Hot stamped products with aluminum-silicon coating

Abstract

The present application relates to a hot stamping formed product with an aluminum-silicon coating, which can ensure that the process window of the weldable current is not less than 1.0 kA. The hot stamping formed product includes a steel substrate and an aluminum-silicon coating covering the steel substrate, the latter including an interdiffusion layer with a body-centered cubic structure adjacent to the steel substrate and an Fe-Al intermetallic compound layer adjacent to the outside thereof; the Fe-Al intermetallic compound layer includes an 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 and a second Si-rich region with a BCC structure adjacent to the outside thereof, and the second Si-rich region has the same average composition of Fe, Al, and Si as the first Si-rich region; 0.05≤A HD / A T ≤0.24, where A T represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; A HD Represents the area occupied by the second Si-rich region.

Description

Technical Field

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

[0002] In the modern automotive industry, hot stamped products with aluminum-silicon (Al-Si) coatings have become an important material for solving the problem of lightweighting and improving vehicle safety performance due to their excellent formability, oxidation resistance, and ultra-high strength. These hot stamped products are made of hot-formed steel with an Al-Si pre-coating through a hot stamping process and usually need to be firmly connected to other components (such as the body-in-white) by welding. Therefore, in order to achieve stable industrial mass production, the weldability of these hot stamped products, especially the process window of the weldable current for resistance spot welding, has received widespread attention. At present, the process window of the weldable current of the material is generally required to be no less than 1.0 kA. However, it is difficult for hot stamped products with Al-Si coatings to stably meet the above-mentioned process window of weldable current.

[0003] On the one hand, for commonly used hot-stamped products with aluminum-silicon coatings, intermetallic compounds in the aluminum-silicon coating have a significant negative impact on welding. This is because the aluminum-silicon coating, formed through interdiffusion between the aluminum-silicon pre-coating and the steel substrate during the hot stamping process, contains intermetallic compounds with a hardness of 500–1000 HV, and the surface of the aluminum-silicon coating is often uneven. Due to these intermetallic compounds and uneven surface, insufficient welding pressure during welding reduces the contact area between the sheets to be welded, leading to a sharp increase in current density, which in turn causes spatter and narrows the process window for weldable current. On the other hand, chemical reactions occur between the end face of the commonly used Cu-Cr-Zr alloy electrode cap and the aluminum-silicon coating on the workpiece surface, forming a hard and highly resistive heterogeneous layer on the end face. This affects the current path between the electrode cap and the workpiece, resulting in higher current density in local high-resistance areas and lower current density in local low-resistance areas. This current density difference leads to uneven heat distribution, which is highly susceptible to spatter during welding and narrows the welding process window. The narrowing of the welding process window not only makes it difficult to ensure the formation and quality of welds, increasing the scrap rate, but also requires more precise control and adjustment of the welding process, resulting in reduced production efficiency and increased unit product production costs. These are all undesirable for enterprises.

[0004] In view of the above, there is a need to ensure that the process window of the weldable current is not less than 1.0 kA, at least in the welding process of hot stamping formed products with aluminum silicon coating. Summary of the Invention

[0005] The present invention is made in view of the above problems existing in the prior art.

[0006] The present invention provides a hot stamping formed product with an aluminum-silicon coating, which can ensure that the process window of the weldable current is not less than 1.0 kA, exhibits good weldability, helps to ensure the yield rate and control production costs.

[0007] The hot stamping formed product with an aluminum-silicon coating according to the present invention comprises a steel substrate and an aluminum-silicon coating covering the steel substrate.

[0008] The aluminum-silicon coating comprises an interdiffusion layer having 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;

[0009] The Fe-Al intermetallic compound layer includes a Fe2Al5 region adjacent to the interdiffusion layer and a first Si-rich region having a body-centered cubic structure;

[0010] The interdiffusion layer includes an α-Fe layer adjacent to the steel substrate and a second Si-rich region having a body-centered cubic structure adjacent to the outside of the α-Fe layer, wherein the second Si-rich region has the same average composition of Fe, Al, and Si as the first Si-rich region;

[0011] Among them, 0.05≤A HD / A T ≤0.24,

[0012] Among them, A T represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; A HD Represents the area occupied by the second Si-rich region.

[0013] Optionally, the first Si-rich region includes, by mass percentage: Si≥3 wt.%.

[0014] Preferably, A HD / A T ≤0.21. Preferably, A HD / A T ≤0.18. Preferably, A HD / A T ≤0.12. Preferably, A HD / A T ≤0.11. Preferably, A HD / A T ≤0.07.

[0015] Optionally, 0.06≤A HD / A T .

[0016] Optionally, the average thickness of the aluminum silicon coating is in the range of 6.0 to 21.0 μm, further optionally not less than 11.0 μm, optionally not less than 13.0 μm, optionally not greater than 18.0 μm, optionally not greater than 15.0 μm. It will be understood that these ranges may be arbitrarily combined.

[0017] Optionally, the average thickness of the aluminum silicon coating is within a range of greater than 21.0 μm and less than or equal to 45.0 μm, optionally not less than 21.5 μm, optionally not less than 25.0 μm, optionally not less than 30.0 μm. Optionally, it is not greater than 40.5 μm, optionally not greater than 38.5 μm, optionally not greater than 35.0 μm, and optionally not greater than 33.5 μm. It will be understood that these ranges may be combined in any manner.

[0018] In order to solve the problem of the process window of weldable current being less than 1.0 kA during the resistance spot welding of hot stamped products with aluminum-silicon coating, the inventors conducted in-depth research on this resistance spot welding process and found that in addition to the high hardness and high resistance of the aluminum-silicon coating, which can significantly affect the welding process window, the deformation of the electrode cap is also an important influencing factor.

[0019] Generally speaking, the commonly used Cu-based electrode caps are subject to wear under the action of welding pressure and high temperature, which is mainly manifested as changes in the morphology of the end face of the electrode cap. The wear mechanisms are generally divided into the following three types: 1) Mechanical wear: In the process of repeatedly applying and releasing welding pressure, the end face of the electrode cap gradually wears 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 "end face enlargement", making it more susceptible to wear; 3) Chemical corrosion: A chemical reaction occurs between the end face of the electrode cap and the workpiece to be welded, thereby generating a heterogeneous layer on the end face, which is difficult to withstand welding pressure and high temperature and is prone to peeling or wear. Regardless of the wear mechanism, the final result presented on a macroscopic level is the "end face enlargement" of the electrode cap. The enlargement of the end face of the electrode cap reduces the current density, allowing the weld nugget to grow stably, which can delay the generation of spatter.

[0020] However, the inventors discovered that when resistance spot welding is performed on hot stamped products with aluminum-silicon coatings, the electrode cap not only experiences an "end face enlargement" phenomenon, but also an "end face shrinkage" phenomenon, an abnormal phenomenon not previously noted in the prior art. During their research, the inventors discovered that a boss appears on the end face of the electrode cap, causing the end face to be abnormally reduced in some areas. Figure 1 A cross-sectional view schematically shows a boss formed on the end surface of the electrode cap in the later stage of its use, Figure 2The presence of the boss (the white portion in the figure) has been confirmed by the inventors' observations of the end face of the electrode cap during actual use. In this case, the contact area between the electrode cap and the workpiece to be welded decreases, meaning the current path becomes smaller, resulting in an increase in the local current density. Therefore, when performing resistance spot welding under fixed welding parameters, although the welding current remains constant, the abnormal reduction in the end face of the electrode cap causes a sudden increase in the current density at the contact interface between the electrode cap and the workpiece to be welded, thereby instantaneously generating a large amount of resistance heat. This unstable, instantaneous high heat can lead to spatter. In other words, the "reduction in the end face" of the electrode cap will cause spatter, which will significantly affect the range of the welding current.

[0021] After discovering this phenomenon, the inventors proposed that the reduction in the electrode cap's end face could be delayed or eliminated by suppressing the formation of bosses. Based on this, through in-depth research into the resistance spot welding process, the inventors discovered that reducing the resistance heat generated by the aluminum-silicon coating is crucial for suppressing boss formation. The specific reasons are explained below.

[0022] As we all know, resistance spot welding is a process in which the workpieces to be welded are melted by resistance heat and then rapidly solidified to form a physical connection. The principle of resistance heat generation follows the basic Joule's law, that is, Q = I 2 RT ,in, Q is the resistance heat, I is the welding current, T For time, and R is the resistance of the workpiece to be welded. I and T When fixed, the resistance of the workpiece to be welded R The resistance heat generated will be determined by the heat transfer. Generally, the resistance of the workpiece to be welded consists of the resistance of the substrate itself and the resistance of the surface coating. Generally speaking, the resistance of the steel substrate is related to its composition and is not affected by the hot stamping heating process. However, the hot stamping heating process can affect the structure of the aluminum-silicon coating, thereby affecting the resistance of the workpiece's surface coating.

[0023] During the hot stamping process, the evolution of the aluminum-silicon coating is as follows:

[0024] (1) In the initial state, the Al-Si pre-plating layer consists of a FeSiAl inhibition layer close to the steel substrate and an Al-Si metal alloy;

[0025] (2) During the hot stamping process, the steel sheet with the aluminum-silicon pre-plated layer undergoes a heating process. During the heating process, Fe diffuses from the steel substrate into the pre-plated layer, while Al diffuses from the pre-plated layer into the steel substrate. Depending on the degree of diffusion, aluminum-silicon coatings with different coating structures are formed.

[0026] Generally speaking, for the aluminum-silicon coating obtained by hot stamping, such as Figure 3 (See Z. Wang, NA Xu, MX Huang, Phase transformation and carbon profile at the interface between Al-Sicoating and steel substrate in a press-hardened steel, aterialia, Volume 20, 2021) as shown in the figure. The coating structure is as follows from the steel substrate (the lowermost part - with a body-centered cubic (BCC) structure) to the outside: (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.

[0027] The Fe-Al intermetallic compound layer may include Fe3Al phase, FeAl phase, FeAl2 phase, Fe2Al5 phase and FeAl3, etc., wherein when the Fe2Al5 phase exists, at least a portion of the Fe2Al5 phase is adjacent to the outside of the interdiffusion layer.

[0028] The interdiffusion layer includes the α-Fe (ferrite) layer and the Fe-Al sublayer adjacent to it, in which the Fe and Al elements are continuously distributed. The closer to the steel matrix, the higher the Fe content and the lower the Al content. The α-Fe layer is mainly composed of α-Fe phase (such as Figure 3 As shown in FIG, the Fe-Al sublayer may contain a small amount of Si and may include an Fe3Al phase. The Fe-Al sublayer may include an Fe3Al phase, an FeAl phase, and the like.

[0029] It should be noted that although Figure 3 There is a dotted line between the ferrite and the Fe-Al sublayer (Fe3Al) immediately outside it, but this dotted line is only applicable to this document because in fact there is no clear boundary between the α-Fe phase and the Fe-Al sublayer immediately outside it. The specific reasons are as follows: the α-Fe phase immediately adjacent to the steel matrix is ​​a disordered solid solution with a BCC structure (the solute atoms in the solid solution are disorderedly distributed), which is significantly different from the martensite structure of the steel matrix. Therefore, there is a clear boundary between the two, which can determine the lower edge of the interdiffusion layer. 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., DO3 structure), which further gradually transitions to the FeAl phase with an ordered BCC structure (i.e., B2 structure). Because of the distance from the α-Fe phase to the Fe-Al sublayer outside it (for example, Figure 3The crystal structures of the Fe3Al phase and FeAl phase shown in FIG5 are both BCC structures, with only the proportions of elements such as Fe, Al, and Si changing. Therefore, in the metallographic image obtained by scanning electron microscopy (SEM), the boundary between the α-Fe layer and the adjacent Fe-Al sublayer in the interdiffusion layer cannot be directly determined. Therefore, in the art, the two are usually collectively referred to as the interdiffusion layer.

[0030] The interdiffusion layer has a different crystal structure from the adjacent Fe-Al intermetallic compound layer, so there is a clear boundary between the two, which can determine the upper edge of the interdiffusion layer. Figure 3 As shown in the figure, based on the relationship between light and dark, combined with compositional analysis (energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD)), the outermost interdiffusion layer is divided into the FeAl phase, and the phase in the adjacent Fe-Al intermetallic compound layer is the Fe2Al5 phase. Due to the different crystal structures of the two, there is a clear boundary between them on the metallographic diagram. Based on this, the steel substrate and interdiffusion layer can be identified from the inside out of the hot stamped product with aluminum silicon coating, and the portion outside the interdiffusion layer is the Fe-Al intermetallic compound layer.

[0031] In aluminum-silicon coatings, the FeAl and Fe2Al5 phases not only have distinct crystal structures but also differ in their Fe, Al, and Si content. These differing atomic ratios of Fe and Al directly lead to differences in their mass fractions. Measurements show that the mass fractions of Fe and Al in these two phases do not fully correspond to the atomic ratios in the molecular formulas. This is primarily due to two factors. Firstly, it relates to the degree of order of each phase. Both the FeAl and Fe2Al5 phases are ordered solid solutions, but their structures differ significantly. The FeAl phase has a B2-ordered structure, resulting in a lower formation energy, more flexible atomic diffusion pathways, and a relatively low degree of order, allowing it to exist stably within a certain composition range. In contrast, the Fe2Al5 crystal structure is complex, with a higher formation energy and a higher degree of order, resulting in a narrower composition window. Second, it relates to the testing method. The elemental fractions in coatings are typically determined using EDS line scanning, which involves scanning a sample along a predetermined path with an electron beam while simultaneously acquiring characteristic X-ray signals to generate elemental line distribution curves. Because the FeAl and Fe2Al5 phases are closely adjacent, the detection signal, depending on the electron beam spot diameter (0.05–2 μm) or detection step size (0.01–10 μm), will be the average of the two-phase mixing region, rather than the true composition of a single phase. This results in the EDS analysis curve at the interface showing a gradual compositional transition and fluctuation, rather than the theoretically observed abrupt change. Therefore, it is generally believed that the Fe2Al5 phase has an orthorhombic structure and contains 35–52 wt.% Fe, 48–65 wt.% Al, and less than 1 wt.% Si by mass. The FeAl phase has a BCC structure and contains 52–80 wt.% Fe, 20–48 wt.% Al, and more than 3 wt.% Si by mass. It is important to note that the Si content in the FeAl phase (above 3 wt.%) is significantly higher than that in the Fe2Al5 phase (less than 1 wt.%) because Si is insoluble in the Fe2Al5 phase and can form a solid solution in the FeAl phase by replacing Al atoms.

[0032] From another perspective, the aluminum-silicon coating of hot stamping products is mainly composed of an α-Fe phase and an Fe-Al phase on its outside. 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 difference in electronegativity, atomic radius, and number of valence electrons between the solute element and the solvent element will affect the resistivity. The resistivity of pure aluminum (Al) at room temperature is about 2.7 μΩ·cm. On the one hand, aluminum has a high free electron density and less electron scattering. 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. It is 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 increases. In particular, the increase in the concentration of the high-resistivity silicon element leads to a significant increase in the resistivity of the solid solution. This is because solute atoms can cause lattice distortion and even changes in the crystal structure, leading to more electron scattering and thus increasing resistivity.

[0033] Without considering Si, the resistivity of phases with higher Al content and more complex crystal structures, such as Fe2Al and Fe2Al5, is theoretically slightly higher (60–70 μΩ·cm) than that of phases with ordered BCC structures, such as Fe3Al and FeAl (55–70 μΩ·cm), and also higher than that of the α-Fe phase (approximately 10 μΩ·cm). However, in Al-Si coatings, the Si content in phases with ordered BCC structures (such as Fe3Al and FeAl) is typically higher (at least 3 wt.%) than in phases with orthorhombic structures (such as Fe2Al and Fe2Al5) (at least less than 1 wt.%). Therefore, the introduction of Si can increase the resistivity of phases with ordered BCC structures by 20–30 μΩ·cm. Although the α-Fe phase also contains some Si, both the Si and Al contents are lower than those of phases with ordered BCC structures, such as Fe3Al and FeAl. Therefore, for each phase of the aluminum-silicon coating, the resistivity follows the following order: Si-rich FeAl phase > Fe2Al5 phase > α-Fe phase.

[0034] It is worth noting that the prior art either considers that the Si-rich layer in the Al-Si coating (which contains 4-8 wt.% Si, 40-70 wt.% Fe, a maximum of 1 wt.% Mn and 20-50 wt.% Al, totaling 100 wt.% in addition to impurities) has a positive effect on the welding properties, such as CN109207861B, or that it is the interdiffusion layer that has a negative impact on welding.

[0035] However, the inventors discovered that this is not the case, because the Si-rich FeAl phase in the aluminum-silicon 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 includes, by mass percentage: 3 wt.% ≤ Si ≤ 13 wt.%, 52 wt.% ≤ Fe ≤ 80 wt.%, 20 wt.% ≤ Al ≤ 48 wt.%; alternatively, 3 wt.% ≤ Si ≤ 13 wt.%, 55 wt.% ≤ Fe ≤ 75 wt.%, 20 wt.% ≤ Al ≤ 40 wt.%. Alternatively, 5 wt.% ≤ Si ≤ 10 wt.%, 60 wt.% ≤ Fe ≤ 70 wt.%, 25 wt.% ≤ Al ≤ 35 wt.%.

[0036] On the one hand, as mentioned above, the aluminum-silicon coating of hot-stamped products is primarily composed of an α-Fe phase and an outer Fe-Al phase. The α-Fe phase is primarily ferrite, with a hardness of approximately 200 HV and good toughness. The Fe-Al phase, on the other hand, primarily comprises the Fe2Al5 phase and the Si-rich FeAl phase. These phases have high hardnesses of 800–1000 HV and 400–600 HV, respectively, and are highly brittle. During resistance spot welding, the Cu electrode cap, under the action of resistance heat, undergoes diffusion reactions with the Fe and Al in the coating, forming Cu-Al compounds or Fe-Al-Cu intermetallic compounds on the electrode cap surface. Furthermore, due to the pressure exerted by the electrode cap and the localized stress concentration caused by the Cu-Al compound (with a hardness of 150–250 HV) or Fe-Al-Cu intermetallic compound (with a hardness of over 300 HV), the hard and brittle Fe-Al phase in the coating is prone to cracks, which propagate toward the steel substrate. When the crack propagates into the α-Fe phase, the crack tip will be blunted and terminated in the α-Fe phase due to the good toughness of the α-Fe phase. Therefore, the coating structure outside the interdiffusion layer will be divided into many parts by the crack, such as Figure 4 shown.

[0037] On the other hand, the melting point of the a-Fe phase is generally around 1538°C, the melting point of the Si-rich FeAl phase is approximately 1310°C, and the melting point of the Fe2Al5 phase is 1169°C. Therefore, as the proportion of the first and second Si-rich regions increases, the resistive heat generated by them gradually increases the coating temperature, causing the lower-melting-point Fe2Al5 phase sandwiched between them to melt. Because cooling water flows through the electrode cap at a certain rate during welding, it removes some of the heat generated by the outer first Si-rich region. Therefore, the temperature of the second Si-rich region immediately adjacent to the a-Fe phase in the entire coating will be higher, which in turn will tend to cause the Fe2Al5 phase adjacent to the second Si-rich region to melt at the interface between the two (i.e., the interface between the interdiffusion layer and the Fe-Al intermetallic compound layer outside it). In this case, due to the change in solid-liquid state, the bonding strength at the interface between the Fe2Al5 phase and the second Si-rich region will be significantly reduced.

[0038] Under the combined effect of the above two aspects, namely, (1) the Fe-Al intermetallic compound layer of the aluminum-silicon coating has been divided into multiple parts due to cracks and (2) the bonding force at the interface between the second Si-rich region and the Fe2Al5 phase is greatly weakened due to the change of solid-liquid state, part of the aluminum-silicon coating is easily detached at the interface between the second Si-rich region and the Fe2Al5 phase (such as Figure 5 ) and adheres to the surface of the electrode cap. In the subsequent multiple dotting processes, under the coupling of pressure and heat, the detached coating part and the electrode cap further diffuse to form Cu-Al-Fe intermetallic compounds, which improves the adhesion of the detached coating part to the electrode cap and makes it difficult to fall off. Therefore, the detached coating part will be firmly adhered to the surface of the electrode cap in the form of a local boss, such as Figure 2 This leads to the aforementioned "end reduction" of the electrode cap due to the local boss.

[0039] In combination with the above findings, the inventors propose to control the area A occupied by the second Si-rich region. HD The total area A occupied by the Fe-Al intermetallic compound layer outside the second Si-rich region and the interdiffusion layer T The proportion of A HD / A T To reduce the coating shedding, thereby reducing the generation of bosses to solve the problem of narrow welding current range caused by the reduction of the end face. In order to slow down the reduction of the end face of the electrode cap and expand the process window of the welding current and ensure the expected strength performance of the hot stamping formed product, the inventors require 0.05≤A HD / A T ≤0.24.A HD / A T The smaller the A, the better it is for improving the convex platform on the electrode cap caused by the coating falling off.HD / A T ≤0.18, more preferably A HD / A T ≤0.11, the boss on the electrode cap caused by plating peeling will be significantly improved, thereby greatly improving the situation where spatter is easily generated during welding due to the "smaller end face" of the electrode, thereby ensuring the expected process window of weldable current, helping to ensure good product yield and control production costs.

[0040] Those skilled in the art will understand that any range or any value within the above intervals is applicable to the present invention. HD / A T It can be any range or any specific value within the range of 0.05-0.24, for example, any range of 0.05-0.23, 0.05-0.22, 0.05-0.20, 0.05-0.15, 0.06-0.22, 0.06-0.12, 0.06-0.11, etc., or any value such as 0.12, 0.14, 0.16, 0.17, 0.19, 0.15, 0.18, 0.20, 0.21, 0.22, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The embodiments, features and advantages of the present invention will become clear from the detailed description taken in conjunction with the accompanying drawings. It should be understood that the drawings only illustrate some embodiments, are not necessarily drawn to scale, and are exaggerated in some areas for clarity of illustration, and therefore should not be considered limiting. In the drawings:

[0042] Figure 1 schematically shows a cross-sectional view of a boss formed on an electrode cap;

[0043] Figure 2 The figure schematically shows the boss existing on the end surface of the electrode cap in actual use;

[0044] Figure 3 The coating structure of the aluminum-silicon coating obtained by hot stamping a hot stamped steel plate with an aluminum-silicon pre-coating layer in the prior art is schematically shown;

[0045] Figure 4 Schematic diagram showing the propagation of cracks in the Al-Si coating under electrode cap pressure;

[0046] Figure 5 The EBSD analysis diagram of the partial peeling of the aluminum-silicon coating under the pressure of the electrode cap is schematically shown;

[0047] 6A to 6D Schematically illustrates the detection process of Si-rich areas in the aluminum-silicon coating according to the present invention;

[0048] Figure 7 Schematically shows an EBSD analysis diagram of SH3 according to an embodiment of the present invention;

[0049] Figure 8 The EBSD analysis diagram of comparative example DB2 is schematically shown. DETAILED DESCRIPTION

[0050] In order to more clearly illustrate the technical solution of the present application, the present invention is described below by means of exemplary embodiments with reference to the accompanying drawings. The following examples or experimental data are intended to illustrate the present invention by way of example. It should be clear to those skilled in the art that the present invention is not limited to these examples or experimental data. The descriptions of the chemical element contents (wt.%) herein all refer to mass percentages. Unless otherwise specified, the preferred embodiments can be freely combined as needed. Unless explicitly stated, all ranges include end values. Those skilled in the art will understand that the data and various parameters described in the examples are merely exemplary and do not constitute a limitation of the present invention.

[0051] The present invention provides a hot stamped product with an aluminum-silicon coating, which is obtained by a hot stamping process from a hot stamped steel plate with an aluminum-silicon pre-coating. It will be understood that the present invention focuses on the control of the coating structure to solve the problems existing in the electrode cap. The specific composition of the steel matrix of the hot stamped product does not affect the realization of the technical effect of the coating structure required to be protected. On the one hand, the steel matrix will not be in direct contact with the electrode cap, and changes in the matrix composition will not affect the interaction between the coating and the electrode cap. On the other hand, changes in the composition of the steel matrix will not change the structural characteristics of the coating. Therefore, there is no need to limit the composition of the steel matrix of the hot stamped product. Based on this, the following only takes the three components of the steel matrix of the hot stamped steel plate 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 matrix may contain the following components in weight percentage: 0.05-0.50 wt.% C, 0.2-5 wt.% Mn, 0-0.004 wt.% B, 0-0.4 wt.% Nb+Ti+V, 0.05-1 wt.% Si, 0.01-1 wt.% Al; the total content of Cr, Mo, Ni, and Cu is less than 5 wt.%; the balance is Fe and impurities.

[0052] As an example, a hot stamped steel sheet having the composition shown in Table 1 is prepared, and the corresponding manufacturing process is as follows:

[0053] a) Steelmaking: Steel is smelted in a vacuum induction furnace, electric furnace or converter according to the composition in Table 1, and cast ingots are produced by continuous casting technology, or directly by thin slab continuous casting and rolling process;

[0054] b) Hot rolling: The steel slab is heated to 1120-1280℃ for hot rolling, with a total hot rolling reduction of more than 50% and a final rolling temperature of more than 800℃ to obtain a hot-rolled steel plate, which is then coiled at a temperature below 700℃ to form a hot-rolled coil. The hot-rolled coil is then pickled to remove the oxide scale generated during the hot rolling process;

[0055] c) Cold Rolling: The pickled hot-rolled coil is cold rolled at a reduction of 30-70% to obtain a cold-rolled coil with a thickness of approximately 1.4 mm. It will be understood that the thickness of the steel plate is not limited to this and can be rolled to a different thickness as required, for example, within the range of 0.5-3.0 mm, such as for vehicle body applications, but not limited thereto;

[0056] d) Hot-dip coating: The cold-rolled steel coils were subjected to continuous annealing and then pre-coated on both sides. The coating solution consisted of 9 wt.% Si, 3 wt.% Fe, with the remainder being Al and impurities. The bath temperature was 680°C, the pre-set temperature of the steel sheet entering the bath (i.e., the temperature at which the steel sheet enters the bath) was 640°C, and the hot-dip coating time was 2-7 seconds. Excess coating solution was then removed by air knife blowing to control the coating weight on both surfaces, achieving 20 g / m². 2 , 40 g / m 2 , 75 g / m 2 Three aluminum-silicon pre-coatings with different single-side coating weights. The average aluminum content in the aluminum-silicon pre-coating is greater than or equal to 60 wt.%.

[0057] 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 Al and impurities; the plating solution temperature can be 650°C-700°C; and the steel plate entering the pot temperature can be 600°C-650°C. For another example, the weight of the single-sided Al-Si pre-coating can be controlled by air knife blowing according to different requirements, for example, selected from 10-80 g / m 2 The weight between.

[0058] Table 1 Chemical composition of the steel matrix of an exemplary hot stamping steel sheet (wt.%), the balance being Fe and impurities

[0059] Element C Si Mn B Al Cr Ti V Matrix 1 0.06 0.21 1.0 0.003 / 0.10 0.04 Base 2 0.21 0.20 1.2 0.003 / 0.12 0.03 / Base 3 0.33 0.30 1.0 0.002 0.35 0.26 / 0.16

[0060] After the above process, hot stamped steel sheets with aluminum-silicon pre-plated layers of different pre-plated layer thicknesses were obtained. Samples with a size of 550*375 mm were prepared from the pre-plated steel sheets, and then each sample was subjected to flat-plate hot stamping simulation. The hot stamping process was as follows:

[0061] 1) Heating process: The hot stamped steel sheet with aluminum-silicon pre-plating layer is placed in a roller hearth furnace in zones 1 to 10 for the same time according to the heating conditions in Table 2. The total residence time is shown in Table 3 for the heating time, and then cooled to above 700°C.

[0062] 2) Hot stamping process: The heated hot stamping steel plate is then transferred to a flat plate special mold within 5-10 seconds and maintained at a pressure of 600 tons for 8 seconds for hot stamping. It is then taken out after cooling to below 100°C in the mold to obtain a hot stamped product with an aluminum silicon coating. The weight of the single-sided aluminum silicon pre-coating is 10-80g / m 2 The average thickness of the obtained aluminum-silicon coating can be 6.0~45.0 μm.

[0063] As mentioned above, changes in heating time mean changes in the degree of diffusion, that is, the proportion of the Si-rich FeAl phase in the coating structure will also change continuously. Therefore, through comparative analysis, the effects of different proportions of Si-rich FeAl phases on resistance spot welding can be understood. 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. Knowing 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.

[0064] Table 2 Heating process of exemplary hot stamping process

[0065]

[0066] The samples that underwent the aforementioned hot stamping process are listed in Table 3, with SH1-SH19 representing examples of the present invention and DB1-DB4 representing comparative examples. Using the following methods for detecting average coating thickness, detecting Si-rich regions in aluminum-silicon coatings, and evaluating resistance spot welding, the average thickness of the pre-coating layer and the average thickness of the aluminum-silicon coating obtained after hot stamping were measured for SH1-SH19 and DB1-DB4, respectively. The proportion of the second Si-rich region in the aluminum-silicon coating was calibrated, and the process window for resistance spot welding was determined. The specific results are listed in Table 3.

[0067] Method for detecting the average thickness of the coating

[0068] According to standard GB / T13298-2015, metallographic specimens are prepared in the thickness direction for hot-stamped steel sheets / hot-stamped products with aluminum-silicon pre-coating on both sides. Within a field of view of at least 500x (e.g., 1000x, with a field of view of 110 μm × 150 μm), a clear boundary between the (pre-)coating and the steel substrate, as well as a clear boundary between the (pre-)coating and the mounting material, must be observed on either side of the steel substrate. The two boundaries of the (pre-)coating can be determined for that side, and the thickness of the (pre-)coating is the distance between them. For each specimen, thickness measurements are taken at least three points on either side and in the center of the field of view, perpendicular to the (pre-)coating thickness. The average thickness is then calculated. The specimens are marked so that the pre-coating and coating thicknesses are measured on the same side.

[0069] Detection method of Si-rich area in aluminum-silicon coating

[0070] A metallographic sample was prepared by taking a cross-section through the thickness of a hot-stamped product with an aluminum-silicon coating. After grinding and physical polishing, the sample was then electrolytically polished, ion-polished, or vibratory-polished. The sample was then examined under a field electron emission microscope (e.g., GEMINI 300) using EBSD at appropriate magnification (e.g., 1000x, field of view: 110 μm × 150 μm) to identify Si-rich regions, while still allowing for visibility of the complete coating structure and a portion of the steel substrate. The detailed process is as follows.

[0071] Figure 6A The diagram schematically shows a portion of the metallographic image obtained by the EBSD function (for illustration, only a portion of the metallographic image is shown. The actual field of view of the metallographic image is approximately 32 μm × 18 μm, and specific measurements are performed based on the detection and calculation of each area within the actual field of view). The steel matrix is ​​at the bottom, and although the steel matrix and its adjacent α-Fe phase are both BCC structures, the microstructure of the steel matrix is ​​obviously different from that of the α-Fe phase, resulting in a clear interface between the steel matrix and the coating, namely the lower edge of the interdiffusion layer. Figure 6A By comparing the Fe2Al5 and BCC crystal structures in the standard database of known crystal structures, the EBSD function automatically calculates the crystal plane spacing and crystal orientation based on the position and spacing of the diffraction spots, thereby determining the Fe2Al5 area and BCC structure area in the coating and marking them with different colors to distinguish them. Figure 6B , which shows the Fe2Al5 region (region 1) and two separated BCC structure regions: one is close to the steel substrate, which is the interdiffusion layer (region 2); the other is closer to the coating surface, which is the first Si-rich region (region 3). In this case, region 2 and region 3 are shown in the same color. It is worth noting that the boundaries between region 1 and regions 2 and 3 are Figure 6A The Fe2Al5 region, as indicated by contrast, coincides with the boundary between the interdiffusion layer and the FeAl region, consistent with the previous discussion. Therefore, in SEM micrographs, the boundary between the interdiffusion layer and the adjacent Fe2Al5 region, as well as the boundary between the Si-rich FeAl region and other regions (such as the illustrated Fe2Al5 region) in the Fe-Al intermetallic compound layer, can be identified by contrast differences. Furthermore, it is noted that the entire interdiffusion layer is identified as a BCC structure, making it difficult to identify the boundary between the α-Fe layer and the second Si-rich region. Therefore, the present inventors determined the second Si-rich region in the interdiffusion layer using the following method.

[0072] Specifically, Figure 6B 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 affecting the average composition measurement of the "reference area". Then, using the TURPHASE function module, based on this average composition, Figure 6B The portion with the same average composition in region 2 (i.e., the interdiffusion layer) is identified as the second Si-rich region, which is marked as region 4 and shown in the same color (e.g., blue) as the first Si-rich region, that is, Figure 6C Furthermore, the second Si-rich region (region 4) is marked with other colors (such as purple) using image processing software, and the image is obtained. Figure 6D .

[0073] Afterwards, Image J software was used to identify the color blocks in the corresponding areas of the coating and to count the areas they occupied. Figure 6D 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 purple part (second Si-rich region, i.e., region 4) + the blue part (first Si-rich region, i.e., region 3) + the green part (Fe2Al5 region, i.e., region 1) in the picture; HDIndicates the area occupied by the second Si-rich region in the interdiffusion layer (i.e., region 4, which is the purple part); in order to ensure the accuracy of the data, the resolution rate must be above 80% during EBSD testing. 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 pixel. Therefore, in order to simplify the calculation, A HD / A T The 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.

[0074] For each metallographic sample, take at least three fields of view in the direction perpendicular to the coating thickness to perform the above process, and then HD / A T Take the average value as the final A of the metallographic sample HD / A T .

[0075] Evaluation methods for resistance spot welding

[0076] The resistance spot welding evaluation of hot stamping products with aluminum-silicon coatings of different thicknesses is mainly to test the process window of their weldable current.

[0077] The process window detection method of the weldable current is as follows: 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 process window of the weldable current is the range between the minimum current and the spatter current. The minimum current is the range for generating a weld nugget with a diameter of 4× t 1 / 2 ( t is the thickness of the product to be welded, which is 1.4 mm in the current embodiment, so 4× t 1 / 2 The minimum current required for the nugget diameter is about 4.73 mm). The spatter current is the current that produces spatter during the welding process. For example: the nugget diameter is 4× t 1 / 2 The minimum current at this time is 5.6 kA, which is the minimum current. If spatter occurs at 7.0 kA, the process window for weldable current is recorded as 7.0-5.6=1.4 kA. Generally, for hot stamping products, the industry requires a process window for weldable current of no less than 1 kA.

[0078] Table 3 Coating structures and corresponding welding results according to the embodiments and comparative examples of the present invention

[0079] serial number Steel base Average thickness of pre-plating layer (μm) Heating time (s) Average thickness of coating (μm) <![CDATA[A HD / A T ]]> Weldable current range (kA) Process window of weldable current (kA) DB1* Matrix 1 11.5 180 11.8 <![CDATA[ 0.04 ]]> / / SH1 Matrix 1 10.4 200 11.9 0.05 6.1~7.8 1.7 SH2 Matrix 1 11.0 230 13.5 0.10 6.1~7.7 1.6 SH3 Matrix 1 10.9 260 13.8 0.12 6.0~7.4 1.4 SH4 Matrix 1 12.2 290 15.3 0.21 6.0~7.2 1.2 DB2 Matrix 1 11.8 330 15.8 <![CDATA[ 0.25 ]]> <![CDATA[ 5.9~6.8 ]]> <![CDATA[ 0.9 ]]> SH5 Matrix 1 18.5 220 20.3 0.08 5.9~7.7 1.8 SH6 Matrix 1 17.9 250 21.6 0.11 5.9~7.4 1.5 SH7 Matrix 1 17.5 280 21.8 0.18 5.8~7.1 1.3 SH8 Matrix 1 18.6 310 23.5 0.21 5.8~6.9 1.1 SH9 Matrix 1 19.4 340 25.2 0.24 5.7~6.7 1.0 DB3 Matrix 1 18.9 380 25.8 <![CDATA[ 0.28 ]]> <![CDATA[ 5.7~6.4 ]]> <![CDATA[ 0.7 ]]> SH10 Matrix 1 25.8 250 29.4 0.07 5.7~7.3 1.6 SH11 Matrix 1 26.4 300 33.4 0.15 5.6~6.9 1.3 SH12 Matrix 1 28.6 350 38.2 0.17 5.5~6.8 1.3 SH13 Matrix 1 27.9 400 40.4 0.24 5.4~6.4 1.0 DB4 Matrix 1 28.6 450 43.5 <![CDATA[ 0.29 ]]> <![CDATA[ 5.4~6.2 ]]> <![CDATA[ 0.8 ]]> SH14 Base 2 12.4 240 14.8 0.12 6.2~7.6 1.4 SH15 Base 2 12.0 280 15.2 0.16 6.1~7.4 1.3 SH16 Base 2 12.3 340 17.0 0.23 6.0~7.0 1.0 SH17 Base 3 18.5 260 22.1 0.14 6.0~7.4 1.4 SH18 Base 3 18.2 300 23.6 0.19 5.9~7.2 1.3 SH19 Base 3 18.8 360 25.8 0.22 5.9~6.9 1.0

[0080] *:DB1 is not qualified due to insufficient heating, so the steel matrix of its hot stamped product is not completely austenitized, and therefore the expected tensile strength of 1000 MPa or more cannot be achieved.

[0081] As shown in Table 3, when resistance spot welding was performed on the hot stamped products SH1 to SH19 according to the embodiment of the present invention, the process window for the weldable current was no less than 1.0 kA, and even reached over 1.5 kA. In contrast, when resistance spot welding was performed on the hot stamped products DB1 to DB4 according to the comparative example, the process window for the weldable current did not reach 1.0 kA. This indicates that the hot stamped products according to the present invention can expand the process window for the weldable current to over 1.0 kA, achieving the desired technical effect. The specific reasons for this are analyzed below.

[0082] The hot stamping products SH1, SH2, SH3, SH4 (abbreviated as SH1-4) of the substrate 1 according to the embodiment of the present invention and the comparative example DB2 all start with the same pre-plating layer single-side weight, i.e., 20 g / m 2 The average thicknesses of the aluminum-silicon pre-coatings were measured to be 10.4 μm, 11.0 μm, 10.9 μm, 12.2 μm, and 11.8 μm, respectively. SH1-4 and DB2 were then subjected to a hot stamping process with heating times of 200 s, 230 s, 260 s, 290 s, and 330 s, respectively. Following this, hot stamping was performed to obtain aluminum-silicon coatings with average thicknesses of 11.9 μm, 13.5 μm, 13.8 μm, 15.3 μm, and 15.8 μm, respectively. Each aluminum-silicon coating consisted of a body-centered cubic interdiffusion layer adjacent to the steel substrate and an Fe-Al intermetallic compound layer adjacent to the interdiffusion layer. The aluminum-silicon coatings of SH1-4 and DB2 were then analyzed using the aforementioned method for detecting Si-rich regions in aluminum-silicon coatings. Taking a field of view of the hot stamping formed product SH3 according to an embodiment of the present invention as an example, the aluminum silicon coating is marked by the above-mentioned Si-rich area detection method. Figure 7As shown, the α-Fe layer with a BCC structure is marked in red, the first Si-rich region with a BCC structure is marked in blue, the Fe2Al5 region with an orthorhombic structure is marked in green, and the second Si-rich region is marked in purple. That is, the Al-Si coating of SH3 includes an interdiffusion layer with a BCC structure adjacent to the steel substrate and an Fe-Al intermetallic compound layer adjacent to the outside of the interdiffusion layer. The Fe-Al intermetallic compound layer includes the Fe2Al5 region adjacent to the interdiffusion layer and the Fe2Al5 region on the coating surface (collectively, region 1 (green)), as well as a first Si-rich region with a BCC structure (region 3 (blue). The interdiffusion layer also includes an α-Fe layer (red) adjacent to the steel substrate and a second Si-rich region (region 4 (purple)) with a BCC structure adjacent to the outside of the α-Fe layer. The second Si-rich region is identified by 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. The identified average composition is: 6.6 wt.% Si, 31.8 wt.% Al, and 61.4 wt.% Fe. Using Image J software, the number of pixels occupied by the second Si-rich region is 22136, while the total number of pixels occupied by the second Si-rich region and the Fe-Al intermetallic compound layer outside is 182042. HD and A T The area ratio is 0.1215, which is about 0.12 with two decimal places. Similarly, two different fields of view are taken on the sample to calculate A HD / A T , respectively, are approximately 0.11 and 0.14, and the average of the three and rounded to two decimal places is 0.12. In contrast, a field of view of the aluminum-silicon coating of DB2 that has undergone the same treatment shows Figure 8 The partition mark of its corresponding layer is consistent with SH3. After analysis, the A HD / A T is about 0.26, and the final average value is about 0.25. Similarly, the A of SH1-4 and DB2 HD / A T The process windows of the weldable current on SH1-4 and DB2 are 1.7 kA, 1.6 kA, 1.4 kA, 1.2 kA and 0.9 kA, respectively, showing a gradually decreasing trend.

[0083] In the case of thicker pre-plating, for example, 17.5~19.4 μm thick pre-plating for SH5-9 and DB3, 25.8~28.6 μm thick pre-plating for SH10-13 and DB4, the A of Al-Si coating HD / AT The process window of the welding current shows the same rule as above. That is, by prolonging the heating process time, the A in the aluminum silicon coating HD / A T As a result, the process window of the welding current decreases.

[0084] The above phenomenon occurs because extending the heating time during hot stamping will promote the formation of the second Si-rich region, thereby obtaining a higher A HD / A T As discussed above, the area A occupied by the second Si-rich region in the interdiffusion layer is HD The total area A occupied by the coating outside the α-Fe layer T The higher the proportion of the second Si-rich region, the higher the resistance heat generated by the second Si-rich region during the resistance spot welding process, and the easier it is to cause the Fe2Al5 region adjacent to it to melt, so that part of the aluminum-silicon coating is peeled off from the aluminum-silicon coating at the boundary between the second Si-rich region and the Fe2Al5 region and adheres to the end face of the electrode cap, resulting in a narrowing of the process window of the weldable current. Therefore, based on the above-mentioned discovery, the inventors proposed to reduce the A HD / A T To expand the process window of weldable current.

[0085] In addition, when the specific composition of the steel substrate of the hot stamping steel sheet changes, it does not affect the technical effect of expanding the process window of the weldable current through the coating structure according to the present invention. For example, SH14-16 according to the present invention adopts the composition of substrate 2, and SH17-19 adopts the composition of substrate 3. According to the results in Table 3, it can be confirmed that the A of the aluminum-silicon coating of substrate 2 (SH14-16) and substrate 3 (SH17-19) is consistent with the technical effect achieved by substrate 1. HD / A T The process window of the weldable current shows the same rule as above and is HD / A T When ≤0.24, the substrates 2 and 3 also achieve the technical effect of expanding the process window of the weldable current. Therefore, it can be seen that the change in the composition of the steel substrate will not affect the technical effect of the coating structure according to the present invention in expanding the process window of the weldable current.

[0086] Combined with the results in Table 3, the present invention requires A HD / A T ≤0.24, so as to ensure that when resistance spot welding is performed on hot stamping formed products with such a coating structure, the process window of the weldable current is not less than 1.0 kA. Optionally, the present invention requires A HD / A T≤0.21, so that the process window of the weldable current is not less than 1.1 kA; Optionally, the present invention requires A HD / A T ≤0.18, so that the process window of the weldable current is not less than 1.3 kA; optionally A HD / A T ≤0.12, so that the process window of the weldable current is not less than 1.4 kA; optionally A HD / A T ≤0.11, so that the process window of the weldable current is not less than 1.5 kA; optionally A HD / A T ≤0.07, so that the process window of weldable current is not less than 1.6 kA.

[0087] On the other hand, it is worth noting that A HD / A T It cannot be infinitely small. According to Table 3, reduce A HD / A T This means reducing the heating process of the hot stamping process. If the heating process is too short, such as DB1 only heating for 180 s, although its A HD / A T The tensile strength of the hot stamped product obtained after hot stamping is less than 1000 MPa, which cannot meet the requirements and becomes a defective product. In contrast, according to the SH1 of the present invention, a hot stamped product that meets the strength requirements is obtained under extended heating time, and its A HD / A T The process window of weldable current is expanded to 1.7 kA. Therefore, in order to ensure that the steel matrix is ​​fully austenitized during the heating process to ensure the strength of the product, the present invention requires 0.05≤A HD / A T , optionally, to further ensure strength, 0.06≤A HD / A T It will be understood that the above tensile strength is an expected property related to the composition of the steel matrix and is not necessarily not less than 1000 MPa, and may also be less than 1000 MPa.

[0088] In view of the above two reasons, the present invention requires 0.05≤A HD / A T ≤0.24. Within this range, the range can be further narrowed according to the above required range, for example, optionally 0.05≤A HD / A T ≤0.21, optionally 0.05≤A HD / A T≤0.18, optionally 0.05≤A HD / A T ≤0.12, optionally 0.05≤A HD / A T ≤0.11, optionally 0.05≤A HD / A T ≤0.07, optionally 0.06≤A HD / A T ≤0.21, optionally 0.06≤A HD / A T ≤0.18, optionally 0.06≤A HD / A T ≤0.12, optionally 0.06≤A HD / A T ≤0.11, optionally 0.06≤A HD / A T ≤0.07.

[0089] It is worth pointing out that the results of thick Al-Si coatings are not comparable to those of thin Al-Si coatings. For example, SH9 and SH13 have coating thicknesses of 25.2 µm and 40.4 µm, respectively. HD / A T The values ​​are the same, with an average value of 0.24. HD / A T In terms of the ratio, the resistance of the Al-Si coating of SH13 is greater, which will generate more resistive heat, which should result in a smaller process window for soldering current than SH9. However, as listed in Table 3, the process window for soldering current for both SH9 and SH13 is 1.0 kA. This is because, according to the diffusion growth law of the coating, the thicker the coating thickness, the corresponding A HD will also increase, that is: at the same A HD / A T The value of SH13 with thicker coating is better than that of SH9 with thinner coating. HD Correspondingly, the resistance of the overall aluminum-silicon coating of SH13 is also greater. According to Joule's law Q = I 2 RT Compared with SH9 (minimum current is 5.7 kA), SH13 with larger resistance can obtain enough heat to form a weld nugget diameter of 4× at a smaller minimum current (5.4 kA) t 1 / 2Correspondingly, the spatter current corresponding to the SH13 (6.4 kA) is also smaller than the spatter current of the SH9 (6.4 kA). Since the process window of weldable current is the range between the spatter current and the minimum current, although the current ranges of the SH9 and SH13 differ due to the difference in overall resistance, they ultimately have the same process window of weldable current and are not comparable.

[0090] Optionally, the average thickness of the aluminum-silicon coating is in the range of 6.0-21.0 μm, further optionally not less than 11.0 μm, optionally not less than 13.0 μm; further, the average thickness of the aluminum-silicon coating is optionally not greater than 18.0 μm, optionally not greater than 15.0 μm. It will be understood that these ranges can be combined in any manner, for example, the average thickness of the aluminum-silicon coating is in the range of 11.0-21.0 μm, optionally 13.0-21.0 μm, optionally 6.0-18.0 μm, optionally 11.0-18.0 μm, optionally 13.0-18.0 μm, optionally 6.0-15.0 μm, optionally 11.0-15.0 μm, optionally 13.0-15.0 μm.

[0091] Optionally, the average thickness of the aluminum-silicon coating is within a range greater than 21.0 µm and less than or equal to 45.0 µm, optionally not less than 21.5 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 40.5 µm, optionally not greater than 38.5 µm, optionally not greater than 35.0 µm, optionally not greater than 33.5 µm. It will be understood that these ranges can be combined in any manner, for example, the average thickness of the aluminum silicon coating is in the range of greater than 21.0 μm and less than or equal to 40.5 μm, optionally 21.5-40.5 μm, optionally 25.0-40.5 μm, optionally 30.0-40.5 μm, optionally greater than 21.0 μm and less than or equal to 38.5 μm, optionally 21.5-38.5 μm, optionally 25.0-38.5 μm, optionally 30.0-38.5 μm, optionally greater than 21.0 μm and less than or equal to 35.0 μm, optionally 21.5-35.0 μm, optionally 25.0-35 μm, optionally 30.0-35.0 μm, optionally greater than 21.0 μm and less than or equal to 33.5 μm, optionally 21.5-33.5 μm. µm, optionally 25.0~33.5 µm, optionally 30.0~33.5 µm.

[0092] In summary, by controlling the structure of the aluminum-silicon coating of the hot stamping product, namely A HD / A TThe ratio of 1.0kA to 1.0kA can not only ensure the strength of the hot stamping product, but also ensure that the process window of the welding current during the resistance spot welding process is not less than 1.0kA. This helps to reduce the consumption of the electrode cap and save production costs.

[0093] The detailed description and accompanying drawings support and describe the present invention, but the scope of the present invention is limited only by the claims. Although some modes or embodiments for carrying out the present invention have been described in detail, various alternative designs and embodiments exist for practicing the present invention as defined in the appended claims. It should be clear to those skilled in the art that the present invention is not limited to these embodiments and that various modifications may be made without departing from the scope of protection of the present invention.

Claims

1. A hot stamping formed product with an aluminum-silicon coating, comprising a steel substrate and an aluminum-silicon coating covering the steel substrate. in, The aluminum-silicon coating includes an interdiffusion layer having 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 having a body-centered cubic structure; The interdiffusion layer includes an α-Fe layer adjacent to the steel substrate and a second Si-rich region having a body-centered cubic structure adjacent to the outside of the α-Fe layer, wherein the second Si-rich region has the same average composition of Fe, Al, and Si as the first Si-rich region; Among them, 0.05≤A HD / A T ≤0.24, Wherein, on the cross section in the thickness direction of the hot stamping formed product with aluminum silicon coating, A T represents the total area occupied by the second Si-rich region and the Fe-Al intermetallic compound layer; A HD Represents the area occupied by the second Si-rich region.

2. The hot stamped product according to claim 1, wherein A HD / A T ≤0.21。 3. The hot stamped product according to claim 1, wherein A HD / A T ≤0.18。 4. The hot stamped product according to claim 1, wherein A HD / A T ≤0.12。 5. The hot stamped product according to claim 1, wherein A HD / A T ≤0.11。 6. The hot stamped product according to claim 1, wherein A HD / A T ≤0.07。 7. The hot stamped product according to any one of claims 1 to 6, wherein 0.06≤A HD / A T 。 8. The hot stamped product according to claim 1, wherein The average thickness of the aluminum-silicon coating is in the range of 6.0 to 21.0 μm.

9. The hot stamped product according to claim 8, wherein The average thickness of the aluminum-silicon coating is not less than 11.0 μm.

10. The hot stamped product according to claim 8, wherein The average thickness of the aluminum-silicon coating is not less than 13.0 μm.

11. The hot stamped product according to any one of claims 8 to 10, wherein The average thickness of the aluminum-silicon coating is no more than 18.0 μm.

12. The hot stamped product according to any one of claims 8 to 10, wherein The average thickness of the aluminum-silicon coating is no more than 15.0 μm.

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

14. The hot stamped product according to claim 13, wherein The average thickness of the aluminum-silicon coating is not less than 21.5 μm.

15. The hot stamped product according to claim 13, wherein The average thickness of the aluminum-silicon coating is not less than 25.0 μm.

16. The hot stamped product according to claim 13, wherein The average thickness of the aluminum-silicon coating is not less than 30.0 μm.

17. The hot stamped product according to any one of claims 13 to 16, wherein The average thickness of the aluminum-silicon coating is no more than 40.5 μm.

18. The hot stamped product according to any one of claims 13 to 16, wherein The average thickness of the aluminum-silicon coating is no more than 38.5 μm.

19. The hot stamped product according to any one of claims 13 to 16, wherein The average thickness of the aluminum-silicon coating is no more than 35.0 μm.

20. The hot stamped product according to any one of claims 1 to 6, wherein The first Si-rich region comprises Si≥3 wt.% by mass.

21. The hot stamped product according to claim 20, wherein The first Si-rich region comprises, by mass percentage, 3 wt.%≤Si≤13 wt.%, 52 wt.%≤Fe≤80 wt.%, and 20 wt.%≤Al≤48 wt.%.

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