensity of Cr Mn spinel obtained by XRD. Fig. 5. XRD patterns of samples with/without Cr addition after annealing ISIJ

4 Fig. 7. SEM images of surfaces of annealed samples. (a) Cr-free-1.7 mass% Mn and (b) 0.6 mass% Cr-1.7 mass% Mn. Fig. 8. Effect of amount of Cr addition on contact angle of molten Zn and Zn Al. Fig. 8, it is important to clarify the morphology of the oxides and intermetallic compounds (IMC) at the interface between the Zn Al coating and the steel substrate. Therefore, cross-sectional TEM observation and EDS line analysis were carried out. Figure 9 shows the results of the cross-sectional TEM and EDS line analysis of the interface between the coating and the samples (a) without Cr addition and (b) containing 0.6 mass% Cr, respectively. The results of the EDS line analysis shown in Fig. 9 were concentration profiles of each element measured across each oxide at the interface in the direction perpendicular to the interface from the coating layer toward the steel substrate. In the case without Cr addition shown in Fig. 9(a), the oxide was proved to be composed of O and Mn. In contrast, with Cr addition, the oxide included Cr in addition to O and Mn. Moreover, Al was concentrated on the oxide containing Cr, Mn and O. Fig. 9. Results of cross-sectional TEM and EDS line analysis of interface between the coating and the sample. (a) Cr-free and (b) 0.6 mass% Cr-1.7 mass% Mn ISIJ 1626

5 4. Discussion 4.1. Influence of Cr Addition on Selective Surface Oxidation after Annealing Selective surface oxidation is a phenomenon in which Cr and Mn in the steel are oxidized due to the existence of oxygen provided by the thermodynamic deviation of H 2 O in the atmosphere on the interface between the steel surface and the atmosphere. 15) As shown in Figs. 3, 4 and 5, in the case without Cr addition, the selective surface oxides are mainly MnO, but in the case with Cr addition, the selective surface oxides are assumed to be composed of MnCr 2 O 4 which is called Cr Mn spinel, in addition to MnO. Cr Mn spinel is thought to be formed through an interaction between MnO and Cr, which occurs as a result of Cr addition. 11,12) Formation of Cr Mn spinel is also consistent with the thermodynamic assessment, the Cr Mn spinel appears as a stable phase in reported ternary phase diagrams of Mn Cr O. 16,17) Here, let us consider the mechanism of selective surface oxidation thermodynamically. It may be necessary to consider the following reactions regarding the oxidation of Cr- and Mn-bearing steel in an atmosphere with H 2 H 2 O. Mn HO 2 MnO H2... (1) 2Cr MnO 3H O MnCr O 3H... (2) Cr 3HO Cr O 3H... (3) Fe HO 2 FeO H2... (4) ph 2 O/pH 2 of each oxide was calculated using the above Eqs. (1), (3) and (4) for an Fe-1.7 mass% Mn-0.6 mass% Cr alloy annealed in an atmosphere of 5 vol% H 2 N 2. 18) The results are shown in Fig. 10. In the present study, these calculations were conducted on the supposition that the activities of Mn and Cr were equal to the atomic ratio of each element. 18) Although ph 2 O/pH 2 of MnCr 2 O 4 could not be calculated using the above the Eq. (2), it can be assumed that it may exist at each temperature between that of Cr 2 O 3 (3) and that of MnO (1) in Fig. 10. log(ph 2 O/ ph 2 ) in the annealing atmosphere could also be calculated at approximately 2.5, as the atmosphere had a temperature of 800 C, dew point of 35 C and hydrogen concentration of 5 vol%. 19) As shown in Fig. 10, when log(ph 2 O/pH 2 ) equals 2.5, the reaction in the Eq. (4) should proceed to the left; that is, the reaction means reduction of FeO. In contrast, the reactions of (1) (3) proceed to the right; that is, these reactions are oxidations of Mn and Cr. Therefore, Mn and Cr existed as oxides such as MnO and MnCr 2 O 4, whereas Fe was not an oxide but rather was a metal on the very surface. These results correspond to the XRD results shown in Fig. 5. As shown in Fig. 4, the fact that the atomic ratio of Fe was almost the same even when the amount of Cr addition increased indicates that the exposure ratio of the metallic Fe was also almost the same regardless of Cr addition, and there were many granular MnO and MnCr 2 O 4 particles on the surface. This corresponds to the SEM observation results shown in Fig. 7. According to the SEM observation, MnCr 2 O 4 was a relatively flat oxide compared to MnO. As a result, the difference of surface morphology in the cases with and without Cr addition can be represented schematically as shown in Fig Influence of Cr Addition on Wettability of Steel with Molten Zn Al In the case with Al addition to molten Zn Al, the contact angle decreased from 130 to 105 as the amount of Cr addition was increased up to 0.6 mass%, as shown in Fig. 8. This means that the wettability of the annealed sample with the molten Zn Al may be enhanced by adding Cr to Mn-bearing steel. As shown in Fig. 3, the amount of Mn selective surface oxide did not change greatly when the amount of Cr addition increased, whereas Cr selective surface oxidation increased. In addition, the exposure ratio of the metallic Fe was almost the same regardless of the amount of Cr addition as shown in Fig. 4. This indicates that the wettability did not depend on the amount of Mn selective surface oxidation. Fig. 10. Equilibrium ph 2O/pH 2 of various oxides on sample with annealing in H 2 H 2O mixed gas atmosphere. Fig. 11. Schematic illustrations of surface of annealed samples. (a) Cr-free-1.7 mass% Mn and (b) Cr-added 1.7 mass% Mn ISIJ

6 Fig. 12. Schematic illustrations of mechanism by which the wettability on sample containing Cr and Mn is affected. Furthermore, this influence of Cr addition on the contact angle was not observed without Al. Hence, Al in the molten Zn Al is considered to play an important role in the change in the wettability. Thermodynamically, Al should be oxidized more easily than other elements such as Mn, Cr and Fe. 20) In other words, it may be possible for Al to reduce MnO and Cr Mn spinel, which is a so-called aluminothermic reaction, 21 24) and this may lead to a reduction of the contact angle. Cr Mn spinel can presumably react with Al more easily than MnO, as Cr is less oxidizable than Mn according to an Ellingham diagram. 20) Moreover, reduction of MnO by Al should be difficult, as the Mn in MnO is difficult to replace by Al due to its NaCl-type crystal structure. Al oxide would not fit the NaCl-type structure but would form corundum or spinel. 14,25) Al as well as Cr are thought to form a spinel structure with Mn as follows:... (5) MnCr2O4 Al Mn Cr, AlO4 Cr Accordingly, the wettability and reactivity should be improved by an increase in the amount of Cr Mn spinel. As shown in Fig. 6, the amount of Cr Mn spinel increased with increasing Cr addition. It has been indicated that this is the reason why the contact angle decreases with increasing Cr addition up to 0.6 mass%. Eustathopoulos et al. investigated the contact angles of various molten metals on metal oxides and reported that the contact angle tended to decrease when the metal reacted with the oxide. 26) Consequently, the mechanism by which the wettability of the sample containing Cr and Mn is affected can be shown schematically in Fig Conclusion In order to better understand the mechanism of the contribution of Cr addition to the wettability of steel with molten Zn the influence of Cr addition on the selective surface oxidation behavior on 1.7 mass% Mn-bearing high-strength sheet steel was investigated. The main results of the present study can be summarized as follows. The contact angle of the molten Zn Al on the 1.7 mass% Mn-bearing steel tended to decrease as the amount of Cr addition increased up to 0.6 mass% due to an increment of Cr Mn spinel which should react with molten Al much more easily than MnO. REFERENCES 1) K. Yamazaki: Proc. 224th Symp. of the Japan Society for Technology of Plasticity, Tokyo, (2003), 21. 2) T. Sugiyama: J. Jpn. Soc. Technol. Plast., 46 (2005), No. 534, 8. 3) Y. Yamato: 138th and 139th Nishiyama Memorial Seminar, ISIJ, Tokyo, (1994), 89. 4) Y. Kuriyama, M. Takahashi and H. Ohashi: J. Soc. Automot. Eng. Jpn., 55 (2001), No. 4, 51. 5) For example, B. Schuhmacher, T. Heller, M. Steinhorst and W. Warnecke: Proc. Galvatech 07, ISIJ, Tokyo, (2007), ) Y. Hirose, H. Togawa and J. Sumitani: Tetsu-to-Hagané, 68 (1982), ) A. Nishimoto, J. Inagaki and K. Nakaoka: Tetsu-to-Hagané, 68 (1982), ) C. Kato, T. Sekine, S. Umino, T. Yamashita, K. Mochizuki and M. Matsuda: CAMP-ISIJ, 7 (1994), ) M. Isobe, K. Kyono and N. Totsuka: CAMP-ISIJ, 8 (1995), ) J. Mahieu, B. C. DeCooman, J. Maki and S. Claessens: Iron Steelmaker, 29 (2002), ) S. Swaminathan and M. Spigel: Appl. Surf. Sci., 253 (2007), ) P. R. Wilson and Z. Chen: Corros. Sci., 49 (2007), ) W. Mao, R. W. A. Hendrikx and W. G. Sloof: Oxid. Met., (2017), 1, doi: /s ) T. Kawano, Y. Fushiwaki, Y. Nagataki and M. Nagoshi: Surf. Interface Anal., (2018), doi: /sia ) T. Yamazaki: Hyoumen-Gijyutu, 40 (1989), ) I. Jung: Solid State Ion., 177 (2006), ) L. Kjellqvist and M. Selleby: J. Alloy. Compd., 507 (2010), ) Kinzoku Zairyo no Koon Sanka to Koon Fushoku, ed. by Fushoku Boushoku Kyokai, Maruzen, Tokyo, (1982), ) Y. Fushiwaki, Y. Nagataki, H. Nagano, W. Tanimoto and Y. Sugimoto: ISIJ Int., 54 (2014), ) H. T. T. Ellingham: J. Soc. Chem. Ind., 63 (1944), ) P. Drillet, Z. Zermout, D. Bouleau, J. Mataigne and S. Claessens: Proc. 6th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet, Galvatech 04, AIST, Warrendale, PA, (2004), ) R. Meguerian and J. R. McDermid: Proc. 7th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet, Galvatech 07, ISIJ, Tokyo, (2007), ) R. Khondker, A. Mertens and J. R. McDermid: Mater. Sci. Eng., A463 (2007), ) R. Kavitha and J. R. McDermid: Surf. Coat. Technol., 212 (2012), ) T. Kawano and F. U. Renner: Surf. Interface Anal., 44 (2012), ) N. Eustathopoulous and B. Drevet: Mater. Sci. Eng. A, 249 (1998), ISIJ 1628