Effect of Dew Point on Galvanizability in 4 Mass% Al Added Low Density Steel

Transcription

1 ISIJ International, Advance Publication by J-STAGE ISIJ International, ISIJ International, J-Stage Advanced Advance ISIJ Publication International, Publication, ISIJ International, by DOI: J-STAGE, Advance Vol. Publication DOI: 58 (2018), /isijinternational.ISIJINT No. by J-Stage 9International, Vol. 58 (2018), No. 9, pp. 1 8 Effect of Dew Point on Galvanizability in 4 Mass% Al Added Low Density Steel Xinyan JIN, 1,2) * Guangkui HU, 3) Hongwei QIAN 3) and Hua WANG 4) 1) Baosteel Research Institute, Shanghai, China. 2) State Key Laboratory of Development and Application Technology of Automotive Steels, Baosteel, Shanghai, China. 3) Baosteel Cold Rolling Plant, Shanghai, China. 4) Instrumental Analysis & Research Center of Shanghai University, Shanghai, China. (Received on January 16, 2018; accepted on April 23, 2018; J-STAGE Advance published date: July 21, 2018) Hot dip galvanizing experiments were conducted on low density steel to investigate its galvanizability. The panels were annealed at 815 C for 150 s, and the dew point of the annealing atmosphere was varied from 40 C to +10 C. The surface morphologies of the coating and the substrate before dipping and after coating removal were demonstrated by SEM equipped with EDS. The cross sections prepared by common metallographic method and FIB were characterized by SEM and TEM respectively. It is found that the dew point of the annealing atmosphere has a great effect on the galvanizability of 4 mass% aluminium added low density steel. With the dew point increasing, the external Al oxidation converts into internal Al oxidation and an iron particle layer forms on the outmost surface of the matrix. The Al in the zinc bath mainly reacts with the iron particles to form Fe Al Zn inhibition layer at the interface of coating/matrix, resulting in both good wettability and strong coating adhesion of high aluminum low density steel sheet. KEY WORDS: galvanizability; low density steel; selective oxidation; dew point. 1. Introduction Weight reduction of vehicles is well recognized as an effective way to reduce carbon dioxide emission and enhance the fuel efficiency. In last two decades, varieties of advanced high strength steel (AHSS) have been developed and widely used in the body in white. Recently, low density steels, which may have high potential for future automotive applications, have been designed with important aluminium additions obtaining density reductions of 8 10% or higher in comparison with low carbon steels. 1) Latest achievements of the investigation on the fundamentals of processing, phase transformations and microstructure development, deformation characteristics and mechanisms of strengthening, effects of alloying elements, thermodynamics of high Al-containing steels, and alloy development has been briefly reviewed by R. Rana. 2) Moreover, the first industrial coil of low density steel alloyed with approximate 4 mass% of aluminium has been successfully manufactured. 3) However, R. Rana 2) also suggested that many processing issues related to castability, rollability, surface oxidation during thermo-mechanical treatment and coatability in the presence of high alloying additions (Mn and Al) in these steels still need to be studied. It has been already recognized that the external oxidation of the alloying elements, particularly Si, may significantly ruins the wettability of the steel surface after being annealed * Corresponding author: jinxinyan@baosteel.com DOI: and results in poor surface quality or coating adhesion for the galvanized steel. In some transformation induced plasticity (TRIP) steels, Al is used to partially or completely replace Si to improve the active wetting. 4) It is revealed by J. Maki 5) that the silicon free CMnAl TRIP steel shows much improved wetting by the liquid Zn in comparison to a standard CMnSi TRIP steel. Al is expected to be much more easily oxidised internally than both Si and Mn since Al has much lower equilibrium oxygen potential. It is also pointed out that the Al content of the CMnAl TRIP steels should be high enough to form the Al depleted surface layer to improve the galvanizability. 5) However, the addition of Al in CMnAl TRIP steel is usually less than 2 mass%, the galvanizability of the low density steel with much higher aluminium addition has not be reported yet. A recent study on an Al rich Fe Mn Al C low density steel shows that a change of dew point influences both the oxide growing outwards from the surface and the internal oxidation process. 6) For a dew point of 10 C, Mn rich oxide is likely to dominate external growth, while Al participates in internal oxidation. With lower dew points of 30 C and 60 C, the steel becomes covered with continuous film of alumina and internal oxidation does not occur. The Al composition within the range of 2 6 mass% does not alter the general oxidation behaviour significantly. When comparing the limited literatures on the selective oxidation behaviour of Al in the steel, 5,6) it is still a little confused whether the galvanizability of high aluminium low density steel is good or not. A latest study on the selective oxidation of high alu ISIJ

2 minium low density steel 7) has found that a layer of nodular iron is formed on the outermost surface followed by layers of internal Al, Mn oxides and recrystallized ferrite when the dew point is increased to 10 C. Such an attractive surface morphology, which is totally different from other Mn and/or Si strengthening advanced high strength steel, is expected to have a positive effect on the the galvanizability of high aluminium low density steel. In order to find out the above speculation, some laboratory hot dip galvanizing experiments were conducted on a low density steel with 4 mass% Al addition in current work. The primary aim of this study is to gain further understanding of the selective oxidation behaviour of aluminium in matrix and its effect on the galvanizability of low density steel. Table 1. Chemical compositions of the steel sheets (mass%). C Al Mn Si Experimental Procedure The chemical composition of the steel is listed in Table 1. The slabs were processed by convert furnace steel making and continuous casting, and then were reheated and hot rolled between C and approximately 900 C to 3.2 mm and coiled at 600 C. The thickness was finally reduced to nominally 1.7 mm by cold-rolling. Blanks of 220 mm 120 mm were cut from a cold-rolled coil, and then heat-treated in IWATANI hot dipping galvanizing process (HDGP) simulator. Before annealing, the samples were degreased with the industrial degreaser, rinsed with hot water and dried with compressed N 2. Heat patterns, as shown in Fig. 1, were designed to investigate the influence of annealing process parameters on galvanizability of the steel. In the annealing process, the panel was heated with a fixed heating rate of 4 C/s to the soaking temperature of 815 C for 150 s, and cooled down at 10 C/s to the galvanizing temperature of 460 C. The atmosphere in the annealing furnace was N 2 with 5 vol% H 2. The dew point was constant for each test but varied between tests from 40 C to +10 C. In the galvanizing process, the panel was dipped into a zinc bath containing 0.20 mass% dissolved Al and Fe-saturated, which was held at 460 C. After the panel was immersed in the zinc bath for 3 s, it moved out and passed through an air knife to get the coating weight controlled, and was cooled down to room temperature with a cooling rate of 30 C/s by 100% N 2. The schematic diagram of the galvanized panel and the analysis areas are shown in Fig. 2. About two-thirds of the area on the panel was zinc coated, and the remaining one-third was uncoated. Uniform areas in the middle of the galvanized and annealed areas were cut down and analysed. The macro appearance was recorded by digital camera. The coating adhesion was tested by 0T bending. The surface morphologies of both the galvanized coating and the substrate were observed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The inhibition layer formed on the galvanized panel for SEM characterization was exposed by stripping the zinc overlay by 4 vol% HCl solution with C 6 H 12 N 4 inhibitor addition. Cross sectional morphologies of the galvanized areas were prepared by common metallography and observed by SEM. The detailed cross sectional structures of selected samples were characterised using a transmission electron microscopy (TEM) with lamellas prepared using a Fig. 1. Heat pattern of the galvanizing experiments. Fig. 2. Schematic diagram of a galvanized panel and the analysis areas. focused ion beam (FIB) extraction. 3. Results and Discussion Pictures of the galvanized panels prepared using the HDGP simulator are shown in Fig. 3. The sample annealed at a 40 C dew point (Fig. 3(a)) showed a very poor coating quality, there were large uncoated areas at the edges and a great number of large bare spots in the middle of the sample. The size and number of bare spots decreased as the dew point increased to 20 C, however there were still some pin hole bare spots dispersed on the sample (Fig. 3(b)). When the dew point was further increased to 10 C and +10 C, the coating quality was significantly improved. Although there were still some bare spot defects on the left half of the panels, marked by the circles in Figs. 3(c) and 3(d), the right half of the panels showed perfect coating quality. It was considered that these defects on the left half were caused by the residual oxide film on local surface of 2018 ISIJ 2

3 the zinc bath instead of the poor wettability of the substrate surface. It is evident that the galvanizability of the low density steel used in this work can be improved by increasing the dew point of the annealing atmosphere. The results of the 0T bending test are shown in Fig. 4. The coating on the sample annealed at 40 C dew point could be peeled off easily, indicating that its coating adhesion was as poor as its coating quality. When the dew point was increased to 20 C, only a small amount of coating was peeled off and the surface of bended specimen appeared a little rough, as pointed by the arrow in Fig. 4(b). When the dew point was increased to 10 C and +10 C, the coating adhesion was good enough and no coating was peeled off. It is clear that the coating adhesion could also be improved as the dew point was increased. The surface coating morphologies are shown in Fig. 5, Fig. 3. Macro appearances of the hot dip galvanized panels prepared at different dew points. a) 40 C, b) 20 C, c) 10 C, d) +10 C. Fig. 4. Marco appearances of the 0T bended samples prepared at different dew points. a) 40 C, b) 20 C, c) 10 C, d) +10 C. Fig. 5. Surface morphologies of the galvanized coating prepared at different dew points. a) 40 C, b) 20 C, c) 10 C, d) +10 C. Note that the magnification is 300x for a), b) and 100x for c), d) ISIJ

4 in which two different types were observed. In the cases of lower dew points ( 40 C and 20 C), the zinc grains had an irregular shape and did not exhibit visible dendritic structures. While in the cases of higher dew points ( 10 C and +10 C), the zinc grains had a clearly visible dendritic structure. In addition, the size of the dendritic structural zinc grains was several times larger than that of the nondendritic structural zinc grains. As the morphologies shown in Figs. 5(c) and 5(d), which were much close to that of the galvanized coatings produced in industrial continuous hot dip galvanizing lines, the dendritic arms grew laterally from the center of each grain and grew up in six 1110 rapid-growth dendritic directions. 8) However, the growth in these directions of the non-dendritic zinc grains as shown in Figs. 5(a) and 5(b) was suppressed. There are many factors determining the structure of the hot dip galvanized coating, such as bath composition, cooling rate, substrate surface condition, etc. 8) In present work, all other parameters except for the dew point of the annealing atmosphere were fixed. Therefore, it is believed that the substrate surface condition which was influenced by the dew point played a significant role in the nucleation and growth of the zinc coating during the solidification process. In order to clarify the cause of the different galvanizability, coating adhesion and coating morphology, the substrates were revealed when the zinc coating was stripped off by 4 vol% HCl solution with C 6 H 12 N 4 inhibitor addition and the results are shown in Fig. 6. The semi-quantitative analysis results of the typical spectrums for different areas in Fig. 6 are listed in Table 2. Two morphologies with different compositions were observed. The areas detected by spectrum 1 in Fig. 6(a) and spectrum 3 in Fig. 6(b) contained about 5 6 mass% of Al and the balance was Fe, indicating they were the matrix though the concentration of Al was a little higher than the Al in the matrix. While the other areas detected by spectrum 2, 4, 5, 6 in Fig. 6(a) to d contained a much higher concentration of Al, which was in the range of mass%, and they also contained a small amount of Zn and O when being compared to spectrum 1 and 3. It is considered that these areas were covered by inhibition layer formed during hot dipping process. It is obvious that the dew point of the annealing atmosphere had a pronounced effect on the formation of inhibition layer on low density steel. When the dew point was 40 C, the reaction between Al in the molten zinc and the Fe in the substrate was hindered and the coverage of inhibition layer was less than 10%. When the dew point was increased to 20 C, it increased to 30 40%. When the dew point was further increased to 10 C and +10 C, the substrate surface was almost 100% covered by inhibition layer, indicating a promoted reaction between Al and Fe. It has been well recognized that a formation Table 2. Semi-quantitative analysis results of the typical spectrums in Fig. 6 (mass%). Dew point ( C) Spectrum O Al Fe Zn Note Matrix Inhibition layer Matrix Inhibition layer Inhibition layer Inhibition layer Fig. 6. Morphologies of the substrate surface after the zinc coating was stripped. The dew point of the samples were a) 40 C, b) 20 C, c) 10 C, d) +10 C ISIJ 4

5 of continuous inhibition layer means a good wettability of molten zinc on the steel and will result in a good coating adhesion. Therefore, such a change tendency of inhibition layer with the dew point can well explain the improved coating quality and the good coating adhesion when the dew point is increased. In addition, it is necessary to note that the morphologies of the inhibition layer shown in Figs. 6(c) and 6(d) were a little different from the typical inhibition layer formed on most of the commercial galvanized steel. It is believed that the inhibition layer is determined by the substrate surface just before being hot dipped into the zinc bath. Therefore, attention was paid to the annealed areas on the top part of the panels. The surface morphologies of the samples prepared at different dew points are illustrated in Fig. 7. It can clearly be seen that the surface state was obviously changed by dew point. The most significant difference between these four samples were the size and amount of the particles that covered the surface. When the dew point was 40 C, there were only a bit of very small particles dispersed on the surface. In fact, when a cross sectional lamella was prepared by FIB, a continuous external Al oxide film as shown in Fig. 8(a) was observed by STEM. Because its thickness was only nm, it was too thin to be discovered by SEM plane view. When the dew point was increased to 20 C, both the size and the amount of the particles were increased. However, there were still some areas without any particles covered. According to the grain boundaries pointed by the arrows in Fig. 7(b), it seems that whether it was covered by particles or not was related to the grains of the matrix. When the dew point was increased to 10 C and +10 C, most of the surface was covered with a layer of polygonal particles. Some of the particles had been connected to each other while others were separated, and some very small gaps were left. The cross sectional morpho- Fig. 7. Surface morphologies of the uncoated areas prepared at different dew points. a) 40 C, b) 20 C, c) 10 C, d) +10 C. Fig. 8. Cross sectional STEM micrographs of FIB prepared samples annealed at dew point of a) 40 C and b) 10 C ISIJ

6 gloy of the sample annealed at a dew point of 10 C was shown in Fig. 8(b). The outermost layer which was almost pure iron was followed by an internal oxidation zone in the subsurface. The internal oxides were cofirmed to be Al and Mn oxides by EDS. It has been revealed that these particles were pure iron and caused by the overflow of iron atoms from the subsurface due to the significant volume expansion of the internal Al oxidation when the dew point was high enough. 7) Comparing the morphologies in Fig. 8 to those in Fig. 7 with the same dew point, the coverages of the iron particles on the annealed sheets were basically close to that of the inhibition layer on the galvanized sheets respectively. It can be concluded that the formation of inhibition layer must be hindered by the external Al oxide film when the dew point is low, while the formation of inhibition layer will be promoted by the increased amount of pure iron particles formed on the surface when the dew point is increased. As the cross sectional morphologies of the galvanized steel sheets shown in Fig. 9, different coating/substrate interface microstructure can be observed. Corresponding to the effect of dew point on the surface morphologies shown in Figs. 6 and 7, both the coverage of inhibition layer and iron particles were increased with dew point increasing. In addition, the effect of dew point on the internal oxidation can also be observed. When the dew point was 40 C, neither internal oxidation nor iron particles was observed, as a result of which, no inhibition layer was formed at the coating/substrate interface due to the external oxidation of aluminum shown in Fig. 8(a). When the dew point was 20 C, a very thin internal oxidation layer with a thickness of less than 0.5 um appeared in local areas and a bit of inhibition layer was formed at the interface in these areas. When the dew point was further increased to 10 C and +10 C, an internal oxidation layer and an iron particle layer were fully developed in the subsurface and on the outmost surface of the matrix respectively. Moreover, a very thin layer with the deepest contrast in the backscattered electron images, as pointed by the downward arrows in Figs. 9(c) and 9(d), was found at the interface of coating/iron particle. Considering the EDS results in Fig. 5 and Table 2, it is recognized as inhibition layer. Therefore, it is clear that the formations of internal oxidation layer, iron particle layer and inhibition layer are enhanced by dew point increasing. Figure 10 shows a more detailed analysis of the cross sectional structure of the galvanized sample prepared at a Fig. 10. Cross sectional STEM micrographs of the FIB extracted lamella from the coated sample with a dew point of +10 C. Fig. 9. Cross sectional morphologies of the galvanized steel sheets prepared at different dew points. a) 40 C, b) 20 C, c) 10 C, d) +10 C ISIJ 6

7 dew point of + 10 C, which representing the galvanizability improved samples. The elemental mappings of the selected area are shown in Fig. 11 and the semi-quantitative EDS analysis results of the marked phases in Fig. 10 are listed in Table 3. From top to bottom, there were Pt protective layer, zinc coating, inhibition layer, iron particle layer and internal oxidation layer. It is necessary to note that the iron particle layer was free of oxides on the surface and inside the grains. Therefore, continuous inhibition layer containing Fe, Al and Zn and having a thickness of nm covered both the surface of iron particles and the gaps between them. Due to the large coverage of the iron particles on the surface, the reaction between Al in the molten zinc and the iron particles was much more dominant than that between the Al and the matrix surface. The schematic illustration of the effect of dew point increasing on the galvanizability improvement of high aluminum low density steel is shown in Fig. 12. When the steel sheet having 4 mass% of aluminum in the matrix is annealed in a high dew point atmosphere and hot dipped into a zinc bath containing 0.2 mass% dissolved Al, three interesting reactions occur. The first one is the formation of the internal oxidation layer in the subsurface. The selective oxidation behaviour of Al is very similar to those of other alloy elements such as Si, Mn, which have been widely studied. With dew point increasing, the external Al oxidation converts Table 3. Semi-quantitative analysis results of the labeled spectrums in Fig. 10 (mass%). Spectrum Al Fe Zn Note Inhibition layer Iron particle layer Fig. 11. TEM EDS elemental mappings of the selected area by dashed rectangle in Fig. 10. a) Fe, b) Al, c) Zn, d) O and e) Pt. (Online version in color.) ISIJ

8 Fig. 12. Schematic diagram of the galvanizability improvement mechanism of low density steel by dew point increasing. a) annealing stage, b) hot dipping stage. into internal oxidation. The second one is the formation of the iron particle layer on the outmost surface, which is not so popular in other Si and Mn strengthening high strength steels. It has been reported in our previous study 7) that the iron particle layer is also influenced by the annealing temperature and soaking time besides the dew point. And based on the theoretical calculation of the internal oxides volume of different alloy elements, it has been figured out that the formation of the outermost iron particle layer is caused by the overflow of iron atoms from the subsurface due to the significant volume expansion of the internal Al oxides. The third one is the formation of inhibition layer on both the surfaces of iron particles and gaps between them, in which the former is the dominant one. The growth of inhibition layer at the interface is promoted not only by the formations of internal Al oxides in subsurface but also by the iron particles on outmost surface. Due to the above three important reactions, both the wettability and the coating adhesion of hot dip galvanized low density steel are improved significantly by dew point increasing. 4. Conclusion The effect of dew point on the galvanizing behaviour of a high aluminium low density steel was investigated. It is found that the galvanizability of 4 mass% aluminium added low density steel can be effectively improved by increasing the dew point of the annealing atmosphere. The improved coating quality is not only a result of the conversion of external Al oxidation to internal Al oxidation when the dew point was increased, but also a result of the formation of iron particle layer on the outmost surface of the substrate. The iron particles whose sufaces are free of oxides provides large areas for the molten zinc to wet and react with iron to form Fe Al Zn inhibition layer, resulting in both good quality and strong coating adhesion of high aluminum low density steel sheet. REFERENCES 1) I. Zuazo, B. Hallstedt, B. Lindahl, M. Selleby, M. Soler, A. Etienne, A. Perlade, D. Hasenpouth, V. Massardier-Jourdan, S. Cazottes and X. Kleber: JOM, 66 (2014), ) R. Rana: JOM, 66 (2014), ) X. C. Xiong, L. Sun, J. F. Wang, X. Y. Jin, L. Wang, B. Y. Xu, P. Chen, G. D. Wang and H. L. Yi: Mater. Sci. Technol., 32 (2016), ) E. M. Bellhouse and J. R. McDermid: Mater. Sci. Eng. A, 491 (2008), 39. 5) J. Maki, J. Mahieu, B. C. De Cooman and S. Claessens: Mater. Sci. Technol., 19 (2003), ) T. K. Jeong, G. Jung, K. Lee, Y. B. Kang, H. K. D. H. Bhadeshia and D.-W. Suh: Mater. Sci. Technol., 30 (2014), ) X. Y. Jin, Y. X. Xie and H. Wang: Proc. 11th Int. Conf. on Zinc and Zinc Alloy Coated Steel Sheet (Galvatech 2017), The Iron and Steel Institute of Japan,Tokyo, (2017), ) S. Kaboli and J. R. Mcdermid: Metall. Mater. Trans. A, 45 (2014), ISIJ 8