Phase Formation of Al/Si-Coatings during Induction Heating of Boron Alloyed Steel Sheets

Robert Veit1, Harald Hofmann2, Ralf Kolleck1, Peggy Brugger2, Ingo Thomas2, Sascha Sikora2

1Graz University of Technology, Institute Tools and Forming, Inffeldgasse 11, 8010 Graz, Austria

robert.veit@tugraz.at 2ThyssenKrupp Steel Europe AG

Abstract: In Hot stamping of boron alloyed steels Aluminum-Silicon coated sheets cover a high percentage of the applications. The melting temperature of this coating is below the necessary austenitization temperature of the base material. This means, that a diffusion process between base material and coating has to take place during heating, leading to a higher melting temperature of the coating. In conventional heating devices, like roller hearth furnaces, the diffusion process is reached by relatively low heating rates. New technologies, like induction heating, reach very high heating rates and offer great potentials for the application in hot stamping. But also with these technologies the diffusion of iron into the coating has to be assured to reach the same surface conditions as in conventional heating. A new heating concept based on induction heating was introduced by the Institute Tools and Forming, Austria. With this heating device, Aluminum-Silicon coated sheets can be heated within 100 s. This paper will present the results of comparative heating tests. EDX analysis and Electron Backscatter Diffraction (EBSD) were used to describe the phase formation within the coating. Keywords: Phase formation, AlSi coating, hot stamping, boron alloyed steel, 22MnB5, induction heating, EBSD

1. INTRODUCTION

Hot stamping of boron alloyed steels has become one of the most important technologies for the production of high strength car body parts. With this technology complex parts can be produced because the part is formed at high temperature with a austenitic grain structure. In the closed tool the part is cooled down with a minimized critical cooling rate of 27 K/s to reach a transformation from austenite into martensite [1]. The produced components are characterized by a high strength of about 1500 MPa and a residual elongation of about 5%. There are two process variants: direct and indirect hot stamping. The direct process starts with a plain blank that is heated and formed, while the indirect process uses a preformed component.

2. COATING SYSTEMS FOR HOT STAMPING

To reduce abrasive wear in the forming tool and to assure the paintability of the formed component the formation of scale has to be avoided during the heating process. As a result either uncoated materials have to be heated under protective gas or a protective coating has to be applied on the sheet metal. Currently there are only three industrial relevant coating systems available: Zinc, Aluminum/Silicon and x-tec. Zinc coatings can only be used for the indirect hot stamping process because at the austenitization temperature a further forming step would lead to zinc adsorption at the grain boundaries. This would cause delayed cracks in the component [2]. x-tec is a paint system based on nanotechnology which avoids scale formation during heating but offers only a limited corrosion resistance for the formed component. The coating can be used for both process variants of hot stamping [3]. A new coating system (GammaProtect) has been introduced by ThyssenKrupp Steel Europe [4]. It consists basically of electrolytically deposed zinc with a small content of nickel. But it has yet to be shown that this product is capable of fulfilling the requirements of the automotive industry.

3. AL/SI COATED STEELS FOR HOT STAMPING

Because of the minimum number of process steps the direct hot stamping process is the one which is actually preferred in the automobile industry. Here mostly Al/Si-coated blanks are used. The coating consists typically of approx. 87 wt.% aluminum, 10 wt.% silicon and, 3 wt.% iron in average. Figure 1 shows a coating in delivery condition. The upper layer consists of biphasic eutectic aluminum and silicon. The layer below is the τ5-phase (Al8Fe2Si) containing already approx. 31 wt.% iron.

Figure 1. 22MnB5 with Al/Si coating in delivery condition

The melting temperature of the coating is below the austenitization temperature of the base material at 580-600 °C. During the heating of the blanks, Fe diffuses into the coating which leads to an increased melting temperature [5]. It was proved that at

industrial relevant heating rates the liquid phase of the coating cannot be avoided. Circular samples with 5 mm in diameter were observed with a scanning electron microscope. The samples were mounted on a heating stage and heated to 950°C. The heating rate was limited to 50 K/min. Figure 2 illustrates the results of these investigations. At 99 °C the surface of the coating is still similar to the delivery condition. The material shows typical surface textures from the coating process. At 591 °C these textures disappear because the coating reveals a phase transformation into a liquid phase. At 949 °C the coating is resolidified with a completely new grain structure [6].

Figure 2. Heating process observed with a scanning electron microscope

4. HEATING OF AL/SI-COATED SHEETS

The special properties of the Al/Si-coatings are reflected in the currently used furnace technologies. Conventionally continuous furnaces like roller hearth or walking beam furnaces are used for hot stamping. The blank is heated by radiation and convective flow of heat. The heating rate of the blanks is controlled by the speed of the rollers or the walking beams on the one hand and by the temperatures in different consecutive chambers on the other. The sheet thickness as well as the coating and therefore the emission coefficient have a great impact on the heating behaviour and the effective sheet temperature. With a typical length of more than 40 m, conventional furnaces require very large spaces and consume a lot of energy. The energy efficiency of such a furnace reaches only 55% with an optimum load of the furnace [7]. Induction heating devices for the heating of the blanks offer the potential of increased energy efficiency, reduced cost and reduced floor space. An induction heating device consists of two components, a high-frequency generator and an induction coil, the so-called inductor. When the sheet enters the inductor, a current is induced in it. This eddy current is short-circuited and results in heating, depending on the operating frequency, the electrical conductivity and the permeability of the material. Induction heating offers the highest surface power density, so very high heating rates can be reached [8]. It was proved, that with uncoated material the same part properties can be reached than with conventional heating [9]. Al/Si-coated sheets cannot be heated completely by induction

heating because of the magnetic field forces on the liquid coating. This would lead to a dislocation of the coating and a part of the coating can even drip off [10].

5. NEW FURNACE CONCEPT FOR AL/SI COATED STEELS

To avoid the dislocation of the liquid coating a new combined furnace concept was developed. Figure 3 shows the furnace concept which consists of four individual heating stages. The sheet is heated in a longitudinal field inductor (A) to the coating’s melting temperature. During the coating’s liquid phase, the next heating step is completed by an inductively heated muffle furnace (B). This furnace type consists of a steel muffle that surrounds the work piece. This muffle is brought to the desired temperature through induction. It heats the work piece through radiation and a convective flow of heat. Once the liquid coating has re-solidified, the sheet can be brought to its final temperature by a face inductor (C). The last stage, a resistance heating device (D), assures a homogenous heat distribution throughout the workpiece, until the blank is transferred into the forming tool, where no further heating takes place.

Figure 3: Combined induction heating device for Al/Si-coated sheets

6. COMPATATIVE HEATING TESTS - EXPERIMENTAL SETUP

With the new furnace concept, shaped blanks were heated to 930°C within 100s, held on temperature for 25s and were then quenched in water. The reference blank was heated in a conventional chamber furnace to 930°C with a total furnace time of 360s. To compare the two different heating technologies the coatings have been characterised in terms of chemical composition by EDX. The crystallographic orientations and phases of single grains within the coating layer have been determined by Electron Backscatter Diffraction (EBSD). For the EDX-Analysis cross sections etched with 4% alcoholic HNO3 have been used. The EBSD measurements have been carried out with cross

sections polished with colloidal silica in order to minimise surface deformation. For both measurements the LEO 1530 FE-SEM (Carl Zeiss, Germany) equipped with a TSL EBSD system with a Digiview cam and a EDX-system (EDAX Gemini) has been used.

7. MICROSTRUCTURE AND TEXTURE OF THE AL/SI-COATING AFTER CONVENTIONAL AND INDUCTION HEATING

For both cross sections the EDX analysis hints to the existence of five different phases within the tempered coating (see figure 4). According to the ternary phase diagram [11] for Fe, Al and Si a diffusion layer consisting of the iron rich phases α-Fe, Fe3(Al,Si) and Fe(Al, Si) can be found at the interface with the steel substrate. This layer is followed by the phase Fe2Al5 which is the main phase of the coating. In the case of induction heating an additional Si rich layer consisting of Fe3(Al,Si)5 (τ1 phase) is embedded in the Fe2Al5 matrix. For conventional heating on the other hand the Si rich layer consists of Fe3(Al,Si)5 and Fe(Al, Si). This means there is a higher Fe content within this layer near the surface. Additionally the thickness of the interfacial diffusion layer is different for the two heating methods. Both the thicker interfacial diffusion layer and the higher Fe content in the Si-rich layer in the coating with conventional heating indicate a higher grade of alloying compared to induction heating. While both coatings roughly show the same thickness the surface roughness seems to be different. The conventional heating obviously leads to a higher surface roughness.

Figure 4. Phase distribution evaluated by EDX.

The formation of the intermetallic coating layer is controlled by Fe-Al interdiffusion. The diffusion of Fe in Al is faster than that of Al in Fe [12]. This leads to the formation of so called Kirkendall-voids [13] at the interface of phases with different Fe content which can be seen in the interfacial diffusion layer and close to the surface. This mechanism possibly creates the observed higher surface roughness for conventional heating due to the higher alloying grade. A similar mechanism has been proposed for Fe-Al diffusion couples [14, 15].

Figure 5. Colour-coded EBSD phase map. Some Fe2Al5 grains and grain boundaries

show single dots of τ1-phase caused by misindexing of the EBSD patterns. Truly indexed τ1 points are located in the compact yellow grains mainly close to the surface.

As the phases α-Fe (space group Im-3m), Fe(Al, Si) (space group Pm-3m) and Fe3(Al,Si) (space group Fm-3m) all show a cubic structure they can not be separated by EBSD and are all indexed as α-Fe in figure 5. Therefore the EBSD phase determination can only distinguish between cubic α-Fe, orthorhombic Fe2Al5 (space group Cmcm) and triclinic τ1 (space group P-1). In these terms the EBSD phase analysis fits very well to the EDX analysis and the phase distribution from figure 4 is confirmed. As Fe2Al5 represents the main phase of the coating layer, the texture of this phase has been evaluated from the EBSD data. Figure 6 shows the orientations of single grains of all phases in normal and rolling direction. The texture of the Fe2Al5 grains is additionally represented by the pole figure of a 001 plane in figure 7. It reveals a preferential growth of the c-axis (001 direction) in normal direction. This result agrees with several other studies [16, 17, 18] and can be explained by the rapid diffusion path along the c-axis of the Fe2Al5 orthorhombic crystal which shows a high density of vacancies.

Figure 6. EBSD inverse pole figure orientation mapping in Normal – and Rolling

direction for Fe2Al5, α-Fe an τ1. Some uniform coloured grains show multicoloured dots caused by misindexing of the EBSD patterns.

Figure 7. Pole figure of the (001) planes in Fe2Al5.

8. SUMMARY

The comparison of the AS-coating produced with conventional and induction heating shows no significant differences in coating thickness and texture of the main component Fe2Al5. The conventional heating method leads to a slightly higher iron content of the AS-coating. As a consequence of this, a stronger formation of Kirkendall voids is observed in the conventionally heated sample. The voids are mainly located at the interface and at the surface of the AS-coating. Most likely this is the reason for the higher surface roughness in the case of conventional heating.

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