Hot stamping forming product with aluminum-silicon coating

Abstract

The invention relates to a hot stamping forming product with an aluminum-silicon coating. The service life of an electrode cap can be prolonged. The hot stamping forming product comprises a steel substrate and an aluminum-silicon coating covering the steel substrate, wherein the aluminum-silicon coating comprises an interdiffusion layer and an Fe-Al intermetallic compound layer; the interdiffusion layer is adjacent to the steel substrate and has a body-centered cubic structure; the Fe-Al intermetallic compound layer is adjacent to the outer side of the interdiffusion layer; the Fe-Al intermetallic compound layer comprises a Fe2Al5 region adjacent to the mutual diffusion layer and a first Si-rich region with a body-centered cubic structure; the interdiffusion layer comprises an alpha-Fe layer and a second Si-rich region which is adjacent to the outer side of the alpha-Fe layer and has a BCC structure, and Fe, Al and Si in the second Si-rich region and Fe, Al and Si in the first Si-rich region have the same average component; 0.09 < = AH / AT < = 0.52, and AT represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; aH represents the sum of the area occupied by the first Si-rich region and the area occupied by the second Si-rich region.

Description

Technical Field

[0001] The present invention relates to a hot stamping formed product with an aluminum-silicon coating. Background Art

[0002] In modern automotive manufacturing, hot stamping formed products with an aluminum-silicon (Al-Si) coating have become an important material for solving automotive lightweighting and improving safety performance due to their excellent formability, oxidation resistance, and ultra-high strength. The hot stamping formed product is made by hot stamping a hot forming steel with an aluminum-silicon pre-coating, and usually needs to be firmly connected to other components (such as a white body) by welding. Therefore, in order to achieve industrial mass production, the weldability of the hot stamping formed product has received wide attention.

[0003] During resistance spot welding, the wear of the electrode cap will directly affect the welding quality and production cost. Under the action of welding pressure and high temperature, the wear of the commonly used Cu-Cr-Zr alloy electrode cap is mainly manifested as the morphological change of the end face of the electrode cap, and its mechanism is usually divided into the following three types: 1) Mechanical wear: During the repeated application and release of the welding pressure, the end face of the electrode cap is gradually worn due to friction and impact; 2) High-temperature softening: The high temperature generated during welding softens the material near the end face of the electrode cap, which reduces the strength and hardness of the end face, resulting in the phenomenon of "enlarged end face", thus making it more prone to wear; 3) Chemical corrosion: A chemical reaction occurs between the end face of the electrode cap and the workpiece to be welded, resulting in the formation of a heterogeneous layer on the end face, which is difficult to withstand the welding pressure and high temperature and is prone to peeling or wear. Regardless of which wear mechanism, the final result presented macroscopically is that the end face of the electrode cap becomes larger. Under the same welding current and time, compared with the original electrode cap, the electrode cap with an enlarged end face reduces the heat input to the workpiece to be welded, resulting in a smaller or even non-compliant weld nugget size. Therefore, the electrode cap needs to be ground before reaching the standard service life to ensure the welding quality.

[0004] Currently, it is generally required that the service life of the electrode cap is not less than 200 times. However, during the resistance spot welding of hot stamping formed products with an aluminum-silicon coating, the service life of the electrode cap is usually only 80 - 120 times, far lower than the general requirement. On the one hand, this leads to the need to frequently replace the electrode cap, resulting in the suspension of the production line and affecting production capacity; on the other hand, after replacing the electrode, it is necessary to re-calibrate the pressure and current parameters, and improper operation is likely to cause batch quality problems, which is not conducive to the yield rate.

[0005] In view of the above, there is at least a need to extend the service life of the electrode cap in the welding process of hot stamping formed products with an aluminum-silicon coating. Summary of the Invention

[0006] The present invention is made in view of the above problems existing in the prior art.

[0007] The present invention provides a hot stamping formed product with an aluminum-silicon coating, which can extend the service life of the electrode cap, thereby reducing the downtime and calibration frequency, and contributing to improving the production yield and product quality.

[0008] The hot stamping formed product with an aluminum-silicon coating according to the present invention includes a steel substrate and an aluminum-silicon coating covering the steel substrate. Wherein, the aluminum-silicon coating includes an interdiffusion layer with a body-centered cubic structure adjacent to the steel substrate and a Fe-Al intermetallic compound layer adjacent to the outer side of the interdiffusion layer. The Fe-Al intermetallic compound layer includes a Fe2Al5 region adjacent to the interdiffusion layer and a first Si-rich region with a body-centered cubic structure. The interdiffusion layer includes an α-Fe layer adjacent to the steel substrate and a second Si-rich region with a body-centered cubic structure adjacent to the outer side of the α-Fe layer. Wherein, the second Si-rich region has the same average composition of Fe, Al, and Si as that in the first Si-rich region. Wherein, 0.09 ≤ A H / A T ≤ 0.52. Wherein, A T represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; A H represents the sum of the areas occupied by the first Si-rich region and the second Si-rich region.

[0009] Optionally, the first Si-rich region includes, by mass percentage: Si ≥ 3 wt.%.

[0010] Preferably, A H / A T ≤ 0.45. Preferably, A H / A T ≤ 0.34. Preferably, A H / A T ≤ 0.25. Preferably, A H / A T ≤ 0.20. Preferably, A H / A T ≤ 0.16.

[0011] Optionally, 0.10 ≤ A H / A T . Optionally, 0.12 ≤ A H / A T .

[0012] Optionally, the average thickness of the aluminum-silicon coating ranges from 6.0 to 21.0 µm, further optionally not less than 10.0 µm, optionally not less than 12.0 µm, optionally not greater than 18.0 µm, and optionally not greater than 16.0 µm.

[0013] Optionally, the average thickness of the aluminum-silicon coating ranges from greater than 21.0 µm to less than or equal to 45.0 µm, optionally not less than 22.0 µm, optionally not less than 23.0 µm, optionally not less than 25.0 µm, and optionally not less than 30.0 µm, optionally not greater than 42.0 µm, optionally not greater than 40.0 µm, and optionally not greater than 37.0 µm.

[0014] The above aluminum-silicon coating according to the present invention extends the service life of the electrode cap, that is, in the standard test method, the number of dotting times on the aluminum-silicon coating by the electrode cap within a single grinding cycle exceeds 200 times, even exceeds 250 times, and further even exceeds 300 times, showing good electrode life.

[0015] In order to solve the problem of early failure of the electrode cap during the resistance spot welding process of hot stamping formed products with an aluminum-silicon coating, the present inventors have deeply studied this resistance spot welding process and found that the failure of the electrode cap is significantly different from the conventional understanding because the reason for "the end face becoming larger" is not only due to high-temperature softening.

[0016] Specifically, for hot stamping formed products with an aluminum-silicon coating, during the resistance spot welding process, the electrode cap and the aluminum-silicon coating are in direct contact, and the current applied to the electrode cap will inevitably generate a certain amount of resistance heat on the coating through the coating. Under the action of the resistance heat, Al and Fe in the coating will diffuse to the end face of the electrode cap, thereby forming a harder Fe-Al-Cu intermetallic compound layer on the end face of the electrode cap. The present inventors have found that, on the one hand, since the resistance of the Fe-Al-Cu intermetallic compound is greater than the original resistance of the electrode cap (made of Cu), more heat will be generated on the surface of the electrode cap by the Fe-Al-Cu intermetallic compound. This further promotes the diffusion of Al and Fe to the surface of the electrode cap and thus the Fe-Al-Cu intermetallic compound becomes thicker. On the other hand, as Figure 1A shown, the hardness of the Fe-Al-Cu intermetallic compound (ranging from 300 to 550 HV according to different component ratios) is much higher than the hardness of the Cu-based electrode cap (about 100 HV) and can withstand the high temperature on the surface of the electrode cap, making the new end face formed by the Fe-Al-Cu intermetallic compound layer not easily undergo "high-temperature softening" deformation. However, as Figure 1BSchematically shown, during the repeated pressure application in welding, the Fe-Al-Cu intermetallic compound layer (schematically shown by the blue line) will squeeze the Cu-based material adjacent to it, causing internal deformation and extending outward from the periphery of the Fe-Al-Cu intermetallic compound layer covering the electrode cap. The outwardly extended part will also diffuse with Al and Fe to form an Fe-Al-Cu intermetallic compound layer there, resulting in an increase in the end face of the electrode cap. As the Fe-Al-Cu intermetallic compound on the end face thickens, the end face of the electrode cap becomes even larger. Thus, it can be seen that during the resistance spot welding process of the hot stamping formed product with an aluminum-silicon coating, the "increase in end face" experienced by the electrode cap is not only due to the inevitably "high-temperature softening" as commonly known, but also because the hardening effect of the Fe-Al-Cu intermetallic compound formed on the end face promotes the increase in the end face. The combined action of the two accelerates the speed of the increase in the end face of the electrode cap, thus significantly reducing the service life of the electrode cap in the resistance spot welding process of the hot stamping formed product with an aluminum-silicon coating. This is a problem not realized by conventional understanding.

[0017] After discovering the above phenomenon, the present inventor proposed to slow down the "increase in end face" failure of the electrode cap by suppressing the formation of the Fe-Al-Cu intermetallic compound on the electrode cap. On this basis, the present inventor discovered through intensive research on the resistance spot welding process that for alleviating the promotion of "increase in end face" by hardening, reducing the resistance heat generated by the aluminum-silicon coating is crucial, and the specific reasons are explained as follows.

[0018] As is well known, resistance spot welding is a process of melting the workpieces to be welded by resistance heat and then forming a physical connection through rapid solidification. The principle of generating resistance heat follows the basic Joule's law, that is Q = I 2 RT , where Q is the resistance heat, I is the welding current, T is the time, and R is the resistance of the workpieces to be welded. When the welding parameters I and T are fixed, the resistance R of the workpieces to be welded will determine the generated resistance heat. Generally, the resistance of the workpiece includes the resistance of the substrate itself and the resistance of the surface coating. Generally speaking, the resistance of the steel substrate is related to the composition and is not affected by the heating process of hot stamping. However, the heating process of hot stamping will affect the structure of the aluminum-silicon coating, thereby affecting the resistance of the surface coating of the workpiece.

[0019] In the hot stamping process, the evolution law of the aluminum-silicon coating is as follows: (1)In the initial state, the Al-Si pre-coating consists of a FeSiAl inhibition layer adjacent to the steel substrate and an Al-Si metal alloy. (2)During the hot stamping process, the steel sheet with the Al-Si pre-coating undergoes a heating process. During the heating process, Fe diffuses from the steel substrate into the pre-coating, while Al diffuses from the pre-coating into the steel substrate. With different degrees of diffusion, an Al-Si coating with different coating structures is presented.

[0020] Generally speaking, for the Al-Si coating obtained by hot stamping, for example, as Figure 2 (see Z. Wang, N.A. Xu, M.X. Huang, Phase transformation and carbon profile at the interface between Al-Si coating and steel substrate in a press-hardened steel, Materialia, Volume 20, 2021) shows, its coating structure from the steel substrate (the bottom part - with a body-centered cubic (BCC) structure) outwards is in turn: (1) an interdiffusion layer with a BCC structure adjacent to the steel substrate and (2) a Fe-Al intermetallic compound layer adjacent to the outside of the interdiffusion layer.

[0021] The Fe-Al intermetallic compound layer may include Fe3Al phase, FeAl phase, FeAl2 phase, Fe2Al5 phase, FeAl3, etc. Among them, when the Fe2Al5 phase exists, at least part of the Fe2Al5 phase is adjacent to the outside of the interdiffusion layer.

[0022] The interdiffusion layer includes an α-Fe (i.e., ferrite) layer and a Fe-Al sub-layer adjacent to its outside, and the Fe and Al elements show a continuous distribution. The closer to the steel substrate, the higher the Fe content and the lower the Al content. The α-Fe layer is mainly composed of the α-Fe phase (as Figure 2 shown), contains a small amount of Si, and may include the Fe3Al phase. The Fe-Al sub-layer may include the Fe3Al phase, FeAl phase, etc.

[0023] It should be noted that although in Figure 2There is a dashed line between the ferrite and the Fe-Al sublayer (Fe3Al) immediately adjacent to its outer side. However, this dashed line is only applicable to this literature because in reality, there is no clear boundary between the α-Fe phase and the Fe-Al sublayer immediately adjacent to its outer side. The specific reasons are as follows: The α-Fe phase adjacent to the steel matrix is a disordered solid solution with a BCC structure (the solute atoms in the solid solution are disorderly distributed), which is significantly different from the martensite structure of the steel matrix. Therefore, there is a clear boundary between the two, and thus the lower edge of the interdiffusion layer can be determined. As the distance from the steel matrix increases, the α-Fe phase with this disordered BCC structure gradually transitions to the Fe3Al phase with an ordered BCC structure (i.e., the DO3 structure), and further gradually transitions to the FeAl phase with an ordered BCC structure (i.e., the B2 structure). Since the crystal structures of the α-Fe phase to its outer Fe-Al sublayer (for example, the Fe3Al phase and FeAl phase as shown in Figure 2 are all BCC structures, and only the proportions of elements such as Fe, Al, and Si change, in the metallographic image obtained by scanning electron microscopy (SEM), the boundary between the α-Fe layer and the Fe-Al sublayer immediately adjacent to it cannot be directly determined in the interdiffusion layer. Therefore, in the art, the two are usually collectively referred to as the interdiffusion layer.

[0024] The crystal structures of the interdiffusion layer and the part of the Fe-Al intermetallic compound layer adjacent to it are different, so there is a clear boundary between the two, and thus the upper edge of the interdiffusion layer can be determined. As shown in Figure 2 , according to the light and dark relationship therein and combined with compositional analysis (energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD)), the outermost part of the interdiffusion layer is the FeAl phase, and the phase in the Fe-Al intermetallic compound layer adjacent to it is the Fe2Al5 phase. Due to the different crystal structures of the two, there is a clear boundary between the two in the metallographic image. Based on the above, the steel matrix and the interdiffusion layer can be determined from the inside out on the hot stamping formed product with an aluminum-silicon coating, and the part outside the interdiffusion layer is the Fe-Al intermetallic compound layer.

[0025] In the Al-Si coating, in addition to the obvious differences in crystal structures between the FeAl phase and the Fe2Al5 phase, there are also differences in the contents of Fe, Al, and Si elements. Different atomic ratios of Fe and Al will directly lead to differences in their mass ratios. The actual measurement shows that the mass ratios of Fe and Al in these two phases do not exactly match the atomic ratios in the molecular formula, mainly for two reasons: on the one hand, it is related to the degree of order of each phase. Both the FeAl phase and the Fe2Al5 phase are ordered solid solutions, but their structures are significantly different. The FeAl phase belongs to the B2 ordered structure with a lower formation energy, a more flexible atomic diffusion path, and a relatively low degree of order, and can stably exist within a certain composition range. The crystal structure of Fe2Al5 is complex, with a higher formation energy and a higher degree of order, and its composition window is relatively narrow. On the other hand, it is related to the testing method. The element ratio in the coating is usually determined by EDS line scanning. The principle is to use an electron beam to scan the sample along a preset path and simultaneously collect characteristic X-ray signals to generate an element line distribution curve. Since the FeAl phase and the Fe2Al5 phase are adjacent, according to the matching of the electron beam spot diameter (0.05~2μm) or the detection step size (0.01~10μm), the detected signal will be the average value of the mixed region of the two phases, rather than the true single-phase composition, resulting in a gradual transition and range fluctuation of the EDS analysis curve at the interface, rather than a theoretical composition mutation. Therefore, it is generally considered that the Fe2Al5 phase has an orthorhombic crystal structure and contains 35~52wt.% of Fe, 48~65 wt.% of Al, and less than 1 wt.% of Si by mass. The FeAl phase has a BCC structure and contains 52~80wt.% of Fe, 20~48 wt.% of Al, and more than 3 wt.% of Si by mass. It should be noted that the Si content in the FeAl phase (more than 3 wt.%) is significantly higher than that in the Fe2Al5 phase (less than 1 wt.%) because Si is hardly soluble in the Fe2Al5 phase, but can form a solid solution by replacing Al atoms in the FeAl phase.

[0026] From another perspective, the aluminum-silicon coating of hot stamping formed products is mainly composed of α-Fe phase and the Fe-Al phase on its outer side. Among them, the α-Fe phase is a disordered solid solution, and the Fe-Al phase is an ordered solid solution. The solute atoms of both are Al and Si, and the solvent is Fe. Generally speaking, the resistivity of a solid solution is directly related to the type of solute element and the solubility of the solute: (1) The electronegativity difference, atomic radius difference, valence electron number, etc. between the solute element and the solvent element will all affect the resistivity. The resistivity of pure aluminum (Al) at room temperature is about 2.7 μΩ·cm. On the one hand, the free electron density of aluminum is high and electron scattering is less. On the other hand, Al has a simple face-centered cubic (FCC) structure, so the resistivity is low. The resistivity of pure silicon (Si) at room temperature is about 640 μΩ·cm, which belongs to a semiconductor with an extremely low free electron density and a resistivity much higher than that of pure Al. (2) As the concentration of the solute element increases, the resistivity of the solid solution usually rises. Especially when the concentration of the high-resistivity silicon element increases, the resistivity of the solid solution increases significantly. This is because solute atoms will cause lattice distortion and even changes in the crystal structure, resulting in more electron scattering and thus increasing the resistivity.

[0027] Without considering Si, theoretically, the resistivity of phases such as Fe2Al and Fe2Al5 with higher Al content and more complex crystal structures (60 - 70 μΩ·cm) is slightly higher than that of phases such as Fe3Al and FeAl with an ordered BCC structure (55 - 70 μΩ·cm), and also higher than the resistivity of the α-Fe phase (about 10 μΩ·cm). However, in the aluminum-silicon coating, the content of Si in phases with an ordered BCC structure (such as Fe3Al and FeAl) (not less than 3 wt.%) is usually higher than its content in phases with an orthorhombic crystal structure (such as Fe2Al and Fe2Al5) (at least less than 1 wt.%). Therefore, due to the introduction of Si, the resistivity of phases with an ordered BCC structure can be increased by 20 - 30 μΩ·cm. Although the α-Fe phase also contains a certain amount of Si, the Si content and Al content are both lower than those of phases with an ordered BCC structure such as Fe3Al and FeAl. Therefore, for each phase of the aluminum-silicon coating, the resistivity follows the following: Si-rich FeAl phase > Fe2Al5 phase > α-Fe phase.

[0028] It should be noted that the prior art either believes that the Si-rich layer in the aluminum-silicon coating (which contains 4 - 8 wt.% of Si, 40 - 70 wt.% of Fe, at most 1 wt.% of Mn, and 20 - 50 wt.% of aluminum, with a total of 100 wt.%) has a positive impact on welding characteristics, such as CN109207861B, or believes that only the interdiffusion layer in the coating needs to be considered for the negative impact on welding, and the coating structure outside the interdiffusion layer does not need to be considered.

[0029] However, the present inventors have found that this is not the case, because as described above, the Si-rich FeAl phase in the coating exists not only in the interdiffusion layer, denoted as the second Si-rich region, but also in the Fe-Al intermetallic compound layer outside the interdiffusion layer, denoted as the first Si-rich region. Optionally, the first Si-rich region by mass percentage includes: 3 wt.% ≤ Si ≤ 13 wt.%, 52 wt.% ≤ Fe ≤ 80 wt.%, 20 wt.% ≤ Al ≤ 48 wt.%; optionally 3 wt.% ≤ Si ≤ 13 wt.%, 55 wt.% ≤ Fe ≤ 75 wt.%, 20 wt.% ≤ Al ≤ 40 wt.%; optionally, 5 wt.% ≤ Si ≤ 10 wt.%, 60 wt.% ≤ Fe ≤ 70 wt.%, 25 wt.% ≤ Al ≤ 35 wt.%. Generally, in the case of a multi-layer coating structure, the relative proportion of the Si-rich FeAl phase will continuously increase with the increase of the diffusion degree. Combining the above resistivity analysis, whether in the interdiffusion layer or in the Fe-Al intermetallic compound layer, the increase in the total area of the first Si-rich region and the second Si-rich region means an increase in the overall resistance of the aluminum-silicon coating. Therefore, the resistance heat generated when the welding current flows through the aluminum-silicon coating will be more, resulting in an increase in the temperature of the coating. Especially the first Si-rich region, which is closer to the end face of the electrode cap, and the heat generated acts more directly on the diffusion between elements, which promotes the diffusion reaction between the Cu-based electrode cap and Al and Fe, generating Fe-Al-Cu intermetallic compounds on the surface of the electrode cap, thus promoting the gradual failure of the electrode cap due to the increase in the end face size, as described above.

[0030] In view of the above, considering that the resistivity of the α-Fe layer is small, the present inventors propose to control the total area A occupied by the first Si-rich region and the second Si-rich region H in the total area A of the coating part outside the α-Fe layer T to control the resistance heat generated by the aluminum-silicon coating, where A H refers to the sum of the areas occupied by the first Si-rich region and the second Si-rich region, and A T refers to the total area occupied by the second Si-rich region adjacent to the outside of the α-Fe layer and the Fe-Al intermetallic compound layer outside the interdiffusion layer. In order to slow down the increase in the end face size of the electrode cap and extend its service life and ensure the expected performance of the hot stamping formed product, the present inventors require 0.09 ≤ A H / A T ≤ 0.52. 0.09 ≤ A H / A T ensures sufficient heating time so that the steel substrate can be fully austenitized to achieve the expected strength, and A H / A T≤0.52 prevents excessive heat accumulation in the aluminum-silicon coating during the welding process, thereby delaying the failure of the electrode cap caused by "the end face becoming larger", and enabling the service life of the electrode cap to be no less than 200 times.

[0031] Furthermore, the coefficient A of the aluminum-silicon coating of the hot stamping formed product H / A T The smaller it is, the smaller the resistance of the aluminum-silicon coating, so that the surface temperature of the coating and the surface temperature of the electrode cap during the resistance spot welding process can be reduced. Furthermore, the tendency of Fe and Al to diffuse into the electrode cap to form Fe-Al-Cu intermetallic compounds is reduced, and the failure of the electrode cap due to "the end face becoming larger" is slowed down. Therefore, preferably, 0.09 ≤ A H / A T ≤0.45, preferably, 0.09 ≤ A H / A T ≤0.34, more preferably 0.09 ≤ A H / A T ≤0.20, so that when the proportion of "the first Si-rich region + the second Si-rich region" in the coating is smaller, the service life of the electrode cap can correspondingly be no less than 220 times, no less than 250 times, and no less than 300 times. By extending the service life of the electrode cap, the downtime and calibration frequency are effectively reduced, which is beneficial to improving the output and the yield rate.

[0032] Those skilled in the art will understand that any range or any value within the above-mentioned respective ranges is applicable to the present invention. For example, A H / A T can take any range or any specific value within the range of 0.09 to 0.52, for example: any range among 0.10 to 0.50, 0.10 to 0.45, 0.10 to 0.40, 0.10 to 0.34, 0.10 to 0.30, 0.10 to 0.20, etc., or any value such as 0.15, 0.18, 0.20, 0.22, 0.24, 0.28, 0.32, 0.36, 0.40, 0.42, 0.43, 0.45, 0.48, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The embodiments, features, and advantages of the present invention will become clear from the specific embodiments in conjunction with the drawings. It should be understood that the drawings only show some embodiments, not necessarily drawn to scale, and are exaggerated locally for clarity, and should not be considered restrictive. In the drawings: Figure 1A Schematically shows the hardness distribution at the end of the electrode cap; Figure 1B Schematically shows the process of the end of the electrode cap gradually becoming larger; Figure 2Schematically shows the aluminum-silicon coating obtained after hot stamping of a hot stamping steel sheet with an aluminum-silicon pre-coating in the prior art; Figures 3A - 3C Schematically shows the detection process of the Si-rich region in the aluminum-silicon coating according to the present invention; Figure 4 Schematically shows the tensile fracture of two welded hot stamping formed product specimens according to the present invention; Figure 5 and Figure 6 Schematically shows the process of determining the service life of the electrode cap according to Example SH2 of the present invention; Figure 7 Schematically shows the identified coating structure of the hot stamping formed product SH3 according to the embodiment of the present invention. Detailed Description of the Invention

[0034] In order to more clearly illustrate the technical solutions of the present application, the present invention will be described below with reference to the accompanying drawings through exemplary embodiments. The following embodiments or experimental data are intended to exemplarily illustrate the present invention, and those skilled in the art should clearly understand that the present invention is not limited to these embodiments or experimental data. The descriptions of chemical element contents (wt.%) herein all refer to mass percentages. Unless otherwise specified, the preferred solutions can be freely combined as needed. Unless clearly stated, all ranges include the end values. Those skilled in the art will understand that the data and various parameters recorded in the embodiments are only exemplary and do not constitute a limitation to the present invention.

[0035] The present invention provides a hot stamping formed product with an aluminum-silicon coating, which is obtained by subjecting a hot stamping steel sheet with an aluminum-silicon pre-coating to a hot stamping process. It will be understood that the present invention focuses on controlling the coating structure to solve the problems existing in the electrode cap. The specific composition of the steel substrate of the hot stamping formed product does not affect the realization of the technical effects of the claimed coating structure. On the one hand, the steel substrate does not directly contact the electrode cap, and the change in the substrate composition does not affect the interaction between the coating and the electrode cap. On the other hand, the change in the composition of the steel substrate does not change the structural characteristics of the coating. Therefore, it is not necessary to limit the composition of the steel substrate of the hot stamping formed product. Based on this, the following only takes 3 compositions of the steel substrate of the hot stamping steel sheet listed in Table 1 (which remain unchanged after the hot stamping process) as examples to demonstrate the technical effects that can be achieved by controlling the coating structure. Optionally, the steel substrate may contain the following components by weight percentage: 0.05 - 0.50 wt.% C, 0.2 - 5 wt.% Mn, 0 - 0.004 wt.% B, 0 - 0.4 wt.% of Nb + Ti + V, 0.05 - 1 wt.% of Si, 0.01 - 1 wt.% of Al, Cr, Mo, Ni, Cu with a total content of less than 5 wt.%; the balance is Fe and impurities.

[0036] As an example, prepare a hot stamping steel sheet with the composition shown in Table 1, and the corresponding manufacturing process is as follows: a) Steelmaking: Smelt according to the composition in Table 1 by a vacuum induction furnace, an electric furnace or a converter, produce a continuous casting billet using continuous casting technology, or directly adopt a thin slab continuous casting and rolling process; b) Hot rolling: Heat the steel billet to 1120 - 1280 °C for hot rolling, with the total hot rolling reduction ratio above 50% and the finishing rolling temperature above 800 °C to obtain a hot rolled steel sheet, and coil it below 700 °C to form a hot rolled steel coil, and pickle the hot rolled coil to remove the scale generated during hot rolling; and c) Cold rolling: Cold roll the pickled hot rolled coil, with the cold rolling reduction ratio being 30 - 70% to obtain a cold rolled steel coil with a thickness of about 1.4 mm. It will be understood that the thickness of the steel sheet is not limited to this and can be rolled to different thicknesses according to requirements, for example, within the range of 0.5 - 3.0 mm, such as expected to be used for the vehicle body, but not limited to this; d) Hot dip coating: After subjecting the obtained cold rolled steel coil to continuous annealing treatment, pre - coat both sides, where the plating solution is: 9 wt.% Si, 3 wt.% Fe, and the balance is Al and impurities; the plating solution temperature is 680 °C, the predetermined temperature for the steel sheet to enter the plating solution (i.e., the temperature when the steel sheet enters the pot) is 640 °C, and the hot dip coating time is 2 - 7 s. Then, blow off the excess plating solution on the surface through an air knife to control the coating weight on both surfaces, and obtain three different single - side coating weights of aluminum - silicon pre - coatings of 20 g / m 2 、40 g / m 2 、75 g / m 2 respectively. The average aluminum content in the aluminum - silicon pre - coating is greater than or equal to 60 wt.%.

[0037] It will be understood that the hot dip coating process is not limited to the above but can be selected from the following. For example, the plating solution contains by mass: 7 - 12 wt.% Si, less than 4 wt.% Fe, and the balance is Al and impurities; the plating solution temperature can be 650 °C - 700 °C; the temperature when the steel sheet enters the pot is 600 °C - 650 °C. Also, for example, according to different requirements, the weight of the single - side aluminum - silicon pre - coating can be controlled by blowing through an air knife, for example, selected from weights between 10 - 80 g / m 2 .

[0038] Table 1 Chemical composition of the steel matrix of the exemplary hot stamping steel sheet (wt.%, the balance is Fe and impurities) Component C Si Mn B Al Cr Ti V Matrix 1 0.22 0.20 1.2 0.003 / 0.12 0.03 / Matrix 2 0.06 0.21 1.0 0.003 / 0.10 0.04 / Matrix 3 0.34 0.30 1.0 0.003 0.30 0.26 / 0.15 After the above process, a hot stamping steel sheet with an aluminum - silicon pre - coating with different pre - coating thicknesses is obtained. Prepare specimens with dimensions of 300 * 200 mm from it, and then subject each specimen to a flat hot stamping simulation. The hot stamping process is as follows: 1) Heating process: The hot stamping steel sheet with an aluminum-silicon pre-coating is successively retained in a roller hearth furnace for the same period of time from Zone 1 to Zone 10 according to the heating conditions in Table 2. The total retention time is shown in the heating time of Table 3, and then it is cooled to above 700 °C; 2) Hot stamping process: Subsequently, the heated hot stamping steel sheet is transferred to a dedicated flat die within 5 - 10 s and held under a pressure of 600 tons for 8 s for hot stamping. Subsequently, it is cooled to below 100 °C in the die and then taken out to obtain a hot stamping formed product with an aluminum-silicon coating; depending on the thickness of the aluminum-silicon pre-coating, i.e., the weight of the single-sided aluminum-silicon pre-coating is 10 - 80 g / m 2 , the average thickness of the obtained aluminum-silicon coating can be 6.0 - 45.0 μm.

[0039] As mentioned above, the change in the heating time means the change in the degree of diffusion, that is, the proportion of the Si-rich FeAl phase in the coating structure will also change continuously. Thus, through comparative analysis, the influence of different proportions of the Si-rich FeAl phase on resistance spot welding can be known. It will be understood that the method for manufacturing the hot stamping formed product according to the present invention is not limited to the above. Given the expected coating structure, those skilled in the art can select an appropriate product production method from conventional methods as needed. For example, the heating process can be divided into 8 zones.

[0040] Table 2 Heating process of the exemplary hot stamping process The specimens after undergoing the above hot stamping process are listed in Table 3, where SH1 - SH20 are the examples of the present invention, and DB1 - DB4 are the comparative examples. According to the following detection methods for the average thickness of the coating, the detection method for the Si-rich region in the aluminum-silicon coating, and the detection method for the service life of the electrode cap, the average thickness of the pre-coating of SH1 - SH20 and DB1 - DB4 and the average thickness of the obtained aluminum-silicon coating after hot stamping are measured, the proportion of the Si-rich region in the aluminum-silicon coating is calibrated, and the service life of the electrode cap is detected on the corresponding specimens. The specific results are listed in Table 3.

[0041] Detection method for average thickness of coating According to the standard GB / T 13298-2015, metallographic samples are prepared in the thickness direction of hot stamping steel plates / hot stamping formed products with aluminum-silicon precoatings on both sides. On either side of the steel matrix within a field of view of at least 500 times magnification (e.g., 1000 times, with a field of view size of: 110 μm × 150 μm), distinct boundaries between the (pre)coating and the steel matrix and between the (pre)coating and the embedding material can be observed. Thus, for this side, two boundaries of the (pre)coating can be determined, and the thickness of the (pre)coating is the distance between them. For each sample, on either side of the (pre)coating on the steel matrix, at least 3 points are taken for thickness measurement along two sides and the middle within the field of view in a direction perpendicular to the thickness direction of the (pre)coating, and then the average value is taken as the average thickness. The specimens are marked so that the measurement of the precoating thickness and the coating thickness is carried out on the same side.

[0042] Detection method for Si-rich region in aluminum-silicon coating A cross-section is taken in the thickness direction of a hot stamping formed product with an aluminum-silicon coating to make a metallographic sample. After the sample is ground and physically polished, it is then subjected to electrolytic polishing, ion polishing, or vibratory polishing. Subsequently, under a field emission electron microscope (e.g., GEMINI 300), the EBSD function is used to appropriately magnify (such as 1000 times, with a field of view size of: 110 μm × 150 μm) in a situation where the complete coating structure and part of the steel matrix can be seen to identify the Si-rich regions. The specific process is as follows.

[0043] Figure 3A A partial metallographic image obtained by the EBSD function is schematically shown (only a part of the metallographic image is shown for illustration. The actual field of view size of this metallographic image is approximately: 32 μm × 18 μm, and specific measurements are made by detecting and calculating each region within the actual field of view). The steel matrix is at the bottom. Although both the steel matrix and the adjacent α-Fe phase have a BCC structure, the microstructure of the steel matrix is significantly different from that of the α-Fe phase, resulting in a distinct interface between the steel matrix and the coating, i.e., the lower edge of the interdiffusion layer. Based on Figure 3A , by comparing with the standard database of known crystal structures of Fe2Al5 and BCC crystal structures, according to characteristics such as the position and spacing of diffraction spots, the EBSD function automatically calculates the lattice plane spacing and crystal orientation of the crystal, thereby determining the Fe2Al5 region and the BCC structure region in the coating and marking them with different colors for distinction, obtaining Figure 3B , which shows the Fe2Al5 region (region 1) and two spaced-apart BCC structure regions: one layer is close to the steel matrix, which is the interdiffusion layer (region 2); the other layer is closer to the coating surface, which is the first Si-rich region (region 3). At this time, regions 2 and 3 are shown in the same color. It should be noted that the boundaries between region 1 and regions 2 and 3 are related to Figure 3AThe Fe2Al5 region shown by contrast in the SEM micrograph is consistent with the boundary between the interdiffusion layer and the FeAl region, which is consistent with the previous discussion, so that the boundary between the interdiffusion layer and the Fe2Al5 region adjacent thereto and the boundary between the Si-rich FeAl region and other regions (such as the Fe2Al5 region shown) in the Fe-Al intermetallic compound layer can be determined by the difference in contrast in the SEM micrograph. Further, it is noted that the entire interdiffusion layer is identified as a BCC structure, and it is difficult to find the boundary between the α-Fe layer and the second Si-rich region. Therefore, the inventors stipulate that the second Si-rich region in the interdiffusion layer is determined by the following method.

[0044] Specifically, Figure 3B For further processing, a rectangular "reference area" with a length of 0.5~2.0 μm and a width of 0.5~1.0 μm is selected at the center position in the thickness direction of the identified first Si-rich area (area 3), and EDS element analysis is performed on the area to obtain the average composition of Fe, Al and Si contained therein. It should be noted that the selection of the "reference area" should be located in the middle of the first Si-rich area as much as possible, that is, avoiding the boundary of the first Si-rich area, so as to avoid the composition of other phases from affecting the average composition measurement of the "reference area". Then, using the TURPHASE function module, based on this average composition, Figure 3B The portion with the same average composition in region 2 (i.e., the interdiffusion layer) in FIG. 1 is identified as the second Si-rich region, which is recorded as region 4 and is shown in the same color as the first Si-rich region (e.g., blue), that is, Figure 3C .

[0045] Afterwards, Image J software was used to identify the corresponding color blocks in the coating and count the area they occupied. Figure 3C For example, A T A represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer outside the interdiffusion layer, that is, the sum of the areas of the blue part (the first Si-rich region + the second Si-rich region, i.e., region 3 and region 4) + the green part (Fe2Al5 region, i.e., region 1) in the picture; H It represents the sum of the areas occupied by the first Si-rich area and the second Si-rich area (i.e. the blue part in the picture). In order to ensure the accuracy of the data, the resolution rate must be above 80% during the EBSD test. It should be noted that when using Image J software to identify color blocks, the statistical result is the number of pixels, and the area occupied by each pixel is a fixed value. In theory, the area occupied by the color block should be the number of pixels × the area per unit pixel. Therefore, in order to simplify the calculation, A H / A TThe value can be directly calculated using the number of pixels. It will be understood that other image processing software with the above functions can be used to perform the above process.

[0046] For each metallographic sample, at least three fields of view (at the same magnification) are taken in the direction perpendicular to the coating thickness to perform the above process, and then the obtained A H / A T The average value is taken as the final A H / A T .

[0047] Detection method for service life of electrode cap The service life of the electrode cap is tested on hot stamping formed products with different aluminum-silicon coatings.

[0048] The electrode cap pressure is selected as 4.0 kN, the electrode cap end face diameter is selected as 6 mm, the welding time is 380 ms, and the cooling water flow rate is 6 L / min. The welding current starts from 5 kA and increases by 100 A in sequence until spatter occurs, and the current at which spatter occurs is recorded as I max . The welding current is selected as I max -200 A, and two hot stamping formed product specimens with a size of 30*100 mm (such as SH1-SH1, welding current is 8.0 kA) are welded using a new electrode cap to form a fusion core at the overlapping part of the two hot stamping formed product specimens to weld the two specimens. Each group (two pieces) of specimens completes one weld point, and every 50 points is taken as a cycle. As Figure 4 shown, at the 50th point within each cycle, the two welded hot stamping formed product specimens are subjected to tensile fracture. Then as Figure 5 shown, the fusion core diameter is measured in two mutually perpendicular directions to obtain the first value D 1 and the first value D 2 of the fusion core diameter, and then the fusion core diameter D is recorded as ( D 1 + D 2) / 2.

[0049] When it is found that the fusion core size measured at the 50th point within this cycle D is less than 4× t 1 / 2 (where t is the thickness of the hot stamping formed product specimen, which is 1.4 mm in the current embodiment, so 4× t 1 / 2 is approximately 4.73 mm), the dotting is stopped, and the fusion core diameter is measured by successively fracturing at the weld points within this cycle. The fusion core diameter greater than or equal to 4×t 1 / 2 The number of points is recorded as the electrode cap life. For example, as Figure 5 shown, at the 50th point of the 5th cycle (i.e., the 250th point in total), it is found that D <4× t 1 / 2 , then stop the dotting test. Then, successively perform disassembly and measurement of the corresponding D at the 49th, 48th, 47th, 46th.......n (n≥1) points within the 5th cycle until D ≥4× t 1 / 2 (4.73 mm). For example, as Figure 6 shown, it is found that the D measured at the 46th point is about 4.55 mm, which is less than 4× t 1 / 2 , but the D measured at the 45th point is 4.82 mm, which is greater than 4× t 1 / 2 , then the service life of the electrode cap is 50×4 + 45 = 245 times. Usually, the industry requires that the service life of the electrode cap is not less than 200 times.

[0050] Table 3 Coating structures and corresponding welding results of the examples and comparative examples according to the present invention Number Substrate Average thickness of pre-coating (μm) Heating time (s) Average thickness of coating (μm) <![CDATA[A H / A T > Test current / kA Service life of electrode cap DB1* Matrix 1 10.5 180 11.3 <![CDATA 0.08 > / / SH1 Matrix 1 11.3 200 12.2 0.15 8.0 336 SH2 Matrix 1 10.5 230 13.8 0.22 7.8 282 SH3 Matrix 1 11.4 260 15.4 0.33 7.6 268 SH4 Matrix 1 11.3 290 16.0 0.37 7.6 245 SH5 Matrix 1 11.6 330 16.3 0.49 7.5 210 DB2 Matrix 1 12.0 350 16.6 <![CDATA 0.54 > 7.4 <![CDATA 196 > SH6 Matrix 1 18.5 220 20.4 0.16 7.8 326 SH7 Matrix 1 19.4 250 22.0 0.25 7.6 276 SH8 Matrix 1 17.8 280 23.4 0.34 7.4 255 SH9 Matrix 1 19.4 310 24.9 0.45 7.4 228 SH10 Matrix 1 18.7 340 25.4 0.52 7.3 204 DB3 Matrix 1 17.9 380 26.8 <![CDATA 0.69 > 7.2 <![CDATA 135 > SH11 Matrix 1 26.4 250 30.6 0.09 7.6 345 SH12 Matrix 1 27.8 300 32.9 0.20 7.4 302 SH13 Matrix 1 26.6 350 36.5 0.39 7.2 230 SH14 Matrix 1 28.9 400 39.2 0.47 7.0 224 DB4 Matrix 1 28.4 450 41.2 <![CDATA 0.56 > 7.0 <![CDATA 162 > SH15 Matrix 2 16.2 240 20.3 0.16 7.4 317 SH16 Matrix 2 17.1 300 22.8 0.32 7.1 260 SH17 Matrix 2 17.4 360 24.2 0.51 6.9 212 SH18 Matrix 3 27.4 260 31.8 0.18 7.5 303 SH19 Matrix 3 26.4 320 34.7 0.30 7.3 264 SH20 Matrix 3 28.0 380 37.5 0.45 7.0 223 *: Since DB1 is not heated sufficiently, the steel substrate of its hot stamping formed product is not fully austenitized, so it fails to reach the expected tensile strength of more than 1350 MPa for this product and is unqualified.

[0051] As shown in Table 3, when performing resistance spot welding on the hot stamping formed products SH1~SH20 according to the examples of the present invention, the service life of the electrode cap is more than 200 times, and even more than 300 times. In contrast, when performing resistance spot welding on the hot stamping formed products DB1~DB4 of the comparative examples, the service life of the electrode cap does not reach 200 times. This shows that the hot stamping formed products according to the present invention can extend the service life of the electrode cap to more than 200 times, achieving the expected technical effect. The specific reason analysis is as follows.

[0052] The hot stamping formed products SH1, SH2, SH3, SH4, SH5 (abbreviated as SH1-5) of the substrate 1 according to the examples of the present invention and the comparative example DB2 all start from the same pre-coated single-sided weight, that is, 20 g / m 2, and the measured average thicknesses of the aluminum-silicon precoatings are 11.3 μm, 10.5 μm, 11.4 μm, 11.3 μm, 11.6 μm, and 12.0 μm, respectively. Then SH1-5 and DB2 respectively undergo heating processes of 200 s, 230 s, 260 s, 290 s, 330 s, and 350 s in the hot stamping process, and then the hot stamping process is carried out to sequentially obtain aluminum-silicon coatings with average thicknesses of 12.2 μm, 13.8 μm, 15.4 μm, 16.0 μm, 16.3 μm, and 16.6 μm. The aluminum-silicon coatings all include an interdiffusion layer with a body-centered cubic structure adjacent to the steel substrate and a Fe-Al intermetallic compound layer adjacent to the outer side of the interdiffusion layer. Then, the aluminum-silicon coatings of SH1-5 and DB2 are analyzed according to the detection method of the Si-rich region in the above aluminum-silicon coatings. Taking a field of view of the hot stamping formed product SH3 according to the embodiment of the present invention as an example, after the aluminum-silicon coating thereof is marked by the above detection method of the Si-rich region, it is as Figure 7 shown, wherein the α-Fe layer with a BCC structure is marked red, the Si-rich region with a BCC structure is marked blue, and the Fe2Al5 region with an orthorhombic structure is marked green. That is to say, the aluminum-silicon coating of SH3 includes an interdiffusion layer with a BCC structure adjacent to the steel substrate and a Fe-Al intermetallic compound layer adjacent to the outer side of the interdiffusion layer. The Fe-Al intermetallic compound layer includes a part of the Fe2Al5 region adjacent to the interdiffusion layer and a part of the Fe2Al5 region on the surface of the coating (total is region 1 - green) and a first Si-rich region with a BCC structure (region 3 - blue); and the interdiffusion layer includes an α-Fe layer (red) adjacent to the steel substrate and a second Si-rich region with a BCC structure (region 4 - blue) adjacent to the outer side of the α-Fe layer. The second Si-rich region is identified from the average composition of the "reference region" in the first Si-rich region, where the "reference region" is a rectangle with a length of 1 μm and a width of 0.5 μm, and the identified average composition is: 7.6 wt.% Si, 29.7 wt.% Al, 62.6 wt.% Fe. Using Image J software, it is statistically obtained that the pixels occupied by A H and A T are 147936 and 451149 respectively. Therefore, the A H / A T of the aluminum-silicon coating of SH3 is 0.3279, and retaining two decimal places is 0.33. Similarly, two different fields of view are taken on this sample to calculate A H / A T , which are approximately 0.31 and 0.34 respectively. The average value of the three is taken and retaining two decimal places is 0.33. Similarly, the A H / A TThey are 0.15, 0.22, 0.33, 0.37, 0.49, 0.54 respectively, showing a gradually increasing trend. In contrast, on SH1-5 and the upper electrode caps of DB2, the service lives are 336 times, 282 times, 268 times, 245 times, 210 times and 196 times respectively, showing a gradually decreasing trend.

[0053] In the case of a thicker pre-plating layer, for example, the 17.8 - 19.4 μm thick pre-plating layer of SH6-10 and DB3, and the 26.4 - 28.9 μm thick pre-plating layer of SH11-14 and DB4, the A of the aluminum-silicon plating layer H / A T and the service life of the electrode cap show the same law as described above. That is, by extending the time of the heating process, the A in the aluminum-silicon plating layer H / A T increases accordingly, while the service life of the electrode cap decreases accordingly.

[0054] The emergence of the above phenomenon is because, during the heating process of hot stamping, extending the heating time will promote the formation of the Si-rich region, thereby obtaining a higher A H / A T . As discussed above, at the same thickness, the higher the proportion of the total area A H occupied by the Si-rich region in the total area A T of the plating layer outside the α-Fe layer, the higher the resistance heat brought by the Si-rich region during the resistance spot welding process, which is not conducive to the service life of the electrode cap. Therefore, according to the law discovered above, the inventor of the present invention proposes to reduce A H / A T to extend the service life of the electrode cap.

[0055] In addition, when the specific composition of the steel matrix of the hot stamping steel plate changes, it will not affect the technical effect of improving the service life of the electrode cap by means of the plating layer structure according to the present invention. Exemplarily, SH15-17 according to the present invention adopts the composition of matrix 2, and SH18-20 adopts the composition of matrix 3. According to the results in Table 3, it can be confirmed that, consistent with the technical effects achieved by matrix 1, the A of the aluminum-silicon plating layer of matrix 2 (SH15-17) and matrix 3 (SH18-20) H / A T and the service life of the electrode cap show the same law as described above and when A H / A T ≤0.52, matrix 2 and matrix 3 also obtain the technical effect of extending the service life of the electrode cap. Thus, it can be seen that the change in the composition of the steel matrix will not affect the technical effect of extending the service life of the electrode cap by the plating layer structure according to the present invention.

[0056] Combined with the results in Table 3, the present invention requires AH / A T ≤0.52, so as to ensure that when resistance spot welding a hot stamping formed product with such a coating structure, the electrode cap can continuously strike more than 200 times within a single electrode cap grinding cycle. Optionally, the present invention requires that A H / A T ≤0.45, such that the electrode cap can continuously strike more than 220 times within a single electrode cap grinding cycle; optionally A H / A T ≤0.34, such that the electrode cap can continuously strike more than 250 times within a single electrode cap grinding cycle; optionally A H / A T ≤0.25, such that the electrode cap can continuously strike more than 270 times within a single electrode cap grinding cycle; optionally A H / A T ≤0.20, such that the electrode cap can continuously strike more than 300 times within a single electrode cap grinding cycle; optionally A H / A T ≤0.16, such that the electrode cap can continuously strike more than 315 times within a single electrode cap grinding cycle.

[0057] On the other hand, it is worth noting that A H / A T cannot be infinitely small, because reducing A H / A T means reducing the heating process of the hot stamping process. According to Table 3, if the heating process is too short, such as DB1 is only heated for 180 s, although its A H / A T is only 0.08, but its steel matrix cannot be fully austenitized, resulting in the strength of the hot stamping formed product obtained after hot stamping cooling being only 1250 MPa, unable to meet the requirement that the tensile strength of the hot stamping product is not less than 1350 MPa, becoming a defective product. In contrast, according to SH11 of the present invention, it obtains a hot stamping formed product that meets the strength requirements under an extended heating time, and at the same time its A H / A T is 0.09 and also realizes an extended service life of the electrode cap, up to 345 times. Therefore, in order to ensure that the steel matrix is fully austenitized during the heating process to ensure the strength of the product, combining the results of DB1 and SH11, the present invention requires 0.09 ≤ A H / A T . Optionally, in order to further ensure the strength, 0.10 ≤ A H / A T ; optionally 0.12 ≤ A H / A TIt will be understood that the above tensile strength is the expected property related to the composition of the steel matrix, and it is not necessarily not less than 1350 MPa. It may also be less than 1350 MPa. For example, the expected tensile strength of matrix 2 is above 1000 MPa.

[0058] In view of the above two reasons, the present invention requires 0.09 ≤ A H / A T ≤ 0.52. Within this range, any combination can be selected according to the above required ranges to further narrow down this range. For example, optionally, 0.09 ≤ A H / A T ≤ 0.45, optionally 0.09 ≤ A H / A T ≤ 0.34, optionally 0.09 ≤ A H / A T ≤ 0.25, optionally 0.09 ≤ A H / A T ≤ 0.16, optionally, 0.10 ≤ A H / A T ≤ 0.52, optionally 0.10 ≤ A H / A T ≤ 0.45, optionally 0.10 ≤ A H / A T ≤ 0.34, optionally 0.10 ≤ A H / A T ≤ 0.25, optionally 0.10 ≤ A H / A T ≤ 0.16, optionally 0.12 ≤ A H / A T ≤ 0.52, optionally 0.12 ≤ A H / A T ≤ 0.45, optionally 0.12 ≤ A H / A T ≤ 0.34, optionally 0.12 ≤ A H / A T ≤ 0.25, optionally 0.12 ≤ A H / A T ≤ 0.16.

[0059] It should be noted that the results between the thick aluminum-silicon coating and the thin aluminum-silicon coating are not comparable. Taking SH5 and SH14 as examples, the coating thicknesses of the two are 16.3 µm and 39.2 µm respectively, A H / A TThe numerical values are close, being 0.49 and 0.47 respectively. Judging only from these two parameters, at similar ratios, the resistance of the aluminum-silicon coating of SH14 is greater, generating more resistive heat, and thus it should lead to a shorter electrode cap life than that of SH5. However, as listed in Table 3, the service lives of the electrode caps for SH5 and SH14 are 210 and 224 respectively, and the service life of the latter's electrode cap is even longer. This is because, according to the test method, to conduct the electrode cap life test, the test current needs to be determined first. The test current is I max -200 A, where I max is the current for generating spatter. The generation of spatter current is related to the welding heat input, that is, spatter is likely to occur when the heat input is greater than the critical value. The heat input conforms to Joule's law Q = I 2 RT , R is the resistance of the welding material. SH5 and SH14 have the same substrate resistance (the same steel substrate composition), but different coating resistances (different coating thicknesses). Overall, SH14 has a higher resistance, so its corresponding I max current is lower, and the test current for SH14 is 7.0 kA correspondingly. In contrast, the test current for SH5 is 7.5 kA. Therefore, theoretically, SH14 with a larger A H , at a lower test current, and SH5 with a smaller A H , at a larger test current, finally obtain similar electrode lives.

[0060] Optionally, the average thickness of the aluminum-silicon coating is in the range of 6.0 - 21.0 µm, further optionally not less than 10.0 µm, and optionally not less than 12.0 µm; further, the average thickness of the aluminum-silicon coating is optionally not greater than 18.0 µm, and optionally not greater than 16.0 µm. It will be understood that these ranges can be combined arbitrarily. For example, the average thickness of the aluminum-silicon coating is in the range of 10.0 - 21.0 µm, optionally 12.0 - 21.0 µm, optionally 6.0 - 18.0 µm, optionally 10.0 - 18.0 µm, optionally 12.0 - 18.0 µm, optionally 6.0 - 16.0 µm, optionally 10.0 - 16.0 µm, and optionally 12.0 - 16.0 µm.

[0061] Optionally, the average thickness of the aluminum-silicon coating ranges from greater than 21.0 µm to less than or equal to 45.0 µm, optionally not less than 22.0 µm, optionally not less than 23.0 µm, optionally not less than 25.0 µm, optionally not less than 30.0 µm; further, the average thickness of the aluminum-silicon coating is optionally not greater than 42.0 µm, optionally not greater than 40.0 µm, optionally not greater than 37.0 µm. It will be understood that these ranges can be combined arbitrarily. For example, the average thickness of the aluminum-silicon coating ranges from greater than 21.0 µm to less than or equal to 42.0 µm, optionally 22.0 - 42.0 µm, optionally 23.0 - 42.0 µm, optionally 25.0 - 42.0 µm, optionally 30.0 - 42.0 µm, optionally 22.0 - 40.0 µm, optionally 23.0 - 40.0 µm, optionally 25.0 - 40.0 µm, optionally 30.0 - 40.0 µm, optionally 22.0 - 37.0 µm, optionally 23.0 - 37.0 µm, optionally 25.0 - 37.0 µm, optionally 30.0 - 37.0 µm.

[0062] In summary, by controlling the structure of the aluminum-silicon coating of the hot stamping formed product, that is, the ratio of A H / A T it is possible to ensure both the strength of the hot stamping formed product and the service life of the electrode cap during its resistance spot welding is not less than 200 times. This helps to reduce the consumption of the electrode cap and save the production cost.

[0063] The specific embodiments and the drawings support and describe the present invention, but the scope of the present invention is only defined by the claims. Although some ways or embodiments for carrying out the present invention have been described in detail, there are various alternative designs and embodiments for practicing the present invention defined in the appended claims. Those skilled in the art should be clear that the present invention is not limited to these embodiments, and various changes can be made without departing from the protection scope of the present invention.

Claims

1. A hot stamping formed product with an aluminum-silicon coating, which comprises a steel substrate and an aluminum-silicon coating covering the steel substrate. Among them, The aluminum-silicon coating includes an interdiffusion layer with a body-centered cubic structure adjacent to the steel substrate and a Fe-Al intermetallic compound layer adjacent to the outside of the interdiffusion layer. The Fe-Al intermetallic compound layer includes a Fe2Al5 region adjacent to the interdiffusion layer and a first Si-rich region with a body-centered cubic structure. The interdiffusion layer includes an α-Fe layer adjacent to the steel substrate and a second Si-rich region with a body-centered cubic structure adjacent to the outside of the α-Fe layer, wherein the Fe, Al, and Si in the second Si-rich region have the same average composition as those in the first Si-rich region. wherein, 0.09 ≤ A H / A T ≤ 0.52, Among them, A T represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; A H represents the sum of the areas occupied by the first Si-rich region and the second Si-rich region.

2. The hot stamping formed product according to claim 1, wherein A H / A T ≤0.

45.

3. The hot stamping formed product according to claim 1, wherein, A H / A T ≤0.34。 4. The hot stamping formed product according to claim 1, wherein, A H / A T ≤0.25。 5. The hot stamping formed product according to claim 1, wherein A H / A T ≤0.20。 6. The hot stamping formed product according to claim 1, wherein, A H / A T ≤0.16。 7. The hot stamping formed product according to any one of claims 1 to 6, wherein, 0.10 ≤ A H / A T 。 8. The hot stamping formed product according to any one of claims 1 to 6, wherein, 0.12 ≤ A H / A T .

9. The hot stamping formed product according to any one of claims 1 to 6, wherein, The first Si-rich region includes, by mass percentage: Si ≥ 3 wt.%.

10. The hot stamping formed product according to claim 1, wherein, The average thickness of the aluminum-silicon coating is in the range of 6.0 - 21.0 µm.

11. The hot stamping formed product according to claim 10, wherein, The average thickness of the aluminum-silicon coating is not less than 10.0 µm.

12. The hot stamping formed product according to claim 10, wherein, The average thickness of the aluminum-silicon coating is not less than 12.0 µm.

13. The hot stamping formed product according to any one of claims 10 to 12, wherein, The average thickness of the aluminum-silicon coating is not greater than 18.0 µm.

14. The hot stamping formed product according to any one of claims 10 to 12, wherein The average thickness of the aluminum-silicon coating is not greater than 16.0 µm.

15. The hot stamping formed product according to claim 1, wherein, The average thickness of the aluminum-silicon coating is in the range of greater than 21.0 µm and less than or equal to 45.0 µm.

16. The hot stamping formed product according to claim 15, wherein, The average thickness of the aluminum-silicon coating is not less than 22.0 µm.

17. The hot stamping formed product according to claim 15, wherein, The average thickness of the aluminum-silicon coating is not less than 25.0 µm.

18. The hot stamping formed product according to claim 15, wherein, The average thickness of the aluminum-silicon coating is not less than 30.0 µm.

19. The hot stamping formed product according to any one of claims 15 to 18, wherein, The average thickness of the aluminum-silicon coating is not greater than 37.0 µm.

20. The hot stamping formed product according to claim 9, wherein, The first Si-rich region includes, by mass percentage: 3 wt.% ≤ Si ≤ 13 wt.%, 52 wt.% ≤ Fe ≤ 80 wt.%, 20 wt.% ≤ Al ≤ 48 wt.%.

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