Pre-coated steel plate, laser tailor-welded member and hot-stamped member

Abstract

The present invention relates to a pre-coated steel plate for laser tailor welding, comprising a substrate and pre-coating layers arranged on an upper surface and / or a lower surface of the substrate. The pre-coated steel plate comprises a transition region. The transition region comprises a portion extending, from a position 0.5 mm away from an edge to be welded of the pre-coated steel plate, along a direction perpendicular to said edge and toward the middle of the pre-coated steel plate for a continuous width W0.5, W0.5≥500 μm. The average thickness T2 of the pre-coating layers outside the transition region satisfies: 9.2 μm≤T2≤19.9 μm. When viewed in a cross section perpendicular to the direction of said edge, in the transition region, the pre-coated steel plate satisfies: at least one of the pre-plating layers on the upper surface and / or lower surface of the substrate comprises an Al-Si metal alloy phase and a high-surface-tension phase consisting of an Fe-Al intermetallic compound and an α-Fe phase, wherein the proportion of the area of the high-surface-tension phase in the corresponding pre-coating layer ranges from 55% to 95%; and in the corresponding pre-coating layer, the high-surface-tension phase and the Al-Si metal alloy phase are in a disordered staggered distribution.

Description

Pre-coated steel sheets, laser-welded components and their hot-stamped components Technical Field

[0001] This invention relates to a pre-coated steel sheet with an aluminum or aluminum alloy pre-coating for laser welding, and a processing method for obtaining the pre-coated steel sheet. The invention also relates to a laser-welded component and a hot-stamped component. Background Technology

[0002] Due to the urgent needs of modern automotive manufacturing for energy conservation, emission reduction, and safety, advanced high-strength steel is increasingly widely used in vehicle body manufacturing. Cold-formed high-strength steel suffers from problems such as high springback, poor dimensional accuracy, and severe wear of stamping dies during the stamping process. Hot stamping technology involves heating sheet metal to full austenitization, then forming and cooling it in a die. This utilizes the sheet metal's good formability at high temperatures and its ability to achieve ultra-high strength at high cooling rates. Hot stamping effectively solves the springback problem of cold-stamped parts; however, oxidation occurs during the heating process. Bare hot-stamped parts require subsequent shot peening to remove the oxide layer. Shot peening increases production costs, affects dimensional accuracy, and also causes environmental pollution. Al-Si coating technology effectively prevents surface oxidation, therefore, Al-Si pre-coated hot-stamped steel is increasingly becoming the choice for safety structural components in body-in-white.

[0003] In automobile manufacturing, laser welding can join steel sheets of different thicknesses, materials, and even surface finishes. This method allows for the placement of materials with different functions according to the specific needs of the vehicle body, thereby achieving optimized cost, weight, and safety performance. However, for commonly used hot-stamped steel with an aluminum-silicon pre-coating, the initial pre-coating can negatively impact weld performance because it melts into the weld during the laser welding process. The distribution of aluminum content in the resulting weld leads to the following two problems:

[0004] i) In areas with relatively high aluminum content, intermetallic compound regions will form, which are highly brittle after hot stamping. Therefore, cracks are easily induced during the service of the weld, accelerating the failure of the weld.

[0005] ii) In areas with relatively low aluminum content, α-Fe phase will form, which prevents the weld structure from being fully austenitized during hot stamping. As a result, a dual-phase structure of martensite and α-Fe phase will be generated after the hot stamping process. Therefore, regions with different mechanical strengths will be formed in the weld, affecting the mechanical properties of the weld.

[0006] To address some of the aforementioned problems, several attempts have been made in this field, which can be broadly categorized into two approaches: 1. Removing a certain amount of the aluminum-silicon alloy layer or thinning the pre-plating layer through mechanical means to reduce the aluminum content melted into the weld, thereby inhibiting the formation of α-Fe phase and intermetallic compounds in the weld; 2. Supplementing the weld with filler wire welding to "neutralize" the aluminum melted into the weld and reduce its impact on the weld.

[0007] For example, CN101426612B proposes a sheet material consisting of a steel substrate and a pre-plating layer, wherein the pre-plating layer comprises an Al-Si metal alloy layer and mainly includes Fe2Al3, Fe2Al5 and Fe. x Al y Si z The pre-plated Al-Si alloy layer is composed of an intermetallic compound layer that is in contact with the substrate. This patent teaches the removal of the Al-Si alloy layer on both the top and bottom sides of the pre-plated layer using a laser beam directed at the periphery of the plate, while retaining the aforementioned intermetallic compound layer. Although this technique helps reduce the amount of aluminum fused into the weld from the pre-plated layer by removing the Al-Si alloy layer, thereby improving the mechanical properties of the weld, the removal of the Al-Si alloy layer leaves only the intermetallic compound layer in the pre-plated layer. Therefore, the area to be welded is prone to corrosion, which can affect the stability of subsequent welding.

[0008] CN111230301B proposes a method for manufacturing equal-strength welded steel thin plates with aluminum or aluminum alloy coatings. In this method, the coating is not removed or thinned before or during welding. Welding is performed using a pre-set specific welding gap and carbon-manganese steel welding wire with a specific composition. After hot stamping, the weld joint exhibits a tensile strength greater than that of the base metal and an elongation greater than 4%, meeting the application requirements for equal-strength welded components in the hot stamping field. However, this technology requires a pre-set butt joint gap of 0.2-0.5 mm between the two steel plates to be welded, placing high demands on the precision of plate processing and positioning. Furthermore, this document only addresses the welding of equal-strength components, while actual industrial production often involves welding combinations of steel plates with different strengths and thicknesses, making the proposed solution difficult to meet production needs.

[0009] CN104023899B proposes using high-carbon, high-manganese welding wire to control the austenitizing temperature range of the laser-welded area, thereby ensuring that the weld region obtains a fully martensitic structure after hot stamping. Specifically, the carbon content in the welding wire is 0.1 wt.% to 0.8 wt.% higher than that of the coated steel substrate, and the manganese content is 1.5 wt.% to 7.0 wt.% higher than that of the coated steel substrate. However, the addition of higher carbon and manganese content leads to increased weld brittleness and decreased toughness, resulting in insufficient resistance to stress corrosion. Furthermore, this technology does not address the problem of aluminum infiltration near the weld toe.

[0010] It is evident that the existing solutions have various shortcomings and cannot meet the needs of the industry. There is still a need for further improvement. Summary of the Invention

[0011] This invention aims to provide an improved pre-coated steel sheet for laser welding, which overcomes the shortcomings of existing technologies, particularly in producing welds with excellent performance and stable quality after laser welding and hot stamping. This invention also aims to provide a processing method for obtaining the improved pre-coated steel sheet. Furthermore, this invention also provides improved laser-welded components and hot-stamped components.

[0012] The inventors of this invention have discovered that the weld toe is a particularly important area to consider in ensuring weld quality stability. Ensuring the absence of brittle Fe-Al intermetallic compounds with a hardness far exceeding that of the martensitic matrix, as well as high-Al-content α-Fe phases with a hardness far lower than that of the martensitic matrix, at the weld toe plays a crucial role in improving weld performance and enhancing the overall quality stability of the weld.

[0013] For convenience, unless otherwise specified, "aluminum-silicon pre-coating" in this article refers to conventional aluminum-silicon pre-coating, which mainly consists of Al-Si metal alloy and Fe-Al intermetallic compound. The average aluminum content in the pre-coating is higher than 80 wt.%, with a certain amount of Si, and the average iron content is lower than 20 wt.%, and in most cases, the average iron content is lower than 10 wt.%. Under normal circumstances, the aluminum-silicon (Al-Si) pre-coating structure of laser-welded steel plates with aluminum-silicon pre-coating in the coated state usually includes a Fe-Si intermetallic compound layer close to the substrate and an Al-Si metal alloy layer above it. The intermetallic compound layer has a higher melting point and its average thickness is generally less than 10 μm (usually 3-6 μm on average), while the remainder is the Al-Si metal alloy layer above it with a lower melting point.

[0014] In the context of this invention, wt refers to weight.

[0015] According to the Fe-Al binary alloy phase diagram, the ordered FeAl3 intermetallic compound gradually transitions to a completely disordered solid solution FCC face-centered cubic aluminum alloy with increasing aluminum content as the Fe content decreases. This aluminum alloy can contain a certain amount of Si. In this context, "Al-Si metallic alloy phase" refers to a face-centered cubic aluminum alloy with a Fe content below 39 wt.% and a high Al content, which may contain less than 13 wt.% Si. When the Si content in the Al-Si metallic alloy phase is approximately 12.6 wt.%, its melting point is at its lowest point of 577 °C. As the Si content decreases to 10 wt.%, its melting point slightly increases to approximately 600 °C. On the other hand, the ordered FeAl intermetallic compound gradually transitions to a completely disordered solid solution α-Fe phase with increasing Fe content. In this context, "α-Fe phase" refers to a body-centered cubic solid solution with a Fe content greater than 73 wt.% and containing a certain amount of Al, which may contain less than 10 wt.% Si. Its melting point is at least 1310℃, and increases to a maximum of 1538℃ with increasing Fe content. Accordingly, in this context, ordered intermetallic compounds with Fe content between 39 wt.% and 73 wt.% and containing a relatively high amount of aluminum and possibly a certain amount of Si are referred to as "Fe-Al intermetallic compounds". The melting point of Fe-Al intermetallic compounds increases with increasing Fe content, ranging from 1160℃ to 1310℃. The mass percentages of the above elements can be measured under an electron microscope using energy dispersive spectroscopy (EDS) or wave dispersive spectroscopy (WDS). If an average value is required, at least three lines should be scanned in the measured area and the average value should be taken.

[0016] During laser welding, a large amount of heat energy from the welding heat source melts part of the steel substrate material and the pre-plating layer to form a weld. A "heat-affected zone" (HAZ) exists on both sides of the weld. The HAZ refers to the area surrounding the weld affected by the welding heat. Within the HAZ, the closer to the weld, the higher the temperature, and vice versa. In the HAZ closest to the weld, the temperature is higher than the melting point of the pre-plating layer but lower than the melting point of the substrate material. Therefore, the pre-plating layer in this area is also in a molten state, especially the extremely low-melting-point Al-Si metallic alloy phase on the surface. According to the research of this invention, the HAZ is divided into four parts, gradually moving away from the weld pool: a high-temperature zone (above 1310℃), a medium-high temperature zone (1160℃~1310℃), a medium-temperature zone (600℃~1160℃), and a low-temperature zone (below 600℃). Through research, the inventors discovered that the width of a typical high-temperature zone is less than 50 μm, extending approximately 50 μm from the edge of the molten pool towards the center of the steel plate. During welding, a large amount of Al-Si metal alloy phase and Fe-Al intermetallic compounds in the pre-plating layer covering the high-temperature zone melt simultaneously, while the α-Fe phase partially melts. In the medium-high temperature zone, in addition to the melting of the Al-Si metal alloy, the Fe-Al intermetallic compounds partially melt depending on the Fe ratio, while the α-Fe phase does not melt. The width of the medium-high temperature zone is generally less than 100 μm, extending approximately 100 μm from the edge of the high-temperature zone towards the center of the steel plate. Furthermore, the width of the medium-temperature zone can reach approximately 200 μm, extending approximately 200 μm from the edge of the medium-high temperature zone towards the center of the steel plate. Only the Al-Si metal alloy melts, while the Fe-Al intermetallic compounds and the α-Fe phase do not melt. The pre-plating layer in the low-temperature zone does not exhibit melting. The weld pool is agitated by the combined thermo-mechanical action induced by the laser, causing the molten pre-coating to flow uniformly throughout the weld. Simultaneously, the alloy composition of the weld pool (typically with an iron content greater than 90 wt.%) differs significantly from the composition of the pre-coating (typically with an iron content less than 10 wt.%), which is composed mostly of Al-Si metal alloys and a small amount of Fe-Al intermetallic compounds near the weld edge. This results in a large surface tension difference between the molten pool and the pre-coating. Under these conditions, due to the Marangoni effect, the low-surface-tension molten pre-coating tends to flow towards the weld toe of the high-surface-tension molten pool. Furthermore, the wettability between the low surface tension molten pre-coating layer and the high surface tension molten pool is poor. Under the influence of the large surface tension difference, the two cannot achieve good mutual solubility in a very short time. The molten pre-coating layer is easy to remain at the weld toe position, which leads to the formation of an ultra-high aluminum content structure at the weld toe. Even after hot stamping, there are still ultra-high aluminum content Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe.Whether the weld toe contains harder Fe-Al intermetallic compounds or softer α-Fe phases, there is a significant hardness difference between it and the martensitic matrix obtained after hot stamping, making the weld toe highly susceptible to interphase cracking. Furthermore, in non-filler wire welding, the weld toe is located in the concave transition zone from the matrix to the weld, while in filler wire welding, it is located in the reinforcement zone. Regardless of the form, the weld toe is a stress concentration zone within the weld. That is, the weld toe is typically an area where Fe-Al intermetallic compounds and / or α-Fe phases are particularly prone to formation or accumulation. If Fe-Al intermetallic compounds and / or α-Fe phases extend into the weld from the weld toe, the combined effect of stress concentration makes the weld toe exceptionally fragile, easily leading to the failure of the entire weld. Thus, the weld toe becomes a weak point in the weld.

[0017] Accordingly, the present invention aims to reduce or even avoid the generation of Fe-Al intermetallic compounds and / or α-Fe phases at the positions corresponding to the weld toe in the weld, thereby reducing the probability of weld failure.

[0018] The inventors of this invention have noted that, in addition to Al-Si metallic alloy phases and Fe-Al intermetallic compounds, a certain proportion of aluminum-containing α-Fe phase inevitably exists in the pre-plating layer of the steel plate. For convenience, in the context of this invention, the Fe-Al intermetallic compounds and α-Fe phase in the pre-plating layer are collectively referred to as "high surface tension phases." The inventors of this invention have discovered that, during laser welding, if the content of "high surface tension phases" in the pre-plating layer near the weld is high, the generation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe position in the weld of the hot-stamped component can be suppressed or even avoided. The principle is analyzed as follows.

[0019] The conventional pre-coating of steel plates for laser welding includes an Al-Si metal alloy phase, which typically contains less than 39 wt.% Fe, a higher content of aluminum, and about 9 to 10 wt.% silicon. It has a low melting point, typically about 600°C.

[0020] During laser welding, the Al-Si metal alloy phase in the high-temperature, medium-high-temperature, and medium-temperature zones of the heat-affected zone adjacent to the weld is prone to melting. In contrast, the melting point of Fe-Al intermetallic compounds is much higher than that of the Al-Si metal alloy phase, and the melting point and surface tension of Fe-Al intermetallic compounds also increase with the increase of Fe content in the phase composition. Therefore, during laser welding, if the pre-coating layer adjacent to the weld has a high Fe-Al intermetallic compound content, it first helps to reduce the total amount of Al-containing phase that melts in the pre-coating layer (i.e., the amount of Al-Si metal alloy that can melt in the medium-high-temperature zone is reduced). Secondly, the increased Fe content helps to reduce the surface tension difference between the molten pre-coating layer at the weld edge and the molten steel substrate material in the weld, thereby helping to suppress the flow of molten Fe-Al intermetallic compounds in the pre-coating layer to the weld toe at the weld pool interface. Furthermore, even if a small amount of molten Fe-Al intermetallic compounds flow into the weld toe, the surface tension difference between the weld pool and the molten Fe-Al intermetallic compounds is significantly lower than that between the Fe-Al and molten Al-Si metal alloys, resulting in better wettability. That is, unlike the molten Al-Si metal alloy phase, which is less likely to rapidly miscible with the weld pool due to the large surface tension difference, the molten Fe-Al intermetallic compounds readily and rapidly miscible with the weld pool. Therefore, the formation of high-aluminum intermetallic compounds and / or α-Fe phases near the weld toe can be reduced or avoided. Similarly, the high-aluminum α-Fe phase in the pre-coating has a higher melting point than the Fe-Al intermetallic compounds and exhibits higher surface tension after melting. Thus, the α-Fe phase in the pre-coating next to the weld can also inhibit the flow of aluminum components to the weld toe during welding.

[0021] This invention proposes that, in the region near the weld edge of the pre-plated steel plate, the pre-plating layer be configured as follows: This region has a high content of a "high surface tension phase" (i.e., a reduced content of the Al-Si metal alloy phase), and the "high surface tension phase" and the Al-Si metal alloy phase are randomly and interwoven. This differs from existing technologies where the pre-plating layer has a lower content of the "high surface tension phase," and the upper part is mainly composed of the Al-Si metal alloy phase. For convenience, the region on the steel plate where the content and distribution of the "high surface tension phase" in the pre-plating layer are adjusted or controlled is referred to as the "transition zone." The specific location and extent of the transition zone will be explained further later.

[0022] The content of the "high surface tension phase" can be reflected by its area ratio in the pre-coating cross-section. To reduce or prevent Al from melting into the weld and accumulating at the weld toe, the area ratio of the "high surface tension phase" in the pre-coating of the transition zone, observed in a cross-section perpendicular to the direction of the edge of the steel plate to be welded (i.e., the extension direction of the subsequently formed weld), should be no less than 55%, preferably no less than 59%, and more preferably no less than 70%. A higher proportion of the "high surface tension phase" in the pre-coating is beneficial for achieving better suppression of aluminum intrusion at the weld toe.

[0023] According to the present invention, considering the area occupied in the above-mentioned cross section, the proportion of the "high surface tension phase" in the pre-plating layer of the transition zone should not exceed 95%, preferably not exceeding 90%. This is based on the following considerations: (1) Once the proportion of the "high surface tension phase" exceeds 95%, or even reaches 100%, it means that the surface Al-Si metal alloy ratio is less than 5%, and the surface of the pre-plating layer in the transition zone near the welding end face is prone to corrosion. (2) Once the proportion of the "high surface tension phase" exceeds 95%, or even reaches 100%, the surface of the area to be welded will appear black or dark, which will affect the laser absorption rate during subsequent laser welding, and thus affect the stability of the laser welding process. (3) Rapidly heating the surface of the pre-coated steel plate with a laser heat source is the preferred way to obtain different proportions of "high surface tension phase". In this case, in order to obtain a pre-coated structure with a higher proportion of "high surface tension phase", it is necessary to use greater power or slower heating speed to obtain a larger heat input. This will cause a large part of the matrix of the steel plate to undergo phase transformation. After the matrix is ​​heated, it is rapidly cooled to obtain martensitic structure, and the hardness will be much higher than the hardness of the original matrix structure of the steel plate, which will cause the pre-coated steel plate to deform or warp, affecting the subsequent laser welding.

[0024] In addition to the proportion of the "high surface tension phase," its distribution pattern is also important. According to the present invention, in the pre-plating layer of the transition zone, the "high surface tension phase" and the Al-Si metal alloy phase are randomly interspersed, especially in the medium- and high-temperature zones. This random interspersed distribution effectively inhibits the flow of the Al-Si metal alloy to the weld after melting, significantly reducing the total amount of high-Al phase melted in the high-temperature zone. Preferably, in the transition zone, irregular blocky "high surface tension phases" exist on the surface of the pre-plating layer. The above-mentioned distribution pattern of the "high surface tension phase" and its above-mentioned proportion work together synergistically, thereby particularly beneficial in achieving a good balance between inhibiting the accumulation of Al components in the weld toe and preventing corrosion of the steel plate surface.

[0025] To suppress the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe, this invention proposes that the transition zone "completely cover" the area adjacent to the weld that is severely affected by welding heat and where aluminum elements tend to accumulate at the weld toe. This means covering the high-temperature and medium-high-temperature zones of the heat-affected zone (HAZ). The medium-temperature and low-temperature zones are farther from the weld edge and have less impact. In this way, in the high-temperature zone of the HAZ, Al elements will melt into the weld as a high-surface-tension phase with a high proportion of iron content. This phase easily mixes with the weld pool, thereby reducing or preventing the formation of Fe-Al intermetallic compounds and / or α-Fe phases near the weld toe. In the medium-high-temperature zone, only the low-melting-point Al-Si alloy phase and some Fe-Al intermetallic compounds can melt. Since this area also contains unmelted high-melting-point "high-surface-tension phases" distributed disorderedly, the low-surface-tension liquid Al-Si alloy, lacking fluidity, will not flow into the high-temperature zone, thus preventing it from flowing into the weld toe. Furthermore, when there are few Al-Si metal alloy phases and the "high surface tension phases" are randomly and interwoven, the Al-Si metal alloy phases may continue to alloy when the heat-affected zone is heated to form Al-Si metal alloy phases with higher iron content, thereby further suppressing or preventing the Al component from flowing to the weld toe during laser welding.

[0026] Considering the common width of welds and heat-affected zones, at least a portion of the transition zone is located within a region extending 3 mm from the edge of the pre-plated steel plate to be welded toward the center of the steel plate (i.e., perpendicular to the direction of the edge to be welded and the thickness direction of the steel plate, pointing toward the interior of the steel plate).

[0027] The transition zone has a continuous width W, which is measured in the plane of the steel plate (or in the aforementioned cross-section) in a direction perpendicular to the edge to be welded; that is, measured along a direction perpendicular to both the direction of the edge to be welded and the thickness direction of the steel plate. The width of the high-temperature zone in the heat-affected zone is typically no greater than approximately 50 μm, and the width of the portion of the medium-high temperature zone that easily affects the weld is typically no greater than approximately 100 μm. Therefore, theoretically, the width W of the transition zone only needs to be no less than 150 μm to completely cover the high-temperature and medium-high temperature zones to suppress the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe. However, in actual production, it is unlikely that precise control can be made to ensure that the transition zone precisely covers the high-temperature and medium-high temperature zones in the heat-affected zone that severely affect the weld. Moreover, the width of the heat-affected zone is greatly affected by the plate thickness and welding process. For example, changes in laser welding parameters such as spot diameter, welding power, welding speed, and defocusing amount directly affect the width of the weld and the high-temperature and medium-high temperature zones of the heat-affected zone. Therefore, this invention proposes that a transition zone width W of not less than 500 μm can completely ensure the suppression of the formation of Fe-Al intermetallic compounds and / or α-Fe phase at the weld toe, improving the applicability of the steel plate. The position of the transition zone on the steel plate can be appropriately selected according to the expected weld width and heat-affected zone width, so that the high-temperature zone of the heat-affected zone and the medium-high temperature zone that easily affects the weld toe are included within the transition zone. In addition, through extensive research, the inventors have found that for laser-welded steel plates with a thickness of 0.8 mm to 3 mm, the weld width obtained after welding is usually between 1.0 mm and 1.7 mm, and in a few cases, the weld width is between 1.7 mm and 3 mm. When considering the narrowest weld width of about 1.0 mm, in this state, the two steel plates to be welded will melt and form a weld within a range of about 0 to 0.5 mm from the edge to be welded. Therefore, under normal circumstances, the transition zone only needs to be set at a range of more than 0.5 mm from the edge of the steel plate. Therefore, according to the preferred embodiment, the transition zone extends from a position 0.5 mm from the edge of the steel plate to be welded towards the center of the steel plate (i.e., perpendicular to the direction of the edge to be welded and the thickness direction of the steel plate, pointing inwards). According to the preferred embodiment, the transition zone includes a continuous width W extending towards the center of the steel plate, measured from a position 0.5 mm from the edge of the steel plate to be welded. 0.5 The portion, the continuous width W 0.5 The continuous width W is also measured in the plane of the steel plate in a direction perpendicular to the edge to be welded. 0.5 ≥500μm ensures that the formation of Fe-Al intermetallic compounds and / or α-Fe phase at the weld toe can be suppressed under conventional welding process conditions. W is preferred. 0.5 A thickness of ≥1150μm ensures that the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe can be suppressed under any reasonable welding process conditions.

[0028] On the other hand, the inventors discovered that the aforementioned special settings are unnecessary for the pre-plating layer located too far from the weld edge. The inventors noted that areas far from the weld are less affected by welding heat, and Al in these locations is less likely to flow into the weld. The transition zone only needs to ensure coverage of the high-temperature and medium-high-temperature zones of the heat-affected zone; moreover, even in extreme cases, the weld width will not exceed 5 mm, therefore the width W of the transition zone starting 0.5 mm from the edge of the steel plate is... 0.5 It does not need to exceed 2150 μm. Furthermore, if the transition zone is obtained by rapidly heating the pre-coated layer with a laser, the width of the resulting transition zone will be limited by the diameter of the laser spot. For cost savings and process simplification, a width setting of no more than 2150 μm is particularly efficient for using laser scanning to obtain specific tissue structures within the transition zone.

[0029] In some embodiments, the edge of the transition zone can coincide with the edge of the steel plate to be welded, that is, the transition zone extends from the edge of the steel plate to be welded toward the middle of the steel plate. This can facilitate the handling of the steel plate while ensuring that the flow of aluminum elements into the weld is suppressed.

[0030] Furthermore, this invention proposes limiting the thickness of the pre-plating layer. The original aluminum-silicon pre-plating layer typically consists of Fe... x Al y Si zThe pre-plating layer consists of an intermetallic compound layer (typically with an average thickness of 3–6 μm) and an Al-Si metal alloy layer. A thicker pre-plating layer indicates a thicker Al-Si metal alloy layer, meaning a higher total Al content in the pre-plating layer. The inventors discovered that when the average thickness of the pre-plating layer exceeds 20 μm, the total amount of aluminum flowing into the weld toe is relatively high due to the thicker coating. In this case, even with reasonable composition and width of the transition zone, the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe remains significant, leading to unstable weld quality. Furthermore, when the pre-plating layer is extremely thin (below 8 μm), the total aluminum content is very low, resulting in very little aluminum flowing into the weld toe. In this case, the advantages of setting a "transition zone" are less pronounced. When the average thickness of the pre-coating is above 9.2 μm and below 19.9 μm, the transition zone established according to the present invention can achieve significant beneficial effects. When the average coating thickness is 9.2–18.3 μm or even 11.3–18.3 μm, the beneficial effect of the present invention in suppressing the formation of Fe-Al intermetallic compounds and / or α-Fe phase at the weld toe is particularly prominent. For typical pre-coated steel sheets, although the thickness of the original pre-coating may fluctuate at different locations, it is approximately uniform overall. When a transition zone is obtained by locally and rapidly heating the pre-coated steel sheet, the thickness of the pre-coating may change due to further interdiffusion of Fe and Al in the transition zone; however, this further interdiffusion does not change the total Al content contained in the pre-coating. Therefore, in this case, to limit the total Al content in the pre-coating, the thickness of the pre-coating in the "non-transition zone" of the pre-coated steel sheet (which corresponds substantially to the original pre-coating thickness in the transition zone before local rapid heating treatment) should satisfy the above conditions. Accordingly, in the laser-welded components obtained by laser welding of pre-coated steel plates conforming to the present invention, the average thickness of the pre-coated layer outside the "transition zone" of the steel plate is 9.2 μm or more and 19.9 μm or less, preferably 9.2 to 18.3 μm or even 11.3 μm to 18.3 μm. During the hot stamping process, the Al-Si coated laser-welded components with this pre-coated layer thickness first need to ensure sufficient heating and full austenitization; however, due to the thin coating, the migration distance between the pre-coated layer and the substrate interface is shortened during diffusion, and the coating after stamping should not be too thick. Therefore, in the hot-stamped components obtained by hot stamping of these laser-welded components, the average thickness of the coating outside the "transition zone" of the steel plate is 11.4 μm to 24.5 μm. The aluminum-silicon coated hot-stamped steel within this coating thickness range also exhibits excellent toughness due to reduced carbon enrichment at the coating and interface.

[0031] This invention proposes a pre-coated steel plate for laser welding. The pre-coated steel plate includes a substrate and a pre-coated layer disposed on the upper and / or lower surface of the substrate. The pre-coated steel plate includes a transition zone, which includes (but is not limited to) a continuous width W extending from a position 0.5 mm from the edge of the pre-coated steel plate to be welded, in a direction perpendicular to the edge to be welded and toward the center of the pre-coated steel plate. 0.5 The portion wherein the width W 0.5 ≥500μm, preferably W 0.5 ≥1150μm;

[0032] Wherein, the average thickness T2 of the pre-plated layer outside the transition zone satisfies: 9.2μm≤T2≤19.9μm, preferably 9.2μm≤T2≤18.3μm, and preferably 11.3μm≤T2≤18.3μm;

[0033] Specifically, when viewed in a cross-section perpendicular to the direction of the edge to be welded, the pre-plated steel plate in the transition zone meets the following conditions:

[0034] - At least one of the pre-plated layers on the upper and / or lower surfaces of the substrate comprises an Al-Si metallic alloy phase and a "high surface tension phase" composed of Fe-Al intermetallic compounds and an α-Fe phase, wherein the area proportion P of the "high surface tension phase" in the corresponding pre-plated layer is in the range of 55% to 95%, preferably 59% to 90%, more preferably 70% to 90%; and

[0035] - In the corresponding pre-coating layer, the "high surface tension phase" and the Al-Si metal alloy phase are randomly and interspersed.

[0036] The pre-coated steel sheet of the present invention can effectively prevent a large amount of Al from accumulating at the weld toe during laser welding. After hot stamping, the laser-welded component made using the pre-coated steel sheet of the present invention has only a small amount of Fe-Al intermetallic compounds and / or α-Fe phase, or even none at all, at the position corresponding to the weld toe in the weld.

[0037] A "high surface tension phase" can be generated in the pre-plating layer, for example, by allowing the Al-Si metal alloy layer in the conventional pre-plating layer to interdiffusion with Fe in the steel substrate material. Different phase compositions can be obtained in this process, such as Fe2Al7(Si), Fe2Al5, FeAl(Si), etc. According to a preferred embodiment, the pre-plating layer structure in the transition zone can be obtained by rapidly heating the original aluminum-silicon pre-plating layer on the pre-plated steel plate. In other areas of the pre-plated steel plate besides the transition zone ("non-transition zone"), the original aluminum-silicon pre-plating layer can still be retained. As is well known to those skilled in the art, the original aluminum-silicon pre-plating layer typically comprises an upper Al-Si metal alloy phase and a lower Fe-Al intermetallic compound, which, after hot stamping, forms a fully alloyed Fe-Al intermetallic compound layer and an α-Fe layer. Preferably, localized rapid heating is achieved by using a continuous or quasi-continuous laser to perform laser scanning on the surface of the pre-plated steel plate. To obtain a transition zone that meets the requirements of a high surface tension phase ratio and ensures that the treated steel plate does not deform due to overheating, the input of linear energy can be controlled by adjusting variables such as laser power, spot diameter, defocusing amount, duty cycle, and scanning speed. According to a preferred embodiment, for the pre-coated sheet material that has undergone laser scanning, laser cutting can be performed along the scanning direction in the scanned area, either in the center or offset by a certain distance. This method conveniently obtains a pre-coated steel plate with a transition zone extending from the edge to be welded.

[0038] Accordingly, the present invention proposes a steel plate processing method for producing a pre-coated steel plate according to the present invention, the steel plate processing method comprising:

[0039] Provide initial steel sheets with an aluminum-silicon pre-coating;

[0040] A continuous or quasi-continuous laser is used to perform laser scanning on the surface of the initial steel plate in the region corresponding to the transition zone, thereby locally heating the aluminum-silicon pre-plating layer on the initial steel plate.

[0041] In some cases, applying the above-described settings / treatments taught in this invention to only one side of the pre-coated steel sheet (especially the upper surface, i.e., the surface closer to the welding heat source during welding) can significantly improve weld performance and quality stability. Studies have found that the molten pool agitation pattern on the upper surface of the pre-coated steel sheet, closer to the welding heat source, differs from that on the lower surface. Typically, Al intrusion into the weld is more likely to occur on the upper surface, leading to the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the location corresponding to the weld toe. Therefore, treating only the pre-coated layer on the upper surface of the steel sheet can achieve a significant overall improvement. Of course, the above-described settings / treatments can also be applied to the pre-coated layers on both the upper and lower surfaces of the steel sheet to further ensure weld quality.

[0042] According to the present invention, since no local thinning or peeling operation is performed on the pre-plating layer, the thickness of the pre-plating layer in the transition zone is not significantly thinned compared to the thickness of the pre-plating layer in the non-transition zone. On the other hand, when the transition zone is obtained by local rapid heating treatment of the pre-plating layer, elements such as Fe and Al in the pre-plating layer and the substrate diffuse during this heating treatment. Therefore, theoretically, the average thickness T1 of the pre-plating layer in the transition zone of the pre-plated steel sheet will be greater than the average thickness T2 of the pre-plating layer in the non-transition zone. However, in practice, considering that the thickness of the pre-plating layer itself inevitably fluctuates, for the pre-plated steel sheet, the average thickness T1 of the pre-plating layer in the transition zone is at least not less than 90% of the average thickness T2 of the pre-plating layer in the non-transition zone; in some embodiments, the average thickness T1 of the pre-plating layer in the transition zone is not less than the average thickness T2 of the pre-plating layer in the non-transition zone; in some embodiments, the average thickness T1 of the pre-plating layer in the transition zone is more than 10% greater than the average thickness T2 of the pre-plating layer in the non-transition zone. Similarly, in welded components obtained by laser welding, the average thickness T1 of the pre-coating layer in the transition zone of the steel plate is at least 90% of the thickness T2 of the pre-coating layer in the non-transition zone; in some designs, the average thickness T1 of the pre-coating layer in the transition zone is not less than the average thickness T2 of the pre-coating layer in the non-transition zone. Considering the important role of the aluminum-silicon pre-coating layer in preventing oxidation during hot stamping and improving the corrosion resistance of the sheet metal, avoiding significant thinning of the pre-coating layer and the coating layer is beneficial to obtaining more consistent corrosion resistance.

[0043] In this document, unless otherwise stated, the thickness of a pre-coating or coating refers to the thickness of a pre-coating or coating on one surface of a steel sheet.

[0044] To ensure effective cooling during the scanning process and prevent excessive heat input from causing excessive deformation of the steel plate, rapid cooling can be achieved by using a certain amount of inert or non-inert compressed gas.

[0045] Furthermore, due to the rapid heating of a localized area (transition zone) of the pre-plated layer, a phase transformation also occurs in the portion of the substrate material relatively close to the surface within the transition zone. The inventors discovered that when a transition zone meeting the conditions of this invention is obtained through localized rapid heating, martensite forms in a "shallower" region within the transition zone, extending 50 μm from the interface between the pre-plated layer and the substrate towards the substrate. This results in the hardness HV1 of the substrate material in this "shallower" region being more than 10% (HV1 ≥ 110% HV2), or even more than 20% (HV1 ≥ 120% HV2) or more than 76% (HV1 ≥ 176% HV2), or even greater, than the hardness HV2 of the substrate material in the "deeper" region of the substrate, which is more than 50 μm from the interface. The phase transformation of the matrix in the transition zone leads to changes in hardness and also causes some warping at the edges of the plates to be welded. Generally, when the warping Q is greater than 3mm, welding is not recommended. The inventors discovered that when using an electromagnet to attract the plates to be welded, welding can be achieved well for plates of a certain thickness or strength when Q ≤ 2.66mm, preferably Q ≤ 2.35mm. However, when the plate strength is high and the thickness is large, the electromagnet's attraction to the plate is weak, which easily causes misalignment between the two plates to be welded. Further, Q ≤ 1.98mm is preferred. Furthermore, the aforementioned method of rapidly heating a local area (transition zone) of the plate can fully meet the requirement of the warping Q at the welded edge.

[0046] It is worth noting that the present invention does not strictly limit the thickness relationship between the two steel plates to be welded together. That is, the present invention can achieve good results whether the two pre-plated steel plates to be welded have the same or different thicknesses.

[0047] Similarly, the present invention is applicable to both cases where the two steel plates to be welded have the same strength level and cases where the two steel plates to be welded have different strength levels.

[0048] Furthermore, this invention is applicable to a variety of commonly used laser welding methods (especially both filler wire welding and non-filler wire welding) and hot stamping methods. That is, no special adjustments are required to the laser welding and hot stamping processes; the desired results can be achieved using conventional processes.

[0049] As can be seen, the present invention has a wide range of applications and high flexibility, which is another major advantage of the present invention.

[0050] As a non-limiting example, the base material of the pre-coated steel sheet of the present invention may include: 0.05% to 0.45% carbon and 0.5% to 3.5% manganese. Since the pre-coated steel sheet is used for hot stamping, the hardenability of its steel base material is required to be high, and the content of elements in the base material should meet the Mn requirement. eq Mn is preferred when its concentration is between 1% and 3%.eq Between 1.4% and 2.4%, of which Mn eq The formula for manganese equivalent, i.e., Mn eq =1.0*C+1.0*Mn+0.5*Si+0.75*Cr+0.3*Ni+1.75*Mo+1.25*V+0.25*Cu, where C, Mn, Si, Cr, Ni, Mo, V, and Cu represent the weight percentages of the corresponding elements.

[0051] The strength grade of the pre-coated steel sheet refers to its strength grade after hot stamping, which can be selected from 1000MPa, 1500MPa, or 2000MPa. Preferably, the thickness of the pre-coated steel sheet can be 0.7mm to 3.0mm, more preferably 1.0mm to 2.0mm.

[0052] When welding two steel plates of different strength grades, the weld toe of the resulting hot-stamped component is more sensitive to stress concentration on the side of the steel plate with relatively higher strength. This weld toe is more prone to failure due to the formation of Fe-Al intermetallic compounds and / or α-Fe phases in the weld. In contrast, the weld toe on the side of the steel plate with relatively lower strength is less prone to failure. When the two steel plates have the same strength grade but different thicknesses, the resulting hot-stamped component often fails more easily on the side of the steel plate with relatively lower thickness due to the formation of Fe-Al intermetallic compounds and / or α-Fe phases at the weld toe. Therefore, in some cases, applying the above-described settings taught in this invention only to the pre-coating of the steel plate with relatively higher strength or the steel plate with relatively lower thickness can significantly improve weld performance and quality stability. Of course, the above settings can also be applied to both steel plates to further ensure weld quality.

[0053] The present invention also proposes a laser-welded component, which is manufactured by laser welding using a pre-plated steel plate according to the present invention. As described above, under appropriate circumstances, only one side of the two steel plates used to form the laser-welded component may have a pre-plated layer conforming to the structure proposed in the present invention, while still achieving good results.

[0054] This invention also proposes a laser welding method, which includes:

[0055] Provided a pre-coated steel sheet according to the present invention;

[0056] The pre-coated steel plate is laser-welded.

[0057] As a non-limiting example, the laser power used in the laser welding process is 2000-10000W; the welding speed is 2-9m / min; the defocusing amount is -10-+10mm; the spot diameter is 0.5-0.7mm; and the wire filling speed is 0-5m / min.

[0058] As described above, due to the presence of the heat-affected zone (HAZ), the transition zone on the outer edge of the weld is partially or completely affected by welding heat during laser welding. Under the influence of welding heat, the pre-plating layer with a special structure in the transition zone according to the present invention tends to transform into a structure without Al-Si metal alloy phase over a large continuous length and throughout its entire thickness. Specifically, in the laser-welded component according to the present invention, there exists a continuous section of length L1 in which the pre-plating layer does not contain Al-Si metal alloy phase over its entire thickness, where L1 ≥ 150 μm. This is because the proportion of Al-Si metal alloy phase in the pre-plating layer of the transition zone of the present invention is relatively low, so that this portion of the pre-plating layer easily transforms into a structure without Al-Si metal alloy phase over its entire thickness when it is in the high-temperature and medium-high-temperature zones of the HAZ. The high-temperature zone is generally no more than 100 μm, and with L1 ≥ 150 μm, it is ensured that at least in the high-temperature and medium-high-temperature zones, there is no Al-Si metal alloy phase over the entire thickness of the pre-plating layer. In contrast, while conventional Al-Si pre-plating layers may transform to some extent into layers without Al-Si metallic alloy phases throughout their entire thickness in the high-temperature zone of the heat-affected zone, this transformation is less likely to occur in the medium-to-high-temperature zone of the heat-affected zone. Therefore, under the same conditions, compared to conventional Al-Si pre-plating layers, the section of the pre-plating layer in the transition zone of the present invention that is free of Al-Si metallic alloy phases throughout its entire thickness after welding will have a larger continuous length L1. Since the formation of the section that is free of Al-Si metallic alloy phases throughout its entire thickness is closely related to the heating conditions, this section typically extends from the weld edge towards the center of the corresponding steel plate.

[0059] Optionally, a residual transition zone exists further away from the weld edge adjacent to the section of length L1. This residual zone is the portion of the welded steel plate that remains due to insufficient exposure to welding heat. The area of ​​the "high surface tension phase" is not less than 55%, and the corresponding "high surface tension phase" and Al-Si metallic alloy phase are randomly and interwoven. Preferably, irregularly shaped "high surface tension phases" are present on the surface of the corresponding pre-plated layer in the residual transition zone. In a preferred embodiment, at least a portion, or even all, of the residual transition zone is included in a region extending 3 mm from the corresponding edge of the weld towards the center of the corresponding steel plate.

[0060] Accordingly, the present invention also proposes a laser-welded component, the laser-welded component comprising a first steel plate, a second steel plate and a weld between the first steel plate and the second steel plate, wherein at least one of the first steel plate and the second steel plate is a pre-plated steel plate, the pre-plated steel plate comprising a substrate and a pre-plated layer disposed on the upper surface and / or lower surface of the substrate;

[0061] Near at least one edge of the weld, the corresponding pre-plating layer includes a section throughout its thickness that is free of Al-Si metallic alloy phases. This section extends from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding steel plate. When viewed in a direction perpendicular to both the weld extension direction and the steel plate thickness direction, the section has a continuous length L1, where L1 ≥ 150 μm.

[0062] The corresponding pre-plated layer has an average thickness T4 in the following region A: the region A is a region more than 3.0 mm away from the edge of the corresponding weld in a direction perpendicular to the weld extension direction and toward the middle of the corresponding pre-plated steel plate;

[0063] The average thickness T4 satisfies 9.2μm≤T4≤19.9μm, preferably 9.2μm≤T4≤18.3μm, and even more preferably 11.3μm≤T4≤18.3μm.

[0064] Such laser-welded components can be manufactured by laser welding using the pre-plated steel sheet according to the present invention described above.

[0065] As those skilled in the art will understand, the position of each “edge” of the weld can be determined by observing the flatness of the laser-welded component, or by observing the height variation between the weld and the steel plate being welded, or by observing the differences in microstructure between the weld and the steel plate being welded.

[0066] As explained above, in some cases, laser-welded components can effectively improve weld performance by meeting the above conditions near only one edge of the weld (especially at the upper surface of steel plates with high strength or thinness). Of course, the above conditions can also be met near more weld edges to further ensure weld performance.

[0067] The present invention also proposes a hot-stamped forming component, which is made by hot stamping from a laser-welded component according to the present invention.

[0068] Furthermore, the inventors discovered through research that the coating of the hot-stamped component formed by hot stamping using the laser-welded component according to the present invention also has a special morphology. As described above, a portion of the outer side of the weld seam of the laser-welded component according to the present invention is included in the transition zone of the steel plate, and there is a continuous section of length L1 without the Al-Si metal alloy phase within the transition zone. The pre-coating structure of this section is different from the pre-coating structure (Al-Si metal alloy layer + Fe2Al7(Si) layer) in the "non-transition zone" due to the diffusion of Fe and Al elements. During subsequent hot stamping of the laser-welded component, the pre-coating in this section diffuses further with Fe in the base material, and the coating grows further. In contrast, the Al-Si metal alloy phase and Fe2Al7(Si) in the pre-coating of the "non-transition zone" need to absorb some heat first to allow the Al-Si metal alloy phase and Fe in the base material to undergo initial diffusion. Therefore, under the same hot stamping heating conditions, the "transition zone" in laser-welded components will diffuse more fully with the substrate than the "non-transition zone," resulting in a greater coating thickness in the "transition zone" of the same steel plate than in the "non-transition zone" in the final hot-stamped component. For ease of observation, the coating thickness T5 in region B of the hot-stamped component can be used to represent the coating thickness in the transition zone: region B is a distance of 150μm to 300μm from the corresponding weld edge, measured in a direction perpendicular to the weld extension direction and the steel plate thickness direction. For the non-transition zone, the coating thickness T6 in region C, which is further away from the corresponding weld edge than region B, can be measured. According to a preferred embodiment, region C is a region where the lateral distance (measured in a direction perpendicular to the weld extension direction and the steel plate thickness direction) from the corresponding weld edge is greater than 3mm, or even greater than 5mm. According to the present invention, taking into account the unavoidable fluctuations in coating thickness, the coating thickness satisfies T5≥110%T6, preferably T5≥123%T6.

[0069] In the hot-stamped forming component according to the invention, the coating thickness T6 in region C is in the range of 11.4 μm to 24.5 μm, preferably 11.4 μm to 20.9 μm, preferably 13.2 μm to 20.9 μm, which corresponds to the pre-coating thickness of the pre-coated steel sheet according to the invention, which is often obtained after laser welding and hot stamping processes.

[0070] Accordingly, the present invention also proposes a hot stamping forming component, the hot stamping forming component comprising a first steel plate, a second steel plate and a weld between the first steel plate and the second steel plate, wherein at least one of the first steel plate and the second steel plate is a coated steel plate, the coated steel plate comprising a substrate and a coating disposed on the upper surface and / or lower surface of the substrate;

[0071] The hot-stamped component includes a region B and a corresponding region C located near each edge of the weld. Region B is defined as a region 150μm to 300μm away from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding coated steel plate. Region C is a region more than 3.0mm away from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding coated steel plate.

[0072] Wherein, in at least one of the regions B and the corresponding region C, the hot-stamped component meets the following conditions:

[0073] The corresponding coating has an average thickness T5 in region B and an average thickness T6 in region C, wherein T5 ≥ 1.1 × T6, preferably T5 ≥ 1.23 × T6; and 11.4 μm ≤ T6 ≤ 24.5 μm, preferably 11.4 μm ≤ T6 ≤ 20.9 μm, and preferably 13.2 μm ≤ T6 ≤ 20.9 μm.

[0074] Such hot-stamped components can be manufactured using the pre-plated steel sheet according to the invention described above, through laser welding and hot stamping.

[0075] The hot-stamped components proposed in this invention contain only a very small amount, or even none, of Fe-Al intermetallic compounds and / or α-Fe phases that penetrate into the weld at the location corresponding to the weld toe. Consequently, the probability of weld failure is greatly reduced, and the weld performance stability is significantly improved.

[0076] For both laser-welded and hot-stamped components, for ease of reference, the portion of the steel plate not fused into the weld seam is still referred to as "steel plate." Those skilled in the art will readily understand the meanings of "weld seam" and "steel plate" and their relationship. As understood by those skilled in the art, the coating on the steel plate and laser-welded components before hot stamping is referred to herein as a "pre-coating." After hot stamping, this "pre-coating" interdiffusion with the base material of the steel plate transforms into the "coating" of the hot-stamped component.

[0077] In some embodiments, the thickness of the coating in region B is 14–29 μm for T5.

[0078] By utilizing the concept of this invention, on the one hand, the formation of intermetallic compounds and / or α-Fe phase within the weld seam at the weld toe can be reduced or even completely avoided, leading to a more fragile weld seam and thus achieving superior and more reliable weld seam performance. On the other hand, it can still effectively prevent corrosion of the steel plate surface, further improving the stability of weld quality. Moreover, this invention does not impose strict limitations on the specific conditions and types of laser welding operations, making it applicable to various application scenarios and better meeting complex industrial needs. Attached Figure Description

[0079] Figures 1A and 1B schematically illustrate the measurement of the pre-coating thickness and the calibration of the "high surface tension phase" based on cross-sectional metallographic images of the embodiment steel plate H6 taken using an optical microscope.

[0080] Figures 2A and 2B schematically show cross-sectional metallographic images of the embodiment steel plate H8 taken using an optical microscope, and the calibration of the "high surface tension phase" based on the images.

[0081] Figures 3A and 3B schematically illustrate the measurement of the pre-coating thickness and the calibration of the "high surface tension phase" based on the cross-sectional metallographic images of the embodiment steel plate H9 taken using an optical microscope.

[0082] Figures 4A and 4B schematically illustrate the measurement of the pre-coating thickness and the calibration of the "high surface tension phase" based on cross-sectional metallographic images of a comparative steel plate H23 taken using an optical microscope.

[0083] Figure 5A shows an image of a cross-sectional metallographic specimen of a steel plate with a conventional pre-coating, taken using an optical microscope.

[0084] Figure 5B schematically illustrates the identification of the "high surface tension phase" using image recognition software based on Figure 5A.

[0085] Figure 6 is a cross-sectional metallographic image of the laser-welded component of Example SH6 without hot stamping, taken using an optical microscope, near the weld edge.

[0086] Figure 7 is a cross-sectional metallographic image of the laser-welded component of Comparative Example DB1 without hot stamping, taken using an optical microscope, near the weld edge.

[0087] Figure 8 is a cross-sectional metallographic image of the hot-stamped component of Example SH6 near the weld edge, taken using an optical microscope.

[0088] Figures 9A-9C are metallographic images of the cross-section of Example SH6 after hot stamping, taken using an optical microscope, located more than 3 mm from the weld edge.

[0089] Figure 10 is a cross-sectional metallographic image of the hot-stamped component of Comparative DB1 near the weld, taken using an optical microscope.

[0090] Figures 11A-11C are metallographic images of the cross-section of the hot-stamped component of Comparative Example DB1, taken using an optical microscope, located 3 mm from the weld edge.

[0091] Figure 12 is an image of a cross-sectional metallographic specimen of the comparative DB8 laser-welded component near the weld edge, taken using an optical microscope.

[0092] Figure 13 is a cross-sectional metallographic image of a hot-stamped component of the comparative DB8 near the weld edge, taken using an optical microscope.

[0093] Figure 14 is a cross-sectional metallographic image of the hot-stamped component of Example SH6 in the region near the weld edge, taken using an optical microscope.

[0094] Figure 15 is a cross-sectional metallographic image of a sample taken using an optical microscope, schematically showing the measurement of hardness in different regions of the matrix.

[0095] Figure 16 is a cross-sectional metallographic image of the pre-coated steel plate of comparative example H27, taken using an optical microscope.

[0096] Figure 17 is a schematic cross-sectional view of the pre-coated layer according to the present invention.

[0097] Figure 18 is a schematic cross-sectional view of a laser-welded component according to the present invention.

[0098] Figure 19 is a schematic cross-sectional view of a hot-stamped component according to the present invention.

[0099] Figures 20A and 20B show the temperature changes at distances of 50 μm and 150 μm from the weld toe during welding, respectively. Detailed Implementation

[0100] The present invention will now be described in more detail with reference to exemplary embodiments. The following examples or experimental data are intended to illustrate the invention by way of example, and it will be clear to those skilled in the art that the invention is not limited to these examples or experimental data. Descriptions of chemical element content (%) herein refer to weight percentages.

[0101] Figure 17 schematically shows a non-limiting cross-sectional view of a pre-plated steel sheet 1, which is not drawn to scale. The pre-plated steel sheet 1 includes a substrate 11 and pre-plated layers 12 and 13 respectively located on the upper and lower surfaces of the substrate 11. The steel sheet has a transition zone of width W near the edge to be welded, which includes a width W extending from a position 0.5 mm away from the edge to be welded towards the center of the steel sheet. 0.5 The part.

[0102] Figure 18 schematically shows a non-limiting cross-sectional view of a laser-welded component 100, which is not drawn to scale. The laser-welded component 100 includes two steel plates 1 and 2 and a weld 3. The first steel plate 1 includes a substrate 11 and pre-plating layers 12 and 13 respectively located on the upper and lower surfaces of the substrate 11. The second steel plate 2 includes a substrate 21 and pre-plating layers 22 and 23 respectively located on the upper and lower surfaces of the substrate 21. The first steel plate 1 and the second steel plate 2 are connected to each other by the weld 3, forming weld edges 31 and 32 between the weld 3 and the first steel plate 1, and weld edges 33 and 34 between the weld 3 and the second steel plate 2. The transition zone of the steel plate 1 includes a portion extending outward from the weld edge 31, thereby suppressing the inflow of Al elements in the heat-affected zone into the weld and their accumulation at the weld toe during welding. The corresponding pre-plating layer 12 includes a section in the region immediately adjacent to the weld edge 31 that is free of Al-Si metallic alloy phase throughout its entire thickness, extending for a continuous length L1 from the weld edge 31. A residual transition zone exists further away from the weld edge adjacent to this L1-length section, encompassed in a region extending 3 mm from the corresponding edge of the weld in a direction toward the center of the corresponding steel plate. The region located more than 3.0 mm away from the weld edge 31 in the direction toward the center of the corresponding steel plate 1 is designated as "Region A".

[0103] Figure 19 schematically illustrates a non-limiting example of a hot-stamped component 400, which is not drawn to scale. The hot-stamped component 400 includes: a first steel plate 4, which includes a substrate 41 and plating layers 42 and 43 respectively located on the upper and lower surfaces of the substrate 41; a second steel plate 5, which includes a substrate 51 and plating layers 52 and 53 respectively located on the upper and lower surfaces of the substrate 51; and a weld 6 between the first steel plate 4 and the second steel plate 5. Weld edges 61 and 62 are formed between the weld 6 and the first steel plate 1, and weld edges 63 and 64 are formed between the weld 6 and the second steel plate 2. A corresponding region B exists near the weld edge 61, which is a region within a range of 150 to 300 μm away from the weld edge 61 in the direction toward the middle of the corresponding steel plate 4. A region C exists at a location further away from the weld edge 61. Region C may be a region more than 3 mm away from the weld edge 61, or, according to other embodiments, a region more than 5 mm away from the weld edge 61.

[0104] This invention is applicable to various types of aluminum-silicon pre-coated steel sheets with different strength levels commonly found on the market. The base material of the various strength levels of pre-coated steel sheets that are applicable may include, for example, the following main components:

[0105] 2000MPa grade steel plate, C content is 0.28-0.35%, Si content is 0.1-0.5%, Mn content is 0.8-1.5%, Cr content is 0-0.5%, B content is 0.0005-0.004%, Ti content is 0-0.1%, and Al content is 0-0.6%.

[0106] 1500MPa grade steel plate, C content is 0.19-0.24%, Si content is 0.1-0.4%, Mn content is 0.8-1.5%, Cr content is 0-0.5%, B content is 0.0005-0.004%, and Ti content is 0-0.1%.

[0107] 1000MPa grade steel plate, C content is 0.05-0.10%, Si content is 0.05-0.3%, Mn content is 1.0-2.0%, Cr content is 0-0.5%, B content is 0.0005-0.004%, and Ti content is 0-0.1%.

[0108] As an example, three representative aluminum-silicon pre-coated steels of different strength grades were purchased from the market, and their compositions are shown in Table 1.

[0109] Table 1. Matrix material composition (wt.%) of aluminum-silicon pre-coated steel.

[0110] The balance consists of Fe and other unavoidable impurity elements.

[0111] Pre-coating treatment

[0112] For each steel composition shown in Table 1, tests were conducted using steel plates with various base thicknesses and pre-coating thicknesses. Information on the base thickness and pre-coating thickness is shown in Table 2. The pre-coating thicknesses shown in Table 2 are the pre-coating thicknesses on a single side of the steel plate.

[0113] For steel of various strength levels, base thicknesses, and pre-coating thicknesses, clean the upper and lower surfaces of the steel plate with acetone or ethanol to remove surface impurities.

[0114] To obtain a "transition zone," an appropriate area is selected on the side of the steel plate to be welded, within a range of 0-3 mm from the edge of the steel plate, and the upper surface of the steel plate (which is the surface closest to the welding heat source during welding) is rapidly heated. A laser is used as the heat source for scanning. To obtain an embodiment conforming to the present invention, the specific laser line energy density is controlled by adjusting the spot diameter, defocusing amount, laser power, pulse duty cycle, and scanning speed to obtain the welding material sheet of the embodiment. Specific settings are shown in Table 2.

[0115] Using laser cutting equipment, the steel is cut into 150×500mm steel plates through the area scanned by the laser, with the edge to be welded being 500mm long. By using different cutting process settings, transition zones of varying widths W can be obtained on the side of the steel plate to be welded, near the edge to be welded. Each transition zone includes a width W extending from a position 0.5mm from the edge to be welded towards the center of the steel plate. 0.5 The part.

[0116] According to standard GB / T13298-2015, the metallographic structure of the transition zone and non-transition zone of the steel plate is obtained in a section perpendicular to the extension direction of the edge to be welded of the steel plate.

[0117] Observe the distribution of the microstructure in the pre-coating layer, especially whether there is a disordered interlacing distribution of "high surface tension phase" and Al-Si metal alloy phase, and whether there are irregular blocky "high surface tension phase" on the surface of the pre-coating layer.

[0118] To measure the thickness T2 of the pre-coating in the non-transition zone (the area not treated by laser scanning), at least three cross-sectional images of the metallographic specimens were taken using an optical microscope at 1000x magnification in the non-transition zone. The lateral distance between the cross-sectional areas corresponding to each image (measured in a direction perpendicular to the edge of the steel plate to be welded and the thickness direction of the steel plate) was not less than 0.5 mm. The surface of the pre-coating and the interface between the pre-coating and the substrate were clearly visible in the images. The thickness of the pre-coating was measured at the center and near the edges of each image, and the average value of the measurements was taken as the thickness T2 of the pre-coating in the non-transition zone. The thickness T1 of the pre-coating in the transition zone was measured in a similar manner. If the width of the transition zone was insufficient to select multiple cross-sectional areas at the above intervals, the measurement result only needed to be obtained based on one cross-sectional area. That is, a cross-sectional image was taken using an optical microscope at 1000x magnification for one area in the middle of the transition zone, and three points were measured at the center and near the edges of the transition zone within the field of view, and the average value was taken. Figures 1A and 3A schematically illustrate how the thickness of the pre-coating layer in the transition zone is measured.

[0119] The area proportion P of the "high surface tension phase" in the coating structure was statistically determined as follows. Observing the cross-sectional images acquired using an optical microscope at 1000x magnification, the "high surface tension phase" and the Al-Si metal alloy phase in the pre-coating exhibit different colors under the optical microscope; for example, in Figure 1A, the "high surface tension phase" appears light gray, while the Al-Si metal alloy phase appears white. Image processing software (Image J software) was used to identify the "high surface tension phase" in the pre-coating structure based on these acquired images, and the area proportion of the "high surface tension phase" in the pre-coating was calculated. Figures 1B, 2B, 3B, and 4B schematically illustrate the measurement of the area proportion of the "high surface tension phase" in Figures 1A, 2A, 3A, and 4A. Additionally, the distribution of the "high surface tension phase" was observed, and the presence of irregularly shaped "high surface tension phases" on the surface of the pre-coating was investigated. The width W of the transition zone extending from 0.5 mm from the edge of the steel plate to be welded towards the center of the steel plate was measured. 0.5 The measurement method is as follows: using an optical microscope at a magnification of at least 500x, observe the disordered distribution area of ​​the "high surface tension phase" and Al-Si metallic alloy phase near the edge of the steel plate to be welded, and measure the continuous width W of this area extending towards the center of the steel plate from a position 0.5 mm away from the edge to be welded. 0.5 .

[0120] Vickers hardness testing of the substrate: Using a force of 100g, three hardness points were marked within a 50μm range from the interface between the substrate and the coating along the thickness direction of the substrate material, and the average value was taken. The distance between the three hardness points was at least greater than one hardness indentation. Then, in the same manner, hardness testing was performed by marking points in the same way in areas along the thickness direction of the substrate material beyond a 50μm range from the interface between the substrate and the coating. Figure 15 schematically shows the hardness testing of the substrate, which was photographed using an optical microscope at 500x magnification.

[0121] Observe the surface condition of the steel plate after rapid heat treatment, paying attention to any abnormalities such as blackening of the weld transition zone or rusting after a period of time. Place the steel plate on a level surface and use a gap gauge to measure the warpage value Q of the weld edge.

[0122] In Table 2, steel plates H1-H19 represent embodiments conforming to the present invention, and their area percentage P of the "high surface tension phase" and partial width W of the transition zone are shown. 0.5The average thickness T2 of the pre-plating layer, reflecting the total Al content in the pre-plating layer, is within an appropriate range. H20-H22 are comparative examples, and their average pre-plating layer thickness T2 in the non-transition zone is greater than 19.9 μm. H23-H33 are comparative examples, and the proportion P of the "high surface tension phase" area in the transition zone is either too high or too low. H34-H37 are comparative examples; although the proportion P of the "high surface tension phase" area in the transition zone and the average pre-plating layer thickness T2 in the non-transition zone are both within an appropriate range, the width W of the transition zone extending from 0.5 mm from the edge to be welded is... 0.5 Less than 500μm.

[0123] In the example, steel plates H1-H19 underwent appropriate rapid heat treatment and cutting. Observation of these steel plates revealed that in the transition zone, the "high surface tension phase" and Al-Si metallic alloy phase were randomly interspersed within the pre-plating layer. Irregular blocky "high surface tension phase" existed on the surface of the pre-plating layer. The continuous length W of the transition zone extending from 0.5 mm from the edge to be welded towards the center of the steel plate was... 0.5 All are not less than 500 μm. Meanwhile, the "high surface tension phase" has an area proportion P in the range of 55% to 95%, and the thickness T2 of the pre-coated layer in the non-transition region does not exceed 19.9 μm. Comparing the thickness T1 of the pre-coated layer in the transition region and the thickness T2 of the pre-coated layer in the non-transition region, it can be seen that T1 is higher than 90% of T2, that is, no significant thinning of the pre-coated layer occurs in the transition region; many embodiments satisfy T1>T2. Regarding hardness, in the transition region of each embodiment, the "shallower" region within 50 μm of the interface shows a significant increase in hardness compared to the "deeper" region more than 50 μm from the interface, all satisfying HV1≥110%HV2, and many embodiments even satisfying HV1≥176%HV2.

[0124] More specifically, taking Example H6 as an example, see Figure 1A, which is a representative image of the cross-sectional metallographic sample of the steel plate H6 of Example H6 taken in the transition zone using an optical microscope at 1000x magnification. The thickness T1 of the pre-coating was measured to be approximately 12.2 μm. As shown in Figure 1B, the "high surface tension phase" (marked in red in the figure) in the transition zone was identified using image processing software. Statistically, the "high surface tension phase" accounts for 80% of the area of ​​the pre-coating cross-section in the transition zone. As can be seen from Figures 1A and 1B, the "high surface tension phase" and the Al-Si metal alloy phase are randomly interspersed within the pre-coating. Typically, the α-Fe phase in the "high surface tension phase" mainly exists or only exists at the bottom of the pre-coating near the substrate, and its content is very low. Most of the microstructure in the "high surface tension phase," especially the microstructure in other locations, is Fe-Al intermetallic compound. Therefore, in the transition zone according to the present invention, the Fe-Al intermetallic compound and the Al-Si metal alloy phase in the pre-coating are actually randomly interspersed. Furthermore, irregularly shaped "high surface tension phases" exist on the surface of the pre-coating layer, which are actually irregularly shaped Fe-Al intermetallic compounds. Similarly, Figures 2A, 3A, and 4A are representative images of the cross-sectional metallographic specimens of steel plates H8, H9, and H23 of the examples taken in the transition zone using an optical microscope at 1000x magnification. Figures 2B, 3B, and 4B show the identification and statistical analysis of the "high surface tension phases" using image processing software.

[0125] In the transition zone of this application embodiment, although the "high surface tension phase" may be relatively more concentrated in the lower part of the pre-coating layer, and there may even be a thinner "high surface tension phase" layer, there are also a large number of scattered and disordered "high surface tension phases" in the upper part of the coating layer. Therefore, the structure above the thinner "high surface tension phase" layer is no longer considered an "Al-Si metal alloy layer". Therefore, the pre-coating layer in the transition zone is described as a "disordered and interwoven distribution of 'high surface tension phase' and Al-Si metal alloy phase". This is different from the structure of the pre-coating layer in the prior art where the "high surface tension phase" and Al-Si metal alloy phase are clearly layered. Figure 5A shows an image of a metallographic sample of a steel plate with a conventional Al-Si pre-coating layer taken using an optical microscope at 1000x magnification. Figure 5B shows the "high surface tension phase" marked in red using image recognition software based on Figure 5A to more clearly show the distribution of the "high surface tension phase". It is evident that in conventional pre-coating layers, the "high surface tension phase" is concentrated in the lower part of the pre-coating layer, which does not fall under the definition of "disorderly interspersed distribution of the 'high surface tension phase' and Al-Si metal alloy phase" in the sense of this invention.

[0126] Furthermore, warpage measurements of the weld edges in Examples H1-H19 revealed that the degree of warpage increased with the increase in the proportion of the "high surface tension phase," with the maximum warpage value Q not exceeding 2.66 mm. Generally, warpage within 3 mm is within the controllable range of laser welding. No abnormalities were observed in the surface color of the steel plate transition zone. It is evident that by setting a reasonable area proportion of the "high surface tension phase," this invention can ensure excellent oxidation resistance and good morphology of the steel plate.

[0127] Observation of the comparative H23-H33 steel plates shows that the pre-coating thickness T2 in the non-transition zone does not exceed 19.9 μm and the width W of the transition zone is... 0.5 The surface area of ​​the "high surface tension phase" in the pre-coated layer cross-section is not less than 500 μm; however, in the transition zone, the area percentage P of the "high surface tension phase" in the pre-coated layer cross-section does not fall within the range of 55% to 95%. For comparative examples H23, H25, H28, H31, and H33, where the area percentage of the "high surface tension phase" in the pre-coated layer cross-section is less than 55%, no irregular blocky "high surface tension phase" was observed on the surface of the pre-coated layer, and the hardness increase in the "shallower" region of these comparative examples compared to the "deeper" region was not significant, with an increase of less than 10%.

[0128] More specifically, taking Comparative Example H23 as an example (see Figures 4A and 4B), although both a "high surface tension phase" and an Al-Si metal alloy phase exist in the transition zone of its pre-plating layer, the area proportion of the "high surface tension phase" (marked in red in Figure 4B) in the coating cross-section is relatively low. Statistical analysis shows that the area proportion of the "high surface tension phase" in the pre-plating layer cross-section in the transition zone is 44% (<55%). As shown in Figure 4B, no irregular blocky "high surface tension phase" was found on the surface of the pre-plating layer.

[0129] The pre-coating layers in the transition zones of comparative examples H24, H26, H27, H29, H30, and H32 contain an excessively high proportion of "high surface tension phases" (>95%). For example, as shown in Figure 16, the pre-coating layer of H27 has transformed into 100% "high surface tension phases." These comparative examples exhibit a blackish surface color because their pre-coating layers have a high Fe content, making them prone to oxidation and rusting in air, thus affecting the surface quality of the weld edges. Furthermore, the rapid heat treatment applied to the pre-coating layers of steel plates H24, H26, H27, H29, H30, and H32 is quite deep, causing the matrix near the weld edges to transform into a fully martensitic structure. This structure differs significantly from the original matrix structure (ferrite + pearlite), resulting in substantial warping at the steel plate edges. This, in turn, easily leads to misalignment during laser welding. Therefore, the quality of these comparative steel plates is poor, and further welding is unnecessary.

[0130] In comparative examples H20, H21, and H22, the proportion of the "high surface tension phase" in the pre-coating layer of the transition zone falls within the range of 55% to 95%, and the width W of the aforementioned portion of the transition zone... 0.5 The thickness is not less than 0.5 mm, but the coating thickness is greater than 19.9 μm, which results in an excessively high total amount of Al that can flow into the weld.

[0131] In comparative examples H34, H35, H36, and H37, the proportion of the "high surface tension phase" in the pre-coating layer of the transition zone falls within the range of 55% to 95%, but the width W of the aforementioned portion of the transition zone... 0.5 The thickness is less than 0.5mm, which increases the difficulty of subsequent welding using this steel plate. That is, the transition zone may not be able to completely cover the high-temperature and medium-high-temperature zones of the heat-affected zone, resulting in the inability to stably achieve the effect of suppressing Al in the pre-plating layer from flowing into the weld.

[0132] Laser welding

[0133] Following the combination shown in Table 3, the sides of the corresponding steel plates to be welded are laser-buttoned to form laser-welded components. During butt welding, the side of the two steel plates being welded that has undergone laser scanning treatment forms the upper surface that is relatively closer to the welding heat source. Specific welding process parameters are shown in Table 3. The filler wire composition is: 0.31 wt.% carbon, 2.5 wt.% manganese, 0.25 wt.% Si, with the remainder being Fe.

[0134] Table 3 Laser Welding Process Parameters

[0135] After laser welding is completed, the resulting laser-welded components, which have not been hot-stamped, are subjected to microstructure and performance tests.

[0136] Specifically, a section perpendicular to the weld extension direction is taken, and metallographic samples of the weld of the laser-welded component are prepared according to standard GB / T13298-2015. After grinding, polishing, and etching the samples, the pre-plating structure near the weld edge is observed using an optical microscope. For the two welded steel plates with the same strength grade and thickness, the pre-plating structure near the weld edge on the upper surface of either steel plate is observed. For the two welded steel plates with different strength grades, the pre-plating structure near the weld edge on the upper surface of the steel plate with the higher strength grade is observed. For the two welded steel plates with the same strength grade but different thicknesses, the pre-plating structure near the weld edge on the upper surface of the steel plate with the lower thickness is observed.

[0137] The microstructure distribution of the pre-coating near the weld edge of the corresponding steel plate is observed using an optical microscope. The presence of a segment extending from the weld edge that does not contain Al-Si metallic alloy phases across the entire thickness of the pre-coating is noted, and the continuous length L1 of this segment is measured along a direction perpendicular to the weld and pointing towards the center of the steel plate (i.e., perpendicular to both the weld extension direction and the steel plate thickness direction). For cases where the phase composition of the pre-coating cannot be determined under an optical microscope, EDS or WDS analysis can be performed using an electron microscope. The location of the weld edge can be determined by observing the flatness of the laser-welded component or by observing the height change between the weld and the welded steel plate.

[0138] The average thickness T3 of the pre-plating layer within the continuous length L1 segment was measured using an optical microscope in the same manner as the measurement of the pre-plating layer thickness in the transition zone described above. Additionally, the thickness T4 of the pre-plating layer in region A was measured in a similar manner to that described above: Region A is a region measured in a direction perpendicular to the weld extension direction and towards the center of the corresponding pre-plated steel plate, more than 3.0 mm from the edge of the corresponding weld. In this experiment, since region A was not substantially affected during laser welding, the pre-plating layer thickness in region A of the laser-welded component can be considered unchanged from before welding. It was verified that the observation positions selected in the aforementioned steps for measuring the pre-plating layer thickness T2 of the steel plate in the non-transition zone were all located in region A in the resulting laser-welded component; therefore, the measurement results of the corresponding thickness T2 were used as the value of thickness T4 in this experiment. Thickness T3 was compared with thickness T4 (in this experiment, the value of thickness T2 was used), and T3 / T4 was calculated. As explained above, when the two steel plates being welded have different strength grades, the T2 of the steel plate with the higher strength grade was used for calculation; when the two steel plates being welded have the same strength grade but different thicknesses, the T2 of the thinner steel plate was used for calculation. The observation results are shown in Table 4.

[0139] Hot stamping

[0140] After completing the microstructure and performance tests of the laser-welded components, they were subjected to hot stamping. The laser-welded components were heated to 930℃ and held for 6 minutes in a muffle furnace, then transferred to a special mold and held under 10t pressure for 10 seconds to obtain the hot-stamped components.

[0141] A cross-section perpendicular to the weld extension direction was taken, and metallographic samples were prepared from the steel plate and the area near the weld of the hot-stamped component according to standard GB / T13298-2015. The samples were then polished, etched, and observed. Similar to the above, when the two welded steel plates had the same strength grade and thickness, the upper surface of either plate was observed. When the two welded steel plates had different strength grades or different thicknesses, the upper surface of the plate with the higher strength grade or the plate with the lower thickness was observed.

[0142] Metallographic observation is performed on the weld toe area to determine whether Fe-Al intermetallic compounds and / or α-Fe phases have formed at the location corresponding to the weld toe (e.g., by microstructure color), and the depth of their penetration into the weld is measured. The depth of Fe-Al intermetallic compounds and / or α-Fe phases penetrating the weld is the distance measured along the thickness direction of the steel plate, from the interface between the corresponding coating and the substrate to the bottom of the Fe-Al intermetallic compounds and / or α-Fe phases in the weld. As is well known to those skilled in the art, the weld toe location is the boundary between the weld surface and the substrate. For the laser-welded components of aluminum-silicon coated steel plates mentioned in this invention and the hot-stamped components made therefrom, the weld surface does not have a coating, so the weld toe location can be determined based on the boundary between the presence and absence of a coating. In addition, whether it is filler wire welding or non-filler wire welding, the weld area will always have undulations compared to the steel plate, and the weld toe location can also be determined based on the surface transition.

[0143] In addition, the coating on the corresponding steel plate was observed using an optical microscope in the following region B near the weld edge: this region is located 150–300 μm from the weld edge in a direction perpendicular to the weld extension direction and towards the center of the corresponding coated steel plate. The average thickness T5 of the coating in region B was measured by observing region B with an optical microscope at a magnification of at least 500, measuring the coating thickness at three locations within the field of view, and taking the average value. Furthermore, the average thickness T6 of the coating on the corresponding steel plate in region C, which is further away from the weld edge (in this experiment, region C is defined as the area more than 3.0 mm from the weld edge), was measured by taking three cross-sectional photographs with an optical microscope at a magnification of at least 500 in the region beyond 3.0 mm from the weld edge. The lateral distance between the regions corresponding to each pair of cross-sectional photographs was not less than 0.5 mm. The coating thickness was measured at the center and near the edges of each photograph, and the average value of each measurement was taken. The observation results are shown in Table 4.

[0144] According to the ASTM E8 / E8M-09 standard method for tensile testing of metallic materials, A50 tensile specimens were made from hot-stamped components, with the weld seam positioned in the middle of the parallel section of the specimen. Tensile tests were then performed, with 20 specimens for each welded assembly. The number of specimens that fractured at the weld seam was counted. The test results are shown in Table 4.

[0145] Table 4 Performance of welded components and hot-stamped components

[0146] As shown in Table 3, steel plates H1-H19 according to the present invention were combined in different ways to form embodiments SH1-SH26. Comparative examples DB1-DB15 were formed by welding steel plates H20-H37 that do not conform to the present invention. The welds obtained by welding all embodiments and comparative examples were free from defects such as indentations or undercuts that are considered in the art to affect weld performance.

[0147] Examples SH1-SH19 are each obtained by laser welding two steel plates of the same specifications. Examples SH20-SH26 are each obtained by laser welding two steel plates of different specifications, wherein the two steel plates being welded differ in strength level (base material composition), base thickness, and pre-coating thickness.

[0148] Microstructural observation of the laser-welded components of Examples SH1-SH26 without hot stamping revealed that, in the region immediately adjacent to the weld edge, there exists a section of the pre-plated layer that does not contain Al-Si metallic alloy phases throughout its entire thickness, and the continuous length L1 of this section is not less than 150 μm. Experiments showed that the high-temperature and medium-high-temperature zones of the heat-affected zone (HAZ) are typically located within a 150 μm range extending from the weld edge towards the center of the steel plate (see Figures 20A and 20B). The length L1 exceeding 150 μm in these examples reflects that the high-temperature and medium-high-temperature zones of the HAZ are "covered" by a specially treated transition zone, thereby suppressing the flow of Al elements into the weld and their accumulation at the weld toe. In some embodiments (particularly the transition zone width W or the aforementioned partial width W...), 0.5 In a larger embodiment, a residual transition zone exists next to the section that does not contain the Al-Si metal alloy phase. This residual transition zone is not significantly affected by welding heat during the welding process, thus preserving the disordered and interwoven distribution of the "high surface tension phase" and Al-Si intermetallic compounds.

[0149] Microstructural observation of the hot-stamped components formed by hot stamping in Examples SH1-SH26 revealed that the coating thickness T5 in region B was increased by more than 10% compared to the coating thickness T6 in region C, with most examples showing an increase of more than 20%.

[0150] Metallographic analysis of the hot-stamped components of Examples SH1-SH26 showed that a small amount or no Fe-Al intermetallic compounds and / or α-Fe phase were present at the weld toe location in the weld. As seen in Examples SH1-SH26, when the transition zone width W is sufficient, as the proportion P of the "high surface tension phase" in the transition zone of the welded steel plate increases and the average thickness T2 of the pre-plating layer in the non-transition zone decreases, the penetration depth of Al element accumulation at the weld toe gradually decreases; correspondingly, the tensile test results show that the probability of weld fracture also decreases. Although individual weld fractures were observed in the tensile test results of Examples SH1-SH3, SH8, SH12, SH15, SH16, SH21, and SH26, only no more than two weld fractures occurred in 20 samples, and the probability of weld fracture was much lower than in the comparative examples, indicating that the present invention improves the stability of weld performance. Experimental results show that when the average thickness T2 of the pre-coating layer in the non-transition zone does not exceed 19.9 μm, as the proportion P of the "high surface tension phase" in the transition zone of the welded steel plate increases to more than 59%, the penetration depth of the high aluminum phase at the weld toe and the probability of weld fracture are further reduced. When the proportion of the "high surface tension phase" in the transition zone increases to more than 70%, the penetration depth of the Al component in the weld toe and the probability of weld fracture in the hot stamping component approach 0.

[0151] Comparing the weld performance and the width W of the transition zone in each embodiment 0.5 It can be seen that in the width W 0.5 With a width W of not less than 500μm 0.5 There was no significant correlation between further increases in the weld width W and further improvements in weld performance. This is because, for the welding process used in the experiment, a weld width of 500 μm was not significantly affected. 0.5 It is sufficient to cover the high-temperature and medium-high-temperature zones in the heat-affected zone that cause Al to flow into the weld.

[0152] Observing the pre-plating thickness of the laser-welded components in Examples SH1-SH26, it can be seen that the plating thickness T3 in the Al-Si-free section with a length of L1 is greater than the pre-plating thickness T4 in region A.

[0153] More specifically, taking Example SH6 as an example, it is obtained by welding two steel plates H6 together. As shown in Figure 6, metallographic samples of the weld seam of the laser-welded component of Example SH6 were prepared and observed under a light microscope. In the region immediately adjacent to the weld seam edge, the pre-coating layer includes a section whose entire thickness does not contain Al-Si metallic alloy phase, and the continuous length L1 of this section measured from the weld seam edge is 322.9 μm. As shown in Figure 8, after hot stamping, the laser-welded component of Example SH6 was observed. The average thickness T5 of the coating in region B was measured to be 18.3 μm, while as shown in Figures 9A-9C, the average thickness T6 of the coating in region C of Example SH6 was only 13.4 μm. That is, the coating thickness in region B is more than 30% higher than that in region C. Referring to Figure 14, metallographic observation of the weld seam after hot stamping of Example SH6 revealed that no Al element intrusion was observed at the weld toe position. Tensile tests on 20 specimens revealed that none of the welds fractured. This tensile test was intended to examine the probability of weld fracture. While the "0 fractures" result in Example SH6 out of 20 specimens does not guarantee that the welds in Example SH6 will never fracture, it reflects that the probability of weld fracture is very low, approaching zero.

[0154] Observations revealed that, all other things being equal, welded steel plates with different strength grades or thicknesses tend to exhibit more stable performance compared to welded steel plates with identical strength grades and thicknesses. The reason for this is that when welding two steel plates with different strength grades or thicknesses, the strength and thickness of the weld joint fall between the two plates. This means the weld joint is less likely to become the weakest point, making it more prone to fracture from the weaker substrate during tensile testing.

[0155] It is worth noting that, whether it is Examples SH1-SH19 which weld two identical steel plates, or Examples SH20-SH26 which weld two different steel plates, welds with excellent performance and stable quality were obtained. This demonstrates that the present invention can achieve good results in a variety of different applications.

[0156] Comparative examples DB1-DB4, DB8-DB11, and DB13-DB15 were each obtained by laser welding two steel plates of the same specifications, while comparative examples DB5-DB7 and DB12 were each obtained by laser welding two steel plates of different specifications.

[0157] Comparative Example DB1 was obtained by welding two steel plates H23. Because the content of the "high surface tension phase" in the pre-coating layer of the transition zone of steel plate H23 was too low (resulting in a high content of Al-Si metallic alloy phase), the tendency for this pre-coating layer to transform into a section without Al-Si metallic alloy phase throughout its thickness during welding was relatively weak, resulting in a shorter length L1 of the section without Al-Si metallic alloy phase. As shown in Figure 7, metallographic sampling and observation of the weld area of ​​the laser-welded component of Comparative Example DB1 revealed that in the region immediately adjacent to the weld edge, the continuous length L1 of the section of the pre-coating layer without Al-Si metallic alloy phase throughout its thickness, measured from the weld edge, was 111.8 μm, less than 150 μm.

[0158] After hot stamping, the laser-welded components of Comparative Example DB1 were observed. As shown in Figure 10, the average coating thickness T5 in region B was measured to be 13.2 μm; as shown in Figures 11A-11C, the average coating thickness T6 in region C was measured to be 13.5 μm. Clearly, the coating in region B was not significantly thicker than that in region C. Metallographic observation of the weld after hot stamping revealed significant Al element intrusion at the weld toe, with a depth reaching 145.3 μm. The Fe-Al intermetallic compounds and / or α-Fe phases formed by the aluminum accumulation at the weld toe have a significantly different hardness from the matrix, resulting in high stress concentration in this area during tensile testing, making it prone to cracking. Tensile tests on 20 samples revealed weld fracture in 10 of them, indicating poor weld quality stability.

[0159] Comparative Example DB8 was obtained by welding two H34 steel plates together. As shown in Figure 12, metallographic samples were prepared and observed on the weld seam of the laser-welded component of Comparative Example DB8. It was found that in the area immediately adjacent to the weld seam edge, the continuous length L1 of the section where the pre-plating layer does not contain the Al-Si metal alloy phase over its entire thickness, measured from the weld seam edge, was 90.06 μm, which is less than 150 μm.

[0160] After hot stamping, the laser-welded components of Comparative Example DB8 were observed. The average coating thickness T5 in region B was measured to be 19.4 μm, while the average coating thickness T6 in region C was 18.9 μm, with T5 increasing by only 2.6% compared to T6. Metallographic observation of the weld after hot stamping revealed, as shown in Figure 13, significant Al element intrusion at the weld toe, reaching a depth of 271.4 μm. Tensile testing of 20 samples revealed weld fracture in 13 of them, indicating poor weld quality stability. The weld width of DB8 was measured to be approximately 1.67 mm. This indicates that the low width of the transition zone in the H34 steel plate forming DB8 prevents the coverage of the high-temperature and medium-high-temperature zones in the heat-affected zone that easily affect the weld toe. Although the proportion of high surface tension phase P in the steel plate exceeded 55% and the thickness T2 of the pre-coated layer in the non-transition zone was no higher than 19.9 μm, stable and good weld performance could not be achieved.

[0161] Comparative Example DB13 was formed by welding two steel plates H20. The pre-coating thickness T2 of steel plate H20 in the non-transition zone was 22.4 μm, reflecting its large initial pre-coating thickness and correspondingly high total Al content near the weld. Correspondingly, the hot-stamped component of Comparative Example DB13 also had a large coating thickness of 26.6 μm in region C. Therefore, although the area proportion P of the "high surface tension phase" in the transition zone of steel plate H20 was as high as 83%, the aforementioned width W of the transition zone... 0.5 It also meets the requirement of not less than 500μm, but due to the excessive total amount of Al in the pre-plating layer, the phenomenon of Fe-Al intermetallic compounds and / or α-Fe phases forming at the weld toe in the hot stamping components of DB13 is still quite obvious. The Al penetration depth reaches 55.9μm, and 4 out of 20 test samples fractured at the weld.

[0162] Similar to Comparative Examples DB1, DB8, and DB13, the welds of the hot-stamped components of Comparative Examples DB2-DB7, DB9-DB12, and DB14 also showed significant Al element intrusion at the weld toe, resulting in a large intrusion depth. Consequently, the probability of weld fracture in tensile tests was also higher, reflecting poor weld quality stability.

[0163] The above embodiments and experimental data are intended to illustrate the present invention by way of example. Those skilled in the art should understand that the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of protection of the present invention.

Claims

1. A pre-coated steel sheet for laser welding, the pre-coated steel sheet comprising a substrate and a pre-coated layer disposed on the upper surface and / or lower surface of the substrate, wherein, The pre-coated steel plate includes a transition zone, which extends continuously for a width W from a position 0.5 mm from the edge of the pre-coated steel plate to be welded, in a direction perpendicular to the edge to be welded and toward the center of the pre-coated steel plate. 0.5 The portion wherein the width W 0.5 ≥500μm, preferably W 0.5 ≥1150μm; Wherein, the average thickness T2 of the pre-plated layer outside the transition zone satisfies: 9.2μm≤T2≤19.9μm, preferably 9.2μm≤T2≤18.3μm, and preferably 11.3μm≤T2≤18.3μm; Specifically, when viewed in a cross-section perpendicular to the direction of the edge to be welded, the pre-plated steel plate meets the following conditions in the transition zone: - At least one of the pre-plated layers on the upper and / or lower surfaces of the substrate comprises an Al-Si metallic alloy phase and a high surface tension phase composed of Fe-Al intermetallic compounds and an α-Fe phase, wherein the high surface tension phase occupies an area proportion in the corresponding pre-plated layer in the range of 55% to 95%, preferably 59% to 90%, more preferably 70% to 90%; and - In the corresponding pre-coating layer, the high surface tension phase and the Al-Si metal alloy phase are randomly and interspersed.

2. The pre-coated steel sheet according to claim 1, wherein, The average thickness T1 of the pre-plated layer in the transition zone and the average thickness T2 of the pre-plated layer outside the transition zone satisfy: T1≥0.9×T2; preferably, T1≥T2.

3. The pre-coated steel sheet according to claim 1 or 2, wherein, The width W of the portion of the transition zone 0.5 ≤2150μm.

4. The pre-coated steel sheet according to claim 1 or 2, wherein, In the transition zone, there is an irregular blocky high surface tension phase on the surface of the corresponding pre-coated layer.

5. The pre-coated steel sheet according to claim 1 or 2, wherein, In the transition region: In the region within 50 μm of the interface between the substrate and the corresponding pre-coating layer along the thickness direction, the substrate has a hardness of HV1; in the region more than 50 μm of the interface between the substrate and the corresponding pre-coating layer along the thickness direction, the substrate material has a hardness of HV2. Where HV1≥1.12×HV2, preferably, HV1≥1.21×HV2, and preferably, HV1≥1.76×HV2.

6. The pre-coated steel sheet according to claim 5, wherein, The warpage value of the edge to be welded is Q≤2.66mm, preferably Q≤2.35mm, and preferably Q≤1.98mm.

7. The pre-coated steel sheet according to claim 1 or 2, wherein, The matrix comprises: 0.05% to 0.45% C, 0.5% to 3.5% Mn, and the weight percentages of C, Mn, Si, Cr, Ni, Mo, V, and Cu in the matrix satisfy the condition that Mn eq The content is 1% to 3%, preferably Mn eq It ranges from 1.4% to 2.4%. in, Mn eq =1.0*C+1.0*Mn+0.5*Si+0.75*Cr+0.3*Ni+1.75*Mo+1.25*V+0. 25*Cu.

8. The pre-coated steel sheet according to claim 1 or 2, wherein, Outside the transition zone, the pre-plating layer is an aluminum-silicon pre-plating layer, which contains an Al-Si metal alloy phase and an Fe-Al intermetallic compound.

9. A laser-welded component, the laser-welded component comprising a first steel plate, a second steel plate, and a weld between the first steel plate and the second steel plate, wherein, At least one of the first steel plate and the second steel plate is a pre-plated steel plate, the pre-plated steel plate comprising a substrate and a pre-plated layer disposed on the upper surface and / or lower surface of the substrate; Wherein, near at least one edge of the weld, the corresponding pre-plating layer includes a section throughout its entire thickness that does not contain an Al-Si metallic alloy phase. This section extends from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding steel plate. Viewed in a direction perpendicular to both the weld extension direction and the steel plate thickness direction, the section has a continuous length L1, where L1 ≥ 150 μm. The corresponding pre-plated layer has an average thickness T4 in the following region A: the region A is a region more than 3.0 mm away from the edge of the corresponding weld in a direction perpendicular to the weld extension direction and toward the middle of the corresponding pre-plated steel plate; The average thickness T4 satisfies 9.2μm≤T4≤19.9μm, preferably 9.2μm≤T4≤18.3μm, and even more preferably 11.3μm≤T4≤18.3μm.

10. The laser-welded component according to claim 9, wherein, The corresponding steel plate includes a residual transition zone, which is included in a region extending 3 mm from the corresponding edge of the weld in a direction toward the center of the corresponding steel plate; Viewed in a cross-section perpendicular to the direction of the edge to be welded, in the residual transition zone: - The corresponding pre-plating layer includes an Al-Si metal alloy phase and a high surface tension phase composed of Fe-Al intermetallic compounds and α-Fe phase, wherein the high surface tension phase occupies an area of ​​not less than 55% in the corresponding pre-plating layer; -In the corresponding pre-plating layer, the high surface tension phase and the Al-Si metal alloy phase are randomly and interwoven; - Preferably, there is an irregular blocky high surface tension phase on the surface of the corresponding pre-coated layer.

11. The laser-welded component according to claim 9 or 10, wherein, The first pre-coated steel plate and the second pre-coated steel plate have the same or different thickness and / or strength level.

12. The laser-welded component according to claim 9 or 10, wherein, The corresponding pre-plated layer has an average thickness T3 in the section L1 that does not contain Al-Si metal alloy phases over its entire thickness. Wherein, T3≥0.9×T4; preferably, T3≥T4.

13. The laser-welded component according to claim 9 or 10, wherein, The laser-welded component is made by laser welding pre-coated steel plates that meet the following conditions: The pre-plated steel plate includes a transition zone, at least a portion of which is included in a range extending 3 mm from the edge of the pre-plated steel plate to be welded, in a direction perpendicular to the edge to be welded and toward the center of the pre-plated steel plate, and the transition zone has a width W, which is measured in a direction perpendicular to both the direction of the edge to be welded and the thickness direction of the pre-plated steel plate, wherein the width W ≥ 500 μm; Wherein, the average thickness T2 of the pre-plated layer outside the transition zone satisfies: 9.2μm≤T2≤19.9μm, preferably 9.2μm≤T2≤18.3μm, and preferably 11.3μm≤T2≤18.3μm; Specifically, when viewed in a cross-section perpendicular to the direction of the edge to be welded, the pre-plated steel plate meets the following conditions in the transition zone: - At least one of the pre-plated layers on the upper and / or lower surfaces of the substrate comprises an Al-Si metallic alloy phase and a high surface tension phase composed of Fe-Al intermetallic compounds and an α-Fe phase, wherein the high surface tension phase occupies an area proportion in the corresponding pre-plated layer in the range of 55% to 95%, preferably 59% to 90%, more preferably 70% to 90%; and - In the corresponding pre-coating layer, the high surface tension phase and the Al-Si metal alloy phase are randomly and interspersed.

14. The pre-coated steel sheet according to claim 9 or 10, wherein, The corresponding pre-plating layer in region A is an aluminum-silicon pre-plating layer, which contains an Al-Si metal alloy phase and an Fe-Al intermetallic compound.

15. A laser-welded component, wherein the laser-welded component is made by laser welding of the pre-plated steel plate as described in any one of claims 1-8.

16. A hot-stamped forming component, the hot-stamped forming component comprising a first steel plate, a second steel plate and a weld between the first steel plate and the second steel plate, wherein at least one of the first steel plate and the second steel plate is a coated steel plate, the coated steel plate comprising a substrate and a coating disposed on the upper surface and / or lower surface of the substrate; in, The hot-stamped component includes regions B and corresponding regions C located near each edge of the weld. Region B is defined as a region 150μm to 300μm away from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding coated steel plate. Region C is a region more than 3.0mm away from the corresponding edge of the weld in a direction perpendicular to the weld extension direction and toward the center of the corresponding coated steel plate. Wherein, in at least one of the regions B and the corresponding region C, the hot-stamped component meets the following conditions: The corresponding coating has an average thickness T5 in region B and an average thickness T6 in the corresponding region C, wherein T5 ≥ 1.1 × T6, preferably T5 ≥ 1.23 × T6; and 11.4 μm ≤ T6 ≤ 24.5 μm, preferably 11.4 μm ≤ T6 ≤ 20.9 μm, and more preferably 13.2 μm ≤ T6 ≤ 20.9 μm.

17. The hot-stamped component according to claim 16, wherein, The hot-stamped component does not contain Fe-Al intermetallic compounds and / or α-Fe phases that have penetrated into the weld at the location corresponding to the weld toe.

18. The hot-stamped component according to claim 16 or 17, wherein, The first steel plate and the second steel plate have the same or different thickness and / or strength grades.

19. The hot-stamped component according to claim 16 or 17, wherein, The corresponding coating in region C is an aluminum-silicon coating, which contains Fe-Al intermetallic compounds and α-Fe phase.

20. A hot-stamped component, said hot-stamped component being manufactured by hot stamping using a laser-welded component according to any one of claims 9-15.

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