1. 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 stamping were Al–Si coating (Al-10 wt % Si), pure zinc coating (GI), Zn–Ni alloy coating, composite coating, etc., [
16,
17,
18,
19,
20,
21,
22]. The Al–Si coating was the widest application coating in the hot stamping, due to its unique advantages of die protection and substrate corrosion resistance [
23,
24,
25].
The hot stamping began with the process of austenitization of the Al–Si coating boron steel sheet by heating. During the process of austenitization, the diffusion behavior of atoms between the substrate and Al–Si coating was investigated by the scholars. In order to achieve full alloying between the Al–Si coating and the substrate, and guarantee the subsequent processing properties of the hot stamping parts of the Al–Si coating boron steel sheet, Arcelor Mittal Co., Ltd., gave the specific heating process and the microstructure of the Al–Si coating boron steel sheet under partial heating rates conditions [
26]. Windmann and Frank et al. studied the microstructure evolution of Al–Si coating boron steel sheet during the process of austenitization, and found that the Al–Si coating boron steel sheet was gradually transformed to Fe
2Al
7Si, FeAl
4.5Si, Fe
2Al
5, FeAl
3, FeAl
2, Fe
3Al
2Si
3, FeAl, and α-Fe [
27,
28,
29]. Zhang et al. [
30] investigated the effects of different holding times on the microstructure evolution of the austenitization process of Al–Si coating boron steel sheet, and proposed that the driving force of the microstructure evolution process of the Al–Si coating was the diffusion of Fe atoms. They declared that with the Fe content in the A–Si coating increasing, various intermetallic compounds were gradually formed. Fan [
31], Cheng [
32], Wang [
33], Pontevichi [
34], Achar [
35], and Pelcastre [
36] pointed out that the evolution of voids in the Al–Si coating boron steel sheet was related to the diffusion behavior of Fe atoms and Al atoms during the heating process. They also found that the evolution of microcracks in the Al–Si coating boron steel sheet was relevant to the difference of the thermal expansion coefficient of the new formed Fe
xAl
ySi
z intermetallic compounds. The surface roughness of the Al–Si coating boron steel sheet was related to the formation behavior of intermetallic compounds. Liang [
37] found that the heating rates did not significantly influence the microstructure and Fe
xAl
ySi
z intermetallic compounds of the coating when the heating temperature was equal to or less than the eutectic temperature of Al–Si binary compound. Hong [
38] found that when the Al–Si coating was first heated to 570 °C by continues electric current and then heated by pulsed current, the thickness of the intermetallic compound layer in the Al–Si coating was increased by extending the pulse current processing time. The three-layer structures of Fe
2Al
7Si, FeAl
2, and FeAl were obtained in the intermetallic compounds layer. Hong [
38] pointed out that it may be caused by the non-thermal effect of the pulsed current to promote the diffusion of Fe atoms. However, the evolution of microstructure and 3D surface topography of Al–Si coating boron steel sheet during the process of multi-step heating methods, such as two-step methods and three-step methods, was rarely studied in the previous study.
The study was to discuss the effects of two-step heating methods and three-step heating methods on properties of the Al–Si coating boron steel sheet, such as microstructure, phase compositions, Kirkendall voids and 3D surface topography. The heating tests were carried out by the Gleeble-3500 thermal simulator (DSI, St. Paul, MN, USA). The microstructure, phase compositions, and Kirkendall voids of the Al–Si coating boron steel sheet in hot-dipped conditions and after heat treatments were investigated by environmental scanning electron microscope (QUANYA 200, FEI, Eindhoven, The Netherland). The compositions of the intermetallic compounds of the Al–Si coating were analyzed by energy dispersive X-ray spectroscopy (EDS). The 3D surface topography of the Al–Si coating was investigated by 3D optical microscope (VHX-1000C, KEYENCE, Tokyo, Japan). The results of the studies have certain guiding significance for the development of heating process for the Al–Si coating boron steel sheet.
4. 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 Fe3Al2Si3 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.