Inﬂuence of Multi-Step Heating Methods on Properties of Al–Si Coating Boron Steel Sheet

: In this study, the inﬂuence of the multi-step heating methods, such as two-step heating methods and three-step heating methods, on the properties of Al–Si coating boron steel sheet were evaluated by using the Gleeble-3500 thermal simulator. The evolution of microstructure and 3D surface topography of the Al–Si coating were also investigated. The results showed that the heating rates of 50 ◦ C/s, named rapid heating, at the stage of 20–500 ◦ C did not signiﬁcantly inﬂuence the microstructure and 3D surface topography of the Al–Si coating in the two-step heating methods. The results also indicated that the volume fractions of Fe 3 Al 2 Si 3 intermetallic compound, FeAl intermetallic compound and a-Fe phase in the Al–Si coating reduced by rapid heating at the stage of 700–930 ◦ C in the three-step heating methods. The roughness of 3D surface topography of the Al–Si coating increased by rapid heating at the stage of 700–930 ◦ C. Rapid heating at the stage of 700–930 ◦ C had little inﬂuence on the porosity of the Al–Si coating. The results provided a theoretical basis for the popularization and application of rapid heating in the Al–Si coating boron steel sheet.


Introduction
With the increasingly serious of energy and environment crisis, energy saving and emission reducing were the future development direction of the automobile industry. The effective method to achieve energy saving and emission reducing was to reduce the weight of the automobiles which did not lose the structural strength of automobiles. The use of ultra-high strength steel (also called boron steel) became necessary due to meeting the requirements of lightweight and structural strength of the automobiles. The cold forming of ultra-high strength steel had the problems of wrinkling, cracking, springback, poor dimensional accuracy, and high forming force [1]. In order to solve the above problems, the hot stamping method was usually used internationally [2][3][4][5][6][7][8]. The hot stamping included the following process of heating, transportation, forming, and simultaneous quenching. However, the surface of the uncoated ultra-high strength steel was prone to oxidation and decarburization during the hot stamping process of heating, transportation, and forming [9]. The deciduous oxide scale would accelerate the wear of the die surface and affect the life of the die and the forming quality of the parts [10,11]. While the decarburization would reduce the surface strength of the hot stamping parts [12]. The coating boron steel sheet solved the oxidation and decarburization of the uncoating boron steel sheet, which made it ever-increasing used in hot stamping [13][14][15]. At present, the coatings applied to hot The cross section of the Al-Si coating layer and details of the intermetallic compound layer were displayed in Figure 1. As shown in Figure 1, the surface of the Al-Si coating layer was relatively smooth, and the intermetallic compound structure remained continuous and dense. The EDS results of the marked regions in the Al-Si coating layer were given in Table 2. The Al-Si coating layer was composited of Fe 2 Al 7 Si, Fe 3 Al 2 Si 3 , and Fe 2 Al 5 . The 3D surface topography of the Al-Si coating was illustrated in Figure 2. The roughness of the Al-Si coating surface was small, and the height difference between the highest surface and the lowest surface of the Al-Si coating was 7.18 µm.
sults of the studies have certain guiding significance for the development of heating process for the Al-Si coating boron steel sheet.

Materials
The thickness of the Al-Si coating boron steel sheet using in the tests in hot-dipped conditions is 1.8 mm. The chemical compositions of the substrate are displayed in Table  1. The coating was hot-dipped on the substrate in a molten bath containing Al-10 wt % Si. The average coating weight of the top surface of the coated boron steel sheet was about 84 g/m 2 , and that of the bottom surface was about 77.6 g/m 2 [39].
The cross section of the Al-Si coating layer and details of the intermetallic compound layer were displayed in Figure 1. As shown in Figure 1, the surface of the Al-Si coating layer was relatively smooth, and the intermetallic compound structure remained continuous and dense. The EDS results of the marked regions in the Al-Si coating layer were given in Table 2. The Al-Si coating layer was composited of Fe2Al7Si, Fe3Al2Si3, and Fe2Al5. The 3D surface topography of the Al-Si coating was illustrated in Figure 2. The roughness of the Al-Si coating surface was small, and the height difference between the highest surface and the lowest surface of the Al-Si coating was 7.18 μm.     The thickness of the Al-Si coating boron steel sheet using in the tests in hot-dipped conditions is 1.8 mm. The chemical compositions of the substrate are displayed in Table  1. The coating was hot-dipped on the substrate in a molten bath containing Al-10 wt % Si. The average coating weight of the top surface of the coated boron steel sheet was about 84 g/m 2 , and that of the bottom surface was about 77.6 g/m 2 [39].
The cross section of the Al-Si coating layer and details of the intermetallic compound layer were displayed in Figure 1. As shown in Figure 1, the surface of the Al-Si coating layer was relatively smooth, and the intermetallic compound structure remained continuous and dense. The EDS results of the marked regions in the Al-Si coating layer were given in Table 2. The Al-Si coating layer was composited of Fe2Al7Si, Fe3Al2Si3, and Fe2Al5. The 3D surface topography of the Al-Si coating was illustrated in Figure 2. The roughness of the Al-Si coating surface was small, and the height difference between the highest surface and the lowest surface of the Al-Si coating was 7.18 μm.

Experimental Schemes
According to the influence of heating rates, maximum heating temperature, and holding times on the properties of the Al-Si coating [37], new heating tests were designed. The new heating tests included the two-step heating methods ( Figure 3a) and three-step heating methods (Figure 3b). The tests were used to further explore the effects of the multi-step heating methods on the microstructure and 3D surface topography of the Al-Si coating. The tests were implemented by the Gleeble-3500 thermal simulator. After different heat treatment tests, the samples used to characterize microstructure were cut from the middle of the heat treatment samples ( Figure 4). Then, the samples were sanded with the sandpapers and polished with the polishing pastes. The cross-sectional microstructure, porosity, and 3D surface topography of the Al-Si coating after the heat treatments were observed by ESEM/EDS and 3D optical microscope, respectively. According to the influence of heating rates, maximum heating temperature, and holding times on the properties of the Al-Si coating [37], new heating tests were designed. The new heating tests included the two-step heating methods ( Figure 3a) and three-step heating methods (Figure 3b). The tests were used to further explore the effects of the multistep heating methods on the microstructure and 3D surface topography of the Al-Si coating. The tests were implemented by the Gleeble-3500 thermal simulator. After different heat treatment tests, the samples used to characterize microstructure were cut from the middle of the heat treatment samples ( Figure 4). Then, the samples were sanded with the sandpapers and polished with the polishing pastes. The cross-sectional microstructure, porosity, and 3D surface topography of the Al-Si coating after the heat treatments were observed by ESEM/EDS and 3D optical microscope, respectively.

Microstructure Evolution of Al-Si Coating Boron Steel Sheet
After the Al-Si coating boron steel sheet was treated by the two schemes of the twostep heating methods ( Figure 5), microstructure and phase compositions of the Al-Si coating were illustrated in Figure 6. According to the influence of heating rates, maximum heating temperature, and holding times on the properties of the Al-Si coating [37], new heating tests were designed. The new heating tests included the two-step heating methods ( Figure 3a) and three-step heating methods (Figure 3b). The tests were used to further explore the effects of the multistep heating methods on the microstructure and 3D surface topography of the Al-Si coating. The tests were implemented by the Gleeble-3500 thermal simulator. After different heat treatment tests, the samples used to characterize microstructure were cut from the middle of the heat treatment samples ( Figure 4). Then, the samples were sanded with the sandpapers and polished with the polishing pastes. The cross-sectional microstructure, porosity, and 3D surface topography of the Al-Si coating after the heat treatments were observed by ESEM/EDS and 3D optical microscope, respectively.

Microstructure Evolution of Al-Si Coating Boron Steel Sheet
After the Al-Si coating boron steel sheet was treated by the two schemes of the twostep heating methods ( Figure 5), microstructure and phase compositions of the Al-Si coating were illustrated in Figure 6.

Microstructure Evolution of Al-Si Coating Boron Steel Sheet
After the Al-Si coating boron steel sheet was treated by the two schemes of the twostep heating methods ( Figure 5), microstructure and phase compositions of the Al-Si coating were illustrated in Figure 6.  It can be seen from Figure 6 that after the two-step heating methods, a small amo new intermetallic compound of FeAl4.5Si would form in the Al-Si matrix, and the in metallic compound of Fe2Al7Si grew from the substrate regions towards the Al-Si eute compound region. There were Fe3Al2Si3, Fe2Al7Si, Fe2Al5, and FeAl4.5Si (Table 3) in the termetallic layers, detected by EDS, as illustrated in Figure 6. The dense degree of Fe2A formed in scheme A was better than that of scheme B, while the roughness of the A coating surface in scheme B was slightly better than that of scheme A. It can also be s from Figure 6 that compared with the process of heating from 20 to 700 °C via the heat rates of 10 °C/s [37], the microstructure of the Al-Si coating obtained by the two-step h ing methods was almost the same, which indicated that the two-step heating methods not influence the microstructure and intermetallic compounds of the Al-Si coating. I because that at a certain heating temperature, regardless of the heating rates, if the heat temperature was under the phase transition temperature point, no new intermetallic co pounds would formed. The highest temperature at the stage of 20-500 °C was less th It can be seen from Figure 6 that after the two-step heating methods, a small amou new intermetallic compound of FeAl4.5Si would form in the Al-Si matrix, and the int metallic compound of Fe2Al7Si grew from the substrate regions towards the Al-Si eutec compound region. There were Fe3Al2Si3, Fe2Al7Si, Fe2Al5, and FeAl4.5Si (Table 3) in the termetallic layers, detected by EDS, as illustrated in Figure 6. The dense degree of Fe2A formed in scheme A was better than that of scheme B, while the roughness of the Al coating surface in scheme B was slightly better than that of scheme A. It can also be se from Figure 6 that compared with the process of heating from 20 to 700 °C via the heati rates of 10 °C/s [37], the microstructure of the Al-Si coating obtained by the two-step he ing methods was almost the same, which indicated that the two-step heating methods d not influence the microstructure and intermetallic compounds of the Al-Si coating. I because that at a certain heating temperature, regardless of the heating rates, if the heati temperature was under the phase transition temperature point, no new intermetallic co pounds would formed. The highest temperature at the stage of 20-500 °C was less th the melting temperature of Al-Si eutectic compounds [31], the eutectic compounds in It can be seen from Figure 6 that after the two-step heating methods, a small amount new intermetallic compound of FeAl 4.5 Si would form in the Al-Si matrix, and the intermetallic compound of Fe 2 Al 7 Si grew from the substrate regions towards the Al-Si eutectic compound region. There were Fe 3 Al 2 Si 3 , Fe 2 Al 7 Si, Fe 2 Al 5 , and FeAl 4.5 Si (Table 3) in the intermetallic layers, detected by EDS, as illustrated in Figure 6. The dense degree of Fe 2 Al 7 Si formed in scheme A was better than that of scheme B, while the roughness of the Al-Si coating surface in scheme B was slightly better than that of scheme A. It can also be seen from Figure 6 that compared with the process of heating from 20 to 700 • C via the heating rates of 10 • C/s [37], the microstructure of the Al-Si coating obtained by the two-step heating methods was almost the same, which indicated that the two-step heating methods did not influence the microstructure and intermetallic compounds of the Al-Si coating. It is because that at a certain heating temperature, regardless of the heating rates, if the heating temperature was under the phase transition temperature point, no new intermetallic compounds would formed. The highest temperature at the stage of 20-500 • C was less than the melting temperature of Al-Si eutectic compounds [31], the eutectic compounds in the Al-Si coating did not melt yet and remain solid. There was not phase changed in the Al-Si coating below 573 • C [40]. Due to the hindrance of Si in the Al-Si coating, the interdiffusion between Al atoms and the substrate in the Al-Si coating was very slow [41][42][43][44]. Therefore, the heating rates of 50 • C/s, named rapid heating, can be applied to heating the Al-Si coating at the stage of 20-500 • C which did not significantly influence on the microstructure of the Al-Si coating.

Variations of 3D Surface Topography of Al-Si Coating Boron Steel Sheet
After the two schemes of the two-step heating methods, 3D surface topography of the Al-Si coating was shown in Figure 7. It can be seen from Figure 7 that the height differences between the highest surface and the lowest surface of the Al-Si coating through scheme A and scheme B were 17.38 and 16.67 µm, respectively. That is to say, the roughness of the Al-Si coating surface of scheme A was slightly greater than that of scheme B. Compared with the heating rates of 10 • C/s [37], the roughness of the Al-Si coating surface slightly reduced. In other words, the two schemes have little influence on the roughness of the Al-Si coating surface. After the process of two-step heating methods, the types of intermetallic compounds in the Al-Si coating were the same and the differences of 3D surface topography were not large. However, compared with the hot-dipped condition, the roughness of the Al-Si coating surface was greatly changed. It is because that the new FeAl 4.5 Si intermetallic compound generated in the Al-Si matrix and the Fe 2 Al 7 Si intermetallic compound grew from the substrate regions toward the Al-Si eutectic compound regions. According to Fe-Al-Si intermetallic compounds phase diagram [45,46], when the heating temperature was more than 550 • C, Fe atoms diffused into the Al-Si coating layer, new Fe x Al y Si z intermetallic compounds would be generated. Dense intermetallic compounds formed in some regions, while in other regions no or a small number of intermetallic compounds formed. In the regions where dense intermetallic compounds were generated, the volume would reduced, while in the non-generated regions, the volume kept invariant. Due to the influence of atoms diffusion and diffusion times, the distribution of intermetallic compounds in the Al-Si coating would become uneven, which would affect 3D surface topography of the Al-Si coating. The experimental studies showed that rapid heating can be used at 20-500 • C without affecting the properties of the Al-Si coating. tallic compounds formed. In the regions where dense intermetallic compounds wer erated, the volume would reduced, while in the non-generated regions, the volum invariant. Due to the influence of atoms diffusion and diffusion times, the distribution termetallic compounds in the Al-Si coating would become uneven, which would aff surface topography of the Al-Si coating. The experimental studies showed that rapid h can be used at 20-500 °C without affecting the properties of the Al-Si coating.

Microstructure Evolution of Al-Si Coating Boron Steel Sheet
After the four schemes of the three-step heating methods (Figure 8), microstr and phase composition of the Al-Si coating was shown in Figure 9. It can be seen Figure 9 that compared with the hot-dipped condition, the Al-Si coating was transf into Fe3Al2Si3, FeAl2, FeAl, and α-Fe through the four schemes ( Table 4). The dark was the FeAl2 intermetallic compound, and the bright phases were Fe3Al2Si3 interm compound, or FeAl intermetallic compound or α-Fe phase. According to a quant image analysis (as shown in Table 5

Microstructure Evolution of Al-Si Coating Boron Steel Sheet
After the four schemes of the three-step heating methods (Figure 8), microstructure and phase composition of the Al-Si coating was shown in Figure 9. It can be seen from Figure 9 that compared with the hot-dipped condition, the Al-Si coating was transformed into Fe 3 Al 2 Si 3 , FeAl 2 , FeAl, and α-Fe through the four schemes ( Table 4). The dark phase was the FeAl 2 intermetallic compound, and the bright phases were Fe 3 Al 2 Si 3 intermetallic compound, or FeAl intermetallic compound or α-Fe phase. According to a quantitative image analysis (as shown in Table 5), the mean thickness of the Al-Si coating through the four schemes was 39.4, 40.3, 39.8, and 37.5 µm, respectively. and phase composition of the Al-Si coating was shown in Figure 9. It can be seen from Figure 9 that compared with the hot-dipped condition, the Al-Si coating was transformed into Fe3Al2Si3, FeAl2, FeAl, and α-Fe through the four schemes ( Table 4). The dark phase was the FeAl2 intermetallic compound, and the bright phases were Fe3Al2Si3 intermetallic compound, or FeAl intermetallic compound or α-Fe phase. According to a quantitative image analysis (as shown in Table 5), the mean thickness of the Al-Si coating through the four schemes was 39.4, 40.3, 39.8, and 37.5 μm, respectively.   The bright α-Fe phase located near the substrate was a solid solution of Al and Si atoms in the high temperature. The α-Fe phase has the feature of the preferable plasticity  The bright α-Fe phase located near the substrate was a solid solution of Al and Si atoms in the high temperature. The α-Fe phase has the feature of the preferable plasticity which could prevent the growth of the cracks extending to the substrate [47]. The mean thickness of the α-Fe soft phase of the Al-Si coating through the four schemes was 9.7, 8.8, 7.6, and 6.8 µm, separately. The volume fractions of the α-Fe soft phase of the Al-Si coating by the four schemes was 22.68%, 20.53%, 21.32%, 14.06%, respectively. The volume fractions of Fe 3 Al 2 Si 3 intermetallic compound of the Al-Si coating via the four schemes was 11.53%, 9.85%, 7.69%, and 4.97%, in turn. The volume fractions of FeAl intermetallic compound of the Al-Si coating through the four schemes was 9.92%, 8.75%, 6.19%, and 3.71%, respectively. The results also indicated that the thickness of the α-Fe phase gradually reduced and the distribution of Fe 3 Al 2 Si 3 intermetallic compound and FeAl intermetallic compound was less homogenous through the four schemes. It is because that when the heating temperature reached the Fe-Al-Si transformation temperature point, corresponding new intermetallic compounds formed. The number and the distribution of intermetallic compounds were decided by the diffusion times. There was a further alloying process in the Al-Si coating at the stage of 700-930 • C. Because the time of rapid heating was short, the interdiffusion between Fe atoms and Al atoms was insufficient which affected the alloying process. Thus, the number of newly formed intermetallic compounds reduced.
The number and the size of Kirkendall voids through the four schemes gradually increased in the Al-Si coating, except scheme B. The size of Kirkendall voids in scheme D reached maximum. The porosity of the Al-Si coating was 2.92%, 2.53%, 2.96%, and 3.31%, separately. The Kirkendall voids appeared in the Al-Si coating through the four schemes due to the inconsistent diffusion coefficient of Fe atoms and Al atoms [31]. A small amount of micro-cracking appeared in the Al-Si coating through the four schemes owning to the inconsistent thermal expansion coefficient of the new formed intermetallic compounds [48].

Variations of 3D Surface Topography of Al-Si Coating Boron Steel Sheet
After the four schemes of the three-step heating methods, the 3D surface topography of the Al-Si coating was shown in Figure 10. It can be seen from Figure 10 that the roughness of the Al-Si coating surface rose gradually through four schemes, and the height differences between the highest surface and the lowest surface of the Al-Si coating through the four schemes was 11.83, 12.89, 20, and 23.49 µm, respectively. That is to say, scheme D was the largest, and scheme A was smallest. Scheme A and scheme C were smaller than that of scheme B and scheme D, separately. The roughness of the Al-Si coating surface of scheme B was slightly greater than that of scheme A. It is because that there was a severe stage of alloying and phase transformation of the Al-Si coating at the temperature ranging from 700 to 930 • C. However, the heating time of rapid heating was short which led the insufficient interdiffusion between the substrate and the Al-Si coating. It would cause the uneven distribution of newly generated intermetallic compounds, which would affect 3D surface topography of the Al-Si coating. With the heating temperature increasing, more and more Fe-rich intermetallic compounds formed in the Al-Si coating. Due to the high density of the Fe-rich intermetallic compounds (Table 6), the volumes of the regions became smaller, and the changing density of the newly generated intermetallic compounds would also affect 3D surface topography of the Al-Si coating. In addition, the newly generated intermetallic compounds would also influence the diffusion of atoms in the Al-Si coating, making the diffusion of atoms easier in some regions of the Al-Si coating, but hindering in some regions, which would also influence 3D surface topography. Therefore, rapid heating made the larger roughness of the Al-Si coating surface appear in the stage of 700-930 • C. Since there was not phase changed in the Al-Si coating layer in the stage of 20-500 • C, the rapid heating had little influence on the roughness of the Al-Si coating surface. Compared with the two-step heating methods, the roughness of the Al-Si coating surface through the three-step heating methods of scheme A and scheme B became smaller, but scheme C and scheme D became larger. It is because that the Al-Si coating would be conducted in the direction of generating dense intermetallic compounds to reduce the volume and formed a smooth surface to minimize its surface energy. While the formation mechanism of 3D surface topography of the Al-Si coating was complicated. In a word, the heating temperature, the diffusion of atoms, the newly generated intermetallic compounds, the surface tension and the degree of oxidation of the surface jointly determined the changes of 3D surface topography of the Al-Si coating. Table 6. Density values of the common Fe x Al y Si z intermetallic compounds [27].

Intermetallic Compounds Density (g/cm 3 )
Fe 2 Al 7 Si 3.58 Fe 2 Al 8 Si 3.62 FeAl 3 3.9 Fe 2 Al 5 4.11 Fe 3 (AlSi) 5 5.06 FeAl 5.37 came smaller, but scheme C and scheme D became larger. It is because that the Al-Si coating would be conducted in the direction of generating dense intermetallic compounds to reduce the volume and formed a smooth surface to minimize its surface energy. While the formation mechanism of 3D surface topography of the Al-Si coating was complicated. In a word, the heating temperature, the diffusion of atoms, the newly generated intermetallic compounds, the surface tension and the degree of oxidation of the surface jointly determined the changes of 3D surface topography of the Al-Si coating.  Table 6. Density values of the common FexAlySiz intermetallic compounds [27].

Intermetallic Compounds
Density (g/cm 3 ) Figure 10. Influence of three-step heating methods on 3D surface topography of the Al-Si coating boron steel sheet of (a) Scheme A, (b) Scheme B, (c) Scheme C, and (d) Scheme D.

Conclusions
In the two-step heating methods, the heating rates of 50 • C/s, named rapid heating, was used at the temperature range from 20 to 500 • C, which did not significantly influence on microstructure and 3D surface topography of Al-Si coating boron steel sheet. The height differences between the highest surface and the lowest surface of the Al-Si coating were 17.38 and 16.67 µm, respectively. In the three-step heating methods, the mean thickness and volume fractions of the α-Fe soft phase of the Al-Si coating gradually decreased through the four schemes, changing from 9.7 µm and 22.68% to 6.8 µm and 14.06%, separately. The volume fraction of Fe 3 Al 2 Si 3 intermetallic compound decreased, ranging from 11.53% to 4.97%, respectively. While that of FeAl intermetallic compound also decreased, varying from 9.92% to 3.71%, in turn. The overall trend of the porosity of the Al-Si coating was gradually increased, except scheme B, changing from 2.92% to 3.31%. The roughness of 3D surface topography of the Al-Si coating increased gradually. The height difference between the highest surface and the lowest surface of the Al-Si coating varied from 11.83 to 23.49 µm. Data Availability Statement: Data is contained within the article. The data presented in this study are available in this article.