Alloyed hot-dip galvanized steel sheet and method for producing alloyed hot-dip galvanized steel sheet

Abstract

Disclosed is an alloyed hot-dip galvanized steel sheet in which a substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% by mass of Si, the average Fe content in an alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is 10-12% by mass, and the surface phase structural composition of the alloyed hot-dip galvanized layer contains a polygonal zinc-iron alloy phase, the ratio R of the length to the width of the polygonal zinc-iron alloy phase is not less than 1 and not more than Rlt; 2. The alloyed hot-dip galvanized steel plate is good in deep drawability. The invention further discloses a manufacturing method of the alloyed hot-dip galvanized steel plate, the manufacturing method is suitable for the low-silicon steel plate, the alloy phase structure of a steel plate coating is controlled by controlling the air-fuel ratio in the preheating stage and the dew point DP in the soaking stage in the continuous annealing step and the alloying temperature in the alloying step, and therefore the deep drawing performance of the steel plate is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel plate and its manufacturing method, in particular to an alloyed hot-dip galvanized steel plate and a manufacturing method of the alloyed hot-dip galvanized steel plate. BACKGROUND

[0002] The automobile industry requires the use of higher strength steel plates for the purpose of weight reduction and safety. High-strength dual-phase steel has good mechanical properties and use performance, and can be effectively applied to the production and manufacturing of vehicle structural parts. Among them, ultra-high-strength steel with a tensile strength of 780 MPa or above has great potential for weight reduction in terms of weight reduction and safety performance, and can be used for the manufacturing of safety parts, reinforcing parts and structural parts. At the same time, improving the corrosion resistance of the vehicle body has become the goal pursued by users. Alloyed galvanized steel plates have good corrosion resistance, heat resistance, paintability, weldability and rust prevention ability after painting, and also have good scratch resistance due to the high hardness of the alloyed coating.

[0003] The prior art mainly focuses on high-silicon steel for the research of steel plate coating. Since high-silicon steel is not easy to coat, the prior art usually studies how to successfully coat high-silicon steel without studying the coating structure, and the specific method for successful coating is to use high air-fuel ratio and high alloying temperature for coating. Among them, the air-fuel ratio needs to be above 1, not including the end value 1, and a high alloying temperature of above 500℃ is also required.

[0004] For example, patent document CN104040001B discloses an alloyed hot-dip galvanized steel plate with excellent corrosion resistance and paint thickness adhesion. The steel plate has an alloyed hot-dip galvanized layer containing 7-15% Fe by mass, and at least 0.015% of precipitates containing one or more than two selected from Nb, V, Mo and W by mass in the alloyed hot-dip galvanized layer, and the average particle size of the precipitates is 50 nm or less. The invention is to add the above alloying elements, when cracks occur in the coating and the steel plate substrate, to produce specified precipitates in the alloyed hot-dip galvanized layer, which can pin cracks and prevent crack propagation and coating peeling. However, the Nb, V, Mo and W added in the invention belong to noble metals, which is not conducive to cost control in actual application.

[0005] For example, patent document CN111315911A discloses a method for manufacturing a galvannealed steel sheet, which anneals the steel sheet in a full radiation tube furnace including a heating section, a soaking section, a cooling section, and an optional equalizing section, and specifies the hydrogen content of the heating section and the soaking section as 0.1 vol% to 15 vol%, and the dew point as -18°C to 8°C, and also specifies the hydrogen content of the equalizing section as 1 vol% to 30 vol%, and the dew point as lower than -30°C. Then the annealed steel sheet is hot-dip plated in a zinc bath, and alloyed at a temperature of 460°C to 600°C for 1 s to 45 s. However, the Si content of the steel sheet of this patent method is too low, and the dew point temperature of the heating section and the soaking section is high, which is not sufficient to obtain an excellent strength-elongation balance by adding Si.

[0006] For example, patent document CN103732781B proposes an alloyed hot-dip galvanized layer, and the inventors of this patent find that the Fe concentration gradient in the thickness direction of the plated layer has a great influence on the adhesion of the plated layer. Therefore, this patent specifies that the average Fe content in the alloyed hot-dip galvanized layer of the invention is in the range of 8.0-12.0%, and the absolute value of the difference between the Fe content at the position of 1 / 8 of the thickness of the alloyed hot-dip galvanized layer from the interface with the base steel sheet to the outer surface of the alloyed hot-dip galvanized layer and the Fe content at the position of 7 / 8 of the thickness of the alloyed hot-dip galvanized layer is in the range of 0.3-3.0%. However, the Fe content concentration gradient in the thickness direction of the plated layer is not convenient to measure in actual application, and therefore has little guiding significance for actual production.

[0007] In addition, high-silicon steel is also extremely high in cost due to its high silicon content and complex manufacturing process, and is only considered for use in parts with very high elongation requirements. Therefore, it is necessary to study the plated layer of low-silicon steel to improve the performance of low-silicon steel while keeping the cost low. However, the existing technology does not focus on the plated layer of low-silicon steel, and usually only describes its general manufacturing process, and is usually completed by high-temperature heating and alloying, with the alloying temperature set in the range of 540°C to 560°C, without special control of other conditions involved in the steps to control the plated layer performance, and without in-depth study of the microstructure of the manufactured low-silicon steel plated layer.

[0008] With increasing demands for corrosion resistance in the automotive industry, alloyed hot-dip galvanized steel sheets are widely used. After hot-dip galvanizing, the coating is heated to the alloying temperature, causing Fe from the substrate to diffuse into the coating, forming different zinc-iron alloy phases. During zinc-iron diffusion, the zinc-iron alloy phase within the coating forms Γ, Γ1, δ, and ζ phases depending on the Fe content. Compared to pure zinc coatings, zinc-iron alloy phases offer better appearance and corrosion resistance. Automotive steel sheets are mostly used under harsh bending processes (edge ​​bending) and hole expansion during stamping. However, the Zn-Fe alloy formed during alloying is harder and more brittle than the pure zinc phase, making it prone to failure during stamping. The failure modes of alloyed coatings during deformation are mainly two: granular peeling, known as pulverization, and sheet-like peeling, known as delamination. Pulverization is caused by internal failure of the coating, forming granular particles. The particle size is generally smaller than the coating thickness and is usually due to the formation of a thicker Γ phase at the interface between the substrate and the coating. Peeling is the formation of flaky particles due to the failure of adhesion between the coating and the substrate interface. The particle size is generally similar to or larger than the coating thickness, usually caused by the formation of an excessively thick ζ phase on the coating surface, which increases the surface friction. Peeling causes the coating to detach from the substrate, reducing its corrosion resistance and leading to early corrosion and rusting of the base material.

[0009] Therefore, it is necessary to conduct in-depth research on the coating structure of low-silicon steel sheets so that low-silicon steel sheets with lower costs can meet the required tensile strength and elongation while also having good deep-drawing performance. Summary of the Invention

[0010] This invention proposes an alloyed hot-dip galvanized steel sheet to solve the above-mentioned technical problems.

[0011] In a first aspect, embodiments of the present invention disclose an alloyed hot-dip galvanized steel sheet, wherein the substrate of the alloyed hot-dip galvanized steel sheet contains 0 to 1.0% Si by mass, the average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is 10 to 12% by mass, and the surface phase structure composition of the alloyed hot-dip galvanized layer contains a polygonal zinc-iron alloy phase, wherein the length-to-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1 ≤ R < 2.

[0012] Using the above technical solution, the alloyed hot-dip galvanized steel sheet of the present invention has an average Fe content of 10-12% by mass, and the alloyed hot-dip galvanized layer also contains a polygonal zinc-iron alloy phase, resulting in good deep-drawing performance.

[0013] Optionally, the polygonal zinc-iron alloy phase of the alloyed hot-dip galvanized layer is the δ phase.

[0014] Optionally, the average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is in the range of 10.5% to 11.5% by mass.

[0015] Optionally, the proportion of polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer accounts for 95% or more of the total area of ​​the alloy phase in the alloyed hot-dip galvanized layer.

[0016] Optionally, the proportion of polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer accounts for 100% of the total area of ​​the alloy phase in the alloyed hot-dip galvanized layer.

[0017] Optionally, the average grain size of the polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer is less than or equal to 5 μm.

[0018] Optionally, the average grain size of the polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer is in the range of 2 to 3 μm.

[0019] Optionally, the average thickness of the zinc-iron alloy phase with an Fe content exceeding 30% at the interface between the alloyed hot-dip galvanized layer and the substrate is in the range of 500–1000 nm.

[0020] Optionally, the alloyed hot-dip galvanized layer also contains 0.01-1.0% Al by mass.

[0021] Optionally, the substrate of the alloyed hot-dip galvanized steel sheet, by weight, also contains C: 0.05-0.20%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01%, with the remainder including Fe and unavoidable impurities.

[0022] Optionally, the substrate of the alloyed hot-dip galvanized steel sheet may also contain, by weight, one or more of the following: Al: 0.1-1.0%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, Cr: 0.01-0.5%, Mo: 0.05-0.2%, V: 0.005-0.2%, and B: 0.0001-0.10%.

[0023] Optionally, the alloyed hot-dip galvanized steel sheet has a tensile strength ≥780 MPa, an elongation ≥10%, and a coefficient of friction <0.3.

[0024] Second aspect, the present invention also discloses a manufacturing method of an alloyed hot-dip galvanized steel sheet, including a continuous annealing step and an alloying step for the substrate of the alloyed hot-dip galvanized steel sheet containing 0 to 1.0% Si by mass; the continuous annealing step includes a preheating step and a soaking step. Among them, when the heating atmosphere in the preheating step uses air and gas, the air-fuel ratio λ is controlled within the range of 0.80 ≤ λ ≤ 0.90, or when the heating atmosphere in the preheating step uses a mixed gas of N2 and O2, the volume percentage V of O2 therein is controlled within the range of 0 < V ≤ 0.01%; in the soaking step, the dew point DP is controlled within the range of -50 to -30°C; in the alloying step, the alloying temperature is controlled within the range of 500 to 550°C.

[0025] Adopting the above technical solution, by precisely controlling the air-fuel ratio in the preheating stage, the dew point in the soaking stage, and the alloying temperature in the alloying step during the manufacturing process of the steel sheet, it is beneficial for the low-Si steel sheet to be plated and control the surface phase structure in the coating, which is beneficial to form a polygonal zinc-iron alloy phase, thereby improving the deep drawing performance of the steel sheet.

[0026] Optionally, the manufacturing method of the alloyed hot-dip galvanized steel sheet includes the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot dip galvanizing, and alloying;

[0027] Among them, the continuous annealing step includes:

[0028] Preheating: Heat-treat the cold-rolled alloyed hot-dip galvanized steel sheet in a direct-fired heating furnace equipped with a direct-fired burner. When the heating atmosphere in the direct-fired heating furnace uses air and gas, the air-fuel ratio λ is controlled within the range of 0.80 ≤ λ ≤ 0.90, and the alloyed hot-dip galvanized steel sheet is heated to the range of 600 to 750°C; or when the heating atmosphere in the direct-fired heating furnace uses a mixed gas of N2 and O2, the volume percentage V of O2 therein is controlled within the range of 0 < V ≤ 0.01%, and the alloyed hot-dip galvanized steel sheet is heated to the range of 600 to 750°C;

[0029] Soaking: Heat and hold the preheated alloyed hot-dip galvanized steel sheet at a soaking temperature of 700 to 900°C in an atmosphere where the hydrogen concentration is limited to 3 to 25 vol%, the dew point DP is controlled within the range of -50 to -30°C, and the balance is N2 and inevitable impurity gases;

[0030] The hot dip galvanizing step includes: cooling the alloyed hot-dip galvanized steel sheet after continuous annealing and putting it into a zinc bath within the range of 440 to 465°C for hot dip galvanizing;

[0031] [[ID=二十一]]The alloying step includes: putting the alloyed hot-dip galvanized steel sheet after hot dip galvanizing into an alloying furnace for alloying, controlling the alloying temperature within the range of 505 to 550°C, and then cooling and coiling to obtain the required alloyed hot-dip galvanized steel sheet.

[0032] Optionally, the oxidizing atmosphere heated in the preheating step is a mixture of N2 and O2, wherein the volume percentage of O2 is in the range of 0 to 0.005%.

[0033] Optionally, the dew point DP is controlled to be ≥ -40℃ during the preheating step.

[0034] Optionally, the dew point (DP) of the heat equalization step is controlled within the range of -40 to -30°C.

[0035] Optionally, the hydrogen concentration in the homogenization step is controlled within the range of 3 to 10%.

[0036] Optionally, the alloying temperature is controlled within the range of 505–520°C during the alloying step.

[0037] Optionally, the alloying time in the alloying step is controlled within the range of 10 to 30 seconds. Attached Figure Description

[0038] Figure 1 A schematic diagram of the cross-sectional morphology of the steel plate coating of the present invention is shown;

[0039] Figure 2 The SEM image shows the surface morphology of the coating in Embodiment 1 of the present invention.

[0040] Figure 3 This shows a SEM image of the coating cross-sectional morphology of Embodiment 1 of the present invention;

[0041] Figure 4 The SEM image shows the surface morphology of the coating in Comparative Example 1 of the present invention.

[0042] Figure 5 The SEM image shows the cross-sectional morphology of the coating in Comparative Example 1 of the present invention.

[0043] Figure 6 This diagram illustrates the process of determining the aspect ratio of the coating phase structure according to the present invention.

[0044] (Symbol Explanation)

[0045] 1. Steel plate substrate; 2. Coating body; 3. Coating surface layer Detailed Implementation

[0046] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0047] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0048] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0049] In the description of this embodiment, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set up," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this embodiment based on the specific circumstances.

[0050] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0051] Based on existing technologies, it remains unclear how to optimize the structure of low-silicon steel sheet coatings to improve their deep-drawing performance. Through in-depth research, the inventors discovered that the deep-drawing performance (i.e., stamping performance) of hot-dip galvanized alloy coatings primarily depends on the surface phase structure of the coating. For complex deep-drawing parts, poor control of the surface phase structure of the alloy coating can not only cause coating peeling but also lead to stamping cracks in the steel sheet. Extensive experimental research revealed that the key to improving the deep-drawing performance of alloy coatings lies in controlling the formation of a fine-grained, granular, full-δ-phase structure on the coating surface—a polygonal zinc-iron alloy phase—by improving the alloying process. When the coating contains a large amount of polygonal zinc-iron alloy phase, the deep-drawing performance of the low-silicon steel sheet coating is superior. The δ-phase (polygonal zinc-iron alloy phase) has high hardness, which reduces the coefficient of friction during stamping, thereby improving the stamping performance of the steel sheet.

[0052] This invention provides an alloyed hot-dip galvanized steel sheet (GA steel sheet), more specifically a low-silicon steel sheet, which has a reasonably controlled Fe content and phase structure of the alloyed hot-dip galvanized layer (hereinafter referred to as "coating"), thereby giving the steel sheet better deep-drawing performance. Figure 1 The schematic diagram of the cross-sectional morphology of the steel plate coating of the present invention is shown, including a steel plate substrate 1, a coating body 2, and a coating surface layer 3. Figure 3 This is a cross-sectional topographic view of the steel plate coating in this actual embodiment.

[0053] The alloyed hot-dip galvanized steel sheet provided by this invention contains 0-1.0% Si (inclusive of the endpoints 0 and 1.0%) by mass in its substrate. This means the steel sheet may contain no Si or contain no more than 1.0% Si, classifying it as a low-silicon steel. This invention optimizes the surface phase structure of the coating on the low-silicon steel sheet, improving its deep-drawing performance while avoiding increased costs.

[0054] The alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet provided by the present invention further contains, by mass, 10-12% average Fe content (including endpoint values ​​of 10% and 12%), preferably 10.5-11.5% average Fe content (including endpoint values ​​of 10.5% and 11.5%). The average Fe content refers to the average Fe content in the coating. The average Fe content can be measured using the ICP method or by taking the average value of five or more points using EDS.

[0055] When the average Fe content in the coating is in the range of 10-12%, a thick columnar ζ-phase is unlikely to form on the coating surface. The moderate Fe content prevents it from adhering to the mold. When the steel sheet is stamped at this point, the friction between the steel sheet and the mold is moderate, and the coating at the interface is not easily peeled off, which is beneficial for steel sheet processing. Furthermore, a thick Γ-phase will not form at the interface between the steel substrate and the coating, preventing the steel sheet from becoming hard and brittle. This helps control the phase structure of the coating and prevents severe pulverization during stamping, thus minimizing the impact on the steel sheet's performance. Preferably, when the average Fe content in the coating is in the range of 10.5-11.5%, it further ensures the presence of a fine polygonal zinc-iron alloy phase in the coating, reducing the likelihood of a thick Γ-phase forming at the interface between the coating and the substrate. This improves the stamping performance of the steel sheet while also providing good resistance to pulverization.

[0056] The surface phase structure of the alloyed hot-dip galvanized layer provided by this invention also contains a polygonal zinc-iron alloy phase (see reference). Figure 2 The length-to-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1 ≤ R < 2. Wherein, reference... Figure 2 A polygonal zinc-iron alloy phase indicates that the phase structure of the zinc-iron alloy phase exhibits a relatively fine granular form, and SEM (Scanning Electron Microscopy) images reveal that the particles in the alloy phase have relatively long sides (length) and relatively short sides (width). Specifically, the longest distance between the two end faces of the polygonal zinc-iron alloy phase is its length, and the longest distance perpendicular to this length direction is its width. The polygon referred to here is a cross-section obtained by assuming the δ phase to be a two-dimensional shape parallel to the plate surface. The polygon can include various shapes such as quadrilaterals, pentagons, and hexagons, and can also be a polygon with more sides. The aspect ratio R of the polygonal zinc-iron alloy phase is greater than or equal to 1 and less than 2.

[0057] When the aspect ratio R of the polygonal zinc-iron alloy phase satisfies 1≤R<2, it indicates that the Fe content of the zinc-iron alloy phase generated after alloying the coating surface is moderate and meets the requirements. The coating will not peel off easily, which is beneficial to improving the deep drawing performance of the coating.

[0058] The following is combined with Figure 6 This paper introduces methods for determining the aspect ratio and area percentage of polygonal zinc-iron alloy phases.

[0059] Alloyed hot-dip galvanized steel sheets were observed under a SEM (scanning electron microscope) and directly photographed to obtain images such as... Figure 6 The phase structure diagram of the coating surface shown is directly marked on the SEM image, and the length and width of the polygonal zinc-iron alloy phase are measured (reference). Figure 6 (1-10) Calculate the ratio of length to width.

[0060] The aspect ratio range can be obtained by measuring the aspect ratio of each polygonal zinc-iron alloy phase in a photographed phase structure image of the coating surface. In the above embodiments, the aspect ratio R of the polygonal zinc-iron alloy phase is in the range of 1 ≤ R < 2.

[0061] In the hot-dip galvanized steel sheet alloyed coatings described in the above embodiments, the proportion of polygonal zinc-iron alloy phases accounts for more than 95% of the total area of ​​the alloy phases. When the proportion of polygonal zinc-iron alloy phases with 1 ≤ R < 2 is greater than 95% of the total area of ​​the alloy phases on the coating surface, the coefficient of friction of the coating surface is moderate, which is beneficial to improving the deep-drawing performance of the steel sheet. The higher the proportion of polygonal zinc-iron alloy phases in the coating, the more likely it is to reduce the coefficient of friction of the coating surface, resulting in better stamping performance. Preferably, the proportion of polygonal zinc-iron alloy phases accounts for 100% of the total area of ​​the alloy phases, meaning that the surface of the coating is covered with polygonal zinc-iron alloy phases. The area ratio of the phase structure on the coating surface can be analyzed using existing commercial software; specifically, the area of ​​the polygonal zinc-iron alloy phases on one layer of the coating surface can be measured to determine its area ratio. More specifically, surface phase structure images can be captured using SEM and analyzed directly using commercial software. To ensure uniform alloying at different locations on the steel plate, at least three points need to be taken at the edge and center of the strip to determine its area ratio.

[0062] The area ratio of the zinc-iron alloy phase in the coating on the cross-section of the steel plate can also be detected. Specifically, this can be determined as follows: A 15*15mm sample is cut from the alloyed hot-dip galvanized steel plate along the rolling direction (the finished product rolling direction, i.e., the length direction), or a 15*15mm sample is cut from the alloyed hot-dip galvanized steel plate along the transverse direction. These samples are then embedded in epoxy resin, ground and polished, and then etched with a 0.05% nitric acid alcohol solution for 3–5 seconds. Finally, backscattered electron imaging analysis of the coating cross-section is performed using SEM to obtain the area ratio of the zinc-iron alloy phase in the cross-section coating.

[0063] Furthermore, to achieve excellent deep-drawing performance of the steel sheet and its coating, besides obtaining a near-polygonal δ-phase surface phase structure, the grain size of the polygonal δ-phase also affects the surface roughness and coefficient of friction of the alloyed coating. When the size of the polygonal zinc-iron alloy phase is less than or equal to 5 μm, the surface roughness of the coating will not be too high, thus affecting its formability. Preferably, when the size of the polygonal zinc-iron alloy phase is in the range of 2–3 μm (inclusive of the endpoints 2 μm and 3 μm), a lower coefficient of friction can be ensured on the coating surface, which is more conducive to improving the formability of the steel sheet, thereby improving its deep-drawing performance. Here, grain size refers to the average grain size. (Reference) Figure 6The average grain size is determined by measuring the longer side of the polygonal zinc-iron alloy phase (i.e., the length of the polygonal zinc-iron alloy phase in the aforementioned embodiments, which can also be considered as the major axis dimension of the polygonal zinc-iron alloy phase). The measurement method can be the same as the length measurement method in the aforementioned method for determining the aspect ratio of the polygonal zinc-iron alloy phase. More specifically, the average grain size can be determined by measuring the lengths of multiple polygonal zinc-iron alloy phases and calculating the average value.

[0064] Furthermore, at the interface between the alloyed hot-dip galvanized layer and the steel substrate, the average thickness of the zinc-iron alloy phase with an Fe content exceeding 30% is in the range of 500 nm to 1000 nm (inclusive of the endpoints 500 nm and 1000 nm). Here, the zinc-iron alloy phase is the Γ phase. By ensuring that the average thickness of the zinc-iron alloy phase with an Fe content exceeding 30% at the interface between the coating and the steel substrate is in the range of 500 nm to 1000 nm, the coating can possess both good deep-drawing performance and good anti-powdering properties.

[0065] The alloyed hot-dip galvanized coating contains 0.01–1.0% Al by mass. Specifically, steel sheet manufacturing involves an alloying process, which refers to adding other metallic elements to the steel sheet to create an alloy with desired properties, forming an alloyed coating. Diffusion typically occurs during alloying. Through hot-dip galvanizing, a Fe2Al5 barrier layer forms at the interface between the substrate and the coating. During subsequent heating in the alloying process, the Fe2Al5 barrier layer decomposes and diffuses into the coating. Therefore, the alloyed hot-dip galvanized coating contains not only Fe and Zn, but also a certain amount of Al, typically 0.01–1.0% by mass (inclusive of the endpoints 0.01% and 1.0%). When the Al content in the coating falls within this range, it indicates that the Zn-Fe alloying diffusion reaction in the coating is appropriate, and Al does not inhibit the rate of the Zn-Fe alloying reaction, resulting in sufficient alloying and improved deep-drawing performance.

[0066] Furthermore, the main component of the alloyed hot-dip galvanized layer in the above embodiments of the present invention is Zn. It is an alloyed coating formed by the diffusion of zinc and iron with Zn as the main component. The coating contains not only Zn, Fe, and Al, but may also contain one or more of Si, Mn, Ti, Nb, Cr, Mo, V, and B. These other elements, except for Zn, Fe, and Al, generally do not affect the deep drawing performance of the coating.

[0067] Furthermore, in the substrate of the alloyed hot-dip galvanized steel sheet of the above embodiments, by weight, it contains the following elements: C: 0.05-0.20%, Si: 0-1.0%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01%, with the remainder including Fe and unavoidable impurities. Preferably, taking into account the control of steel sheet performance and cost, by weight, the substrate of the alloyed hot-dip galvanized steel sheet of the present invention may also contain one or more of the following elements: Al: 0.1-1.0%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, Cr: 0.01-0.5%, Mo: 0.05-0.2%, V: 0.005-0.2%, B: 0.0001-0.10%.

[0068] The design principles of each chemical element in the alloyed hot-dip galvanized steel sheet of this invention are explained in detail below.

[0069] Carbon (C) is the most fundamental strengthening element in steel and also an austenite stabilizing element. This invention controls the C content in the steel substrate to be within the range of 0.05% to 0.20% by mass (inclusive of the endpoints 0.05% and 0.20%). At this level, the austenite content formed during critical zone annealing of the steel plate is sufficient, improving austenite stability and martensite hardenability, resulting in better strength, plasticity, and weldability of the material.

[0070] Si is a substitutional solid solution alloying element that promotes the enrichment of C in austenite, increasing the stability of austenite and thus improving the strength of the steel plate, and to some extent, its toughness. This invention comprehensively considers the mechanical properties of the steel plate and the surface quality control of the coating, controlling the Si content within the range of 0–1.0% (inclusive of the endpoints 0 and 1.0%). Further, the Si content can be controlled within the range of 0.1–0.8% (inclusive of the endpoints 0.1% and 0.8%). Even further, in the above embodiments of this invention, the Si content is controlled within the range of 0.1–0.45% (inclusive of the endpoints 0.1% and 0.45%). At this point, coarse iron-based carbides are not generated during the alloying process of the coating, the formability of the steel plate is not deteriorated, and the plating applicability of the substrate is guaranteed. Here, plating applicability indicates whether the steel plate is easy to coat; high plating applicability indicates that the steel plate is easy to coat; low plating applicability indicates that the steel plate is not easy to coat.

[0071] Mn can shift the C-curve (i.e., the isothermal transformation curve of supercooled austenite) of steel plates to the right, improving the hardenability of the steel plates. Mn can expand the austenite phase region, reduce Ac3 and Ac1, delay the pearlite phase transformation, and also improve the stability of austenite. Here, Ac3 refers to the actual phase transformation temperature when carbon steel is heated; Ac1 refers to the temperature at which austenite begins to form when steel is heated. Pearlite is a mechanical mixture composed of ferrite and cementite, with mechanical properties between ferrite and cementite, exhibiting high strength, moderate hardness, and a certain degree of plasticity. This invention controls the Mn content in the steel plate substrate to be above 1.5% by mass (including the endpoint value of 1.5%) and below 2.8% (including the endpoint value of 2.8%). At this level, the steel plate has good hardenability, strong strengthening effect, high tensile strength, and reduces the possibility of cracks appearing on the billet during continuous casting, preventing any impact on the weldability of the steel plate.

[0072] Polymer (P) significantly reduces the plasticity and toughness of steel, especially at low temperatures, leading to brittleness. Therefore, the P content must be strictly controlled. This invention controls the P content in the steel substrate to be below 0.02% (inclusive) by weight. At this level, the steel plate is less prone to brittleness at low temperatures.

[0073] Sulfur (S) is an impurity element in steel plates and needs to be maintained at a low level; the lower the S content, the better. S has a detrimental effect on the weldability of steel plates as well as their manufacturability during casting and hot rolling. Specifically, S reacts with Fe in the steel plate to form FeS, which in turn causes hot brittleness problems. In addition, S combines with Mn to form coarse MnS, reducing the ductility of the steel plate. This invention controls the S content in the steel plate substrate to be below 0.01% by mass (including 0.01%) to ensure good ductility of the steel plate.

[0074] Nitrogen (N) readily forms large nitrides in steel sheets, thereby deteriorating the ductility of the steel sheet. This invention controls the N content of the steel substrate to below 0.0100% by mass (including 0.0100%) to prevent severe damage to the ductility of the steel sheet.

[0075] When Al exists in solid solution, it increases the stacking fault energy of the steel plate, thereby inhibiting cementite precipitation and the transformation of the γ phase to martensite, and improving the stability of austenite. Furthermore, Al forms fine, dispersed, insoluble particles with C and N to pin grain boundaries, refining the grain size. This invention controls the Al content in the steel plate substrate within the range of 0.1% to 1.0% (including the endpoints of 0.1% and 1.0%), which provides austenite stability and prevents the formation of large amounts of oxide inclusions in the steel plate, thus preventing interference with continuous casting and reducing the possibility of nozzle blockage during the continuous casting process.

[0076] Ti strengthens the steel by combining with C and N to form Ti(C,N), TiN, and TiC precipitates, and improves the strength of the steel plate by inhibiting the growth of ferrite grains. This invention controls the Ti content of the steel plate substrate within the range of 0.005% to 0.1% by mass (inclusive of the endpoints 0.005% and 0.1%), which not only refines and strengthens the steel but also prevents excessively high content and large size of the aforementioned precipitates, thereby preventing deterioration of the steel plate's formability.

[0077] Similar to Ti, Nb can combine with C and N to form Nb(C,N), and can exert a strengthening effect through precipitates, inhibit ferrite grain growth, and improve the strength of the steel plate through grain refinement (i.e., refining grain size). In this invention, the Nb content in the steel plate substrate is controlled within the range of 0.005% to 0.1% by mass (inclusive of the endpoints 0.005% and 0.1%). At this range, the strengthening effect is significant, the amount of carbide precipitation is moderate, and the formability of the steel plate is good.

[0078] Cr is a ferrite-forming element that promotes the diffusion of carbon into austenite, thereby improving the stability of austenite and reducing the critical cooling rate during annealing. Therefore, Cr can replace some of C and / or Mn and be added to the chemical composition of the steel sheet. In this invention, the Cr content in the steel sheet substrate is controlled within the range of 0.01% to 0.5% by mass (inclusive of the endpoints 0.01% and 0.5%), ensuring that Cr has a significant effect on the steel sheet without compromising its ductility.

[0079] Mo plays a similar role to Cr. Mn can shift the C-curve (i.e., the isothermal transformation curve of supercooled austenite) of steel plates to the right, improving the hardenability of the steel. This invention allows for the appropriate addition of Mo. The Mo content in the steel plate substrate is controlled within the range of 0.05% to 0.2% by mass (inclusive). This significantly improves the strength of the steel plate without affecting the surface quality of the coating, while also preventing excessive Mo addition and thus avoiding excessive increases in manufacturing costs.

[0080] In hot-dip galvanized duplex steel, vanadium (V) mainly exists in the form of VC (vanadium monocarbide), which has a pinning effect, refining grains by pinning grain boundaries. It also disperses and precipitates in ferrite, thereby improving the strength and toughness of the steel plate. In this invention, the V content in the steel plate substrate is controlled within the range of 0.005% to 0.2% by mass (inclusive of the endpoints 0.005% and 0.2%). At this range, the strengthening effect of V on the steel plate is significant, the precipitation of carbonitrides (e.g., V(C,N)) is moderate, and the formability of the steel plate is good.

[0081] The main function of boron (B) is to improve the hardenability of steel. B tends to segregate at austenite grain boundaries, delaying the transformation of austenite to ferrite. Adding a small amount of B to the steel plate can have a significant effect. In this invention, the B content in the steel plate substrate is controlled by mass to be below 0.10% (inclusive), more preferably below 0.005% (inclusive), and simultaneously controlled to be above 0.0001% (inclusive). This ensures that B functions effectively while preventing excessive increases in steel plate strength, thus promoting good plasticity and improving the hardenability of the steel plate.

[0082] In the alloyed hot-dip galvanized steel sheets of the above embodiments, the tensile strength is ≥780 MPa, the elongation is ≥10%, and the coefficient of friction is <0.3. The coefficient of friction can be used to evaluate the deep-drawing performance of the steel sheet; the lower the coefficient of friction, the better the deep-drawing performance. Generally, a coefficient of friction not higher than 0.3 is considered to indicate good deep-drawing performance.

[0083] The coefficient of friction and the experimental method for its determination are described below. The coefficient of friction can be determined using the standard test method for ball friction. The standard test method for ball friction is described in detail below.

[0084] The coefficient of friction was measured using an FPR2200 testing machine. 60mm x 60mm samples were taken from the edge and center of the steel strip, respectively, and immersed in 550HN lubricating oil. After oiling, the samples were left to stand for at least 1 hour. Ball friction was tested using precision bearing steel balls with a diameter of 4.76mm and a hardness of HRC63. The test load was 30N, the test speed was 1rpm, and the test temperature was 60℃. At least 10 revolutions were used. The coefficient of friction was determined by first taking the average value of each of the 2-N revolutions (N being the actual number of revolutions), and then using the maximum value of the selected average value as the material's coefficient of friction.

[0085] Existing technologies for manufacturing steel sheets with good deep-drawing performance typically employ a high air-fuel ratio (specifically, an air-fuel ratio of 1 or higher, but not including 1) and a high dew point. Through in-depth research, the inventors discovered that this manufacturing method, when applied to low-silicon steel sheets, leads to over-alloying of the coating, making the coating extremely prone to peeling and resulting in very poor deep-drawing performance. The inventors have improved the manufacturing method for low-silicon steel by controlling the surface phase structure of the coating on the low-silicon steel sheet, thereby enhancing the deep-drawing performance of the manufactured steel sheet.

[0086] The present invention provides a method for manufacturing an alloyed hot-dip galvanized steel sheet, which is used to manufacture the alloyed hot-dip galvanized steel sheets of the above-mentioned embodiments, and includes the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing, and alloying. The inventors mainly improved the steps of continuous annealing, hot-dip galvanizing, and alloying. Specifically, in the preheating step, when the heating atmosphere is air and gas, the air-fuel ratio λ is controlled within the range of 0.80 ≤ λ ≤ 0.90, or when the heating atmosphere in the preheating step is a mixed gas of N2 and O2, the volume percentage V of O2 therein is controlled within the range of 0 < V ≤ 0.01%; in the soaking step, the dew point DP is controlled within the range of -50 to -30 °C; and in the alloying step, the alloying temperature is controlled within the range of 500 to 550 °C.

[0087] The deep drawing performance of the alloyed hot-dip galvanized steel sheet obtained by the above manufacturing method has been improved. After in-depth research on the alloyed hot-dip galvanized layer (hereinafter referred to as "coating") of the alloyed hot-dip galvanized steel sheet manufactured by the inventors, it is found that the average Fe content in the coating is within the range of 10 to 12% by mass, and the coating also contains a polygonal zinc-iron alloy phase. The proportion of the polygonal zinc-iron alloy phase accounts for more than 95% of the total area of the alloy phases in the coating, and the length-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1 ≤ R < 2. The inventors also conducted multiple repeated experiments and found that when 1 ≤ R < 2 is satisfied, the improvement of the deep drawing performance of the steel sheet is very obvious.

[0088] Among them, smelting, hot rolling, pickling, and cold rolling belong to the steel sheet casting method. The following specifically introduces these steps.

[0089] (1) Smelting: Smelt and continuously cast billets according to the determined steel sheet composition to carry out steel sheet casting.

[0090] (2) Hot rolling: Heat the billet to 1100 to 1250 °C and then control the rolling. The starting rolling temperature is 950 to 1150 °C, the final rolling temperature is 750 to 900 °C, and the thickness of the hot-rolled sheet is less than or equal to 20 mm.

[0091] (3) Pickling: Pickle to remove the scale generated during hot rolling.

[0092] (4) Cold rolling: Cold roll the steel coil to a thickness of less than 2.0 mm, and the cold rolling reduction ratio ≥ 35%.

[0093] Through the above steps, a steel sheet substrate can be manufactured. Table 1 lists the chemical compositions of the steel sheets cast in the present invention, including the mass percentages (%) of the chemical compositions of the steel sheets in the examples and comparative examples, and the rest is Fe.

[0094] Table 1 Mass percentages of chemical compositions of steel sheets in different examples and comparative examples

[0095] C Si Mn P S Al Nb Ti Cr Mo B V A 0.108 0.71 2.14 0.011 0.0004 0.90 / / 0.01 / / / B 0.106 0.19 2.03 0.011 0.0003 0.02 / 0.05 0.03 / / / C 0.135 0.58 2.34 0.010 0.0011 0.26 / 0.02 / 0.06 / / D 0.108 0.37 1.50 0.011 0.0007 / / / / 0.1 0.0017 0.04 E 0.189 0.35 2.49 0.011 0.0015 / 0.04 / / / / / F 0.158 0.34 2.80 0.009 0.0004 / / / 0.04 / / G 0.117 0.54 2.07 0.009 0.0004 / / 0.03 0.03 / / / H 0.055 0.12 1.78 0.009 0.0009 0.05 / / / / 0.0017 0.10 I 0.098 0.45 1.93 0.010 0.0010 / 0.09 / 0.03 / / 0.15 J 0.189 1.35 2.50 0.011 0.0015 / 0.04 / / / / / K 0.098 0.93 1.95 0.010 0.0010 / 0.09 / 0.03 / / 0.15

[0096] This invention improves the deep-drawing performance of steel plates by modifying the manufacturing method, adjusting the process, and controlling the structure and proportion of the alloy phase in the coating. The main improvements are made to the continuous annealing, hot-dip galvanizing, and alloying steps. The preferred steel plates are those with a Si content of 0–1.0% by mass. After cold rolling to obtain the steel plate substrate, continuous annealing, hot-dip galvanizing, and alloying steps are performed sequentially.

[0097] (5) Continuous annealing: In the continuous annealing step, preferably, a continuous hot annealing galvanizing line with a preheating section, a soaking section and a plating section is used, so that the steel strip passes through the preheating section and the soaking section respectively, and the annealing process ends before reaching the plating section, and then the plating process is carried out in the plating section.

[0098] (a) Preheating: Continuous annealing includes a preheating section, which specifically involves heat-treating the rolled hard plate in a direct-fired heating furnace equipped with a direct-fired burner, controlling the air-fuel ratio or the oxygen volume content in the furnace, and heating the steel plate to a range of 600 to 750°C (inclusive of the endpoints 600°C and 750°C).

[0099] In this heating furnace, an atmosphere of air and fuel gas mixture can be selected, in which case the air-fuel ratio needs to be controlled; alternatively, an atmosphere of nitrogen and oxygen mixture can be selected, in which case the oxygen volume content needs to be controlled. Only one of these two options can be chosen; they cannot be used simultaneously. These two options are described below.

[0100] When the atmosphere in the heating furnace is air and fuel gas, the air-fuel ratio in the heating furnace is controlled within the range of 0.80 to 0.90 (inclusive of the endpoint values ​​of 0.80 and 0.90).

[0101] Specifically, the air-fuel ratio refers to the volume of air contained in a unit volume of gas mixture (m³). 3The air-fuel ratio is the ratio of the volume of air to the volume of air theoretically required for complete combustion of the fuel gas (fuel gas) contained in a unit volume of gas mixture. The fuel gas can be, for example, natural gas, methane, or similar gases. When the air-fuel ratio is in the range of 0.80 to 0.90, a pre-oxidized layer of appropriate thickness can be formed on the steel plate surface during the preheating process. In the reducing atmosphere of the soaking zone, the pre-oxidized layer forms a reduced iron layer, which is beneficial for improving the plating applicability of the steel plate surface and ensures sufficient diffusion of Zn-Fe during subsequent alloying. However, since the formation of the δ phase requires a high temperature, a high alloying temperature is needed to obtain a δ phase with a volume percentage exceeding 95%. If the air-fuel ratio is too high, at the subsequent high-temperature alloying temperature, the ζ phase formed at the interface during hot-dip plating will diffuse too fully at the high alloying temperature, forming the Γ phase, resulting in over-alloying of the coating. Therefore, the manufacturing method of the present invention requires the selection of a weaker pre-oxidation atmosphere in the preheating section, so as to utilize the partial external oxides formed on the interface to suppress the diffusion of Zn-Fe to form the ζ phase in the early stage of immersion plating, thus controlling the upper limit of the air-fuel ratio to 0.90.

[0102] When the atmosphere in the heating furnace is an oxidizing atmosphere achieved by using a mixture of N2 and O2, as mentioned above, since the manufacturing method of the present invention requires a weaker oxidizing atmosphere in the preheating section, the volume percentage of O2 is controlled within the range of 0-0.01% (excluding the endpoint value 0, including the endpoint value 0.01%). More preferably, the volume percentage of O2 is within the range of 0-0.005% (excluding the endpoint value 0, including the endpoint value 0.005%), which can further ensure that the partial external oxides formed on the interface are within a suitable range.

[0103] In addition, the heating temperature of the steel plate in the preheating section is 600-750℃, which can form a pre-oxidized layer of appropriate thickness, so that the steel plate can form an effective pure iron layer before galvanizing to improve its plating and alloying ability, and can also prevent the formation of an excessively thick oxide layer, prevent the formation of external oxidation of Si or Mn oxides, and avoid a decrease in the adhesion of the coating.

[0104] Furthermore, controlling the dew point of the preheating section to ≥-40℃ is beneficial for forming the required pre-oxidized layer thickness on the steel plate surface.

[0105] (b) Soaking: In the above embodiments, the continuous annealing step further includes a soaking section. Specifically, the steel plate is heated and held at a soaking temperature of 700 to 900°C in an atmosphere with a hydrogen concentration of 3 to 25% by volume, a dew point DP of -50 to -30°C, and the balance being N2 and unavoidable impurities.

[0106] When the H2 concentration is in the range of 3–25 vol% (inclusive of the endpoints 3 vol% and 25 vol%), there is sufficient hydrogen to suppress the amount of Fe oxidation without excessive cost. Preferably, the H2 content is controlled in the range of 3–10 vol% (inclusive of the endpoints 3 vol% and 10 vol%), as a hydrogen concentration within this range is sufficient to suppress the amount of Fe oxidation.

[0107] The dew point (DP) of the soaking zone is in the range of -50 to -30°C, preferably -40 to -30°C (inclusive of the endpoints -40°C and -30°C). To obtain an alloyed coating with over 95% δ-phase on the surface, it is necessary to utilize some external oxides at the interface between the steel sheet and the coating to suppress the formation of the ζ-phase during the initial immersion plating stage. Controlling the dew point (DP) within the range of -50 to -30°C satisfies the requirement of utilizing some external oxides at the interface between the reduced iron layer and the steel sheet to suppress the formation of the ζ-phase during the initial immersion plating stage. When the dew point (DP) is controlled within the range of -40 to -30°C, the steel sheet, while possessing good plating applicability, can further suppress the formation of the ζ-phase during the initial immersion plating stage, which is more conducive to controlling the alloy phase of the coating and improving deep-drawing performance.

[0108] By comprehensively controlling the air-fuel ratio in the preheating section and the dew point in the soaking section during the continuous annealing stage, the activity of the steel plate surface can be regulated, which is conducive to the formation of fine and uniform polygonal zinc-iron alloy phases. This is beneficial to the formation of an alloyed hot-dip galvanized layer with an average Fe content of 10-12% by mass and an aspect ratio of 1≤R<2 under the alloying temperature conditions of 505-550℃ in the subsequent alloying step.

[0109] (6) Hot-dip galvanizing: In the above embodiments, a hot-dip galvanizing step is required after the continuous annealing step. Specifically, the steel plate needs to be cooled and then immersed in a zinc bath at a temperature of 440 to 465°C (inclusive of the endpoints 440°C and 465°C) for hot-dip galvanizing.

[0110] When the temperature of the zinc bath is controlled within the range of 440 to 465°C (inclusive of the endpoints 440°C and 465°C), the viscosity of the zinc bath is moderate, which is beneficial for controlling the thickness of the coating and will not affect the appearance and alloying of the steel plate. In addition, the zinc bath is not easy to evaporate, which can reduce the possibility of vaporized Zn adhering to the grate, thereby minimizing the risk of zinc ash falling off. This can further prevent zinc ash from adhering to the surface of the strip steel after falling off, thus preventing defects in the strip steel.

[0111] (7) Alloying: In the above embodiments, an alloying step is performed after the hot-dip galvanizing step. Specifically, the steel plate is placed in an alloying furnace for alloying. The alloying temperature range is controlled between 500 and 550°C (inclusive of the endpoints 500°C and 550°C). Then, it is cooled by air mist, water mist, or other methods. The average cooling rate is not less than 15°C / s, preferably 20 to 40°C / s. After that, it is air-cooled, water-quenched, leveled, tensioned, post-treated, trimmed, oiled, slit, and coiled to obtain the desired alloyed hot-dip galvanized steel plate with a polygonal zinc-iron alloy phase. The alloying time is within 10 to 30 seconds.

[0112] At this point, it can be ensured that Zn and Fe can interdiffusion to obtain an alloyed coating with more than 95% δ phase, that is, the proportion of polygonal zinc-iron alloy phase accounts for more than 95% of the total area of ​​alloy phase in the alloyed hot-dip galvanized layer, ensuring that the coating is fully alloyed without over-alloying. The area ratio of the polygonal phase is measured by taking surface phase structure images using SEM, and the results can be directly analyzed using commercial software. To ensure uniform alloying at different locations on the steel plate, at least three points each should be taken at the edge and middle of the strip to measure its area ratio.

[0113] Preferably, the alloying temperature of the present invention is controlled within the range of 505 to 550°C. More preferably, the alloying temperature of the present invention is controlled within the range of 505 to 520°C (inclusive of the endpoints 505°C and 520°C). At this temperature, it helps to make the proportion of the polygonal zinc-iron alloy phase closer to 100% of the total area of ​​the alloy phase in the alloyed hot-dip galvanized layer, which is more conducive to improving deep-drawing performance.

[0114] Furthermore, the alloying time varies on different continuous hot-dip galvanizing production lines. The appropriate alloying time can be selected by comprehensively considering the length of the alloying furnace, the thickness of the strip, and the production speed of the strip. The manufacturing method of this invention controls the alloying time within the range of 10–30 seconds (inclusive of the endpoints 10 and 30 seconds), preventing over-alloying and obtaining alloyed hot-dip galvanized steel sheets with a surface polygonal zinc-iron alloy phase ratio exceeding 95%. Through comprehensive control of alloying temperature and alloying time, the aspect ratio R of the columnar zinc-iron alloy phase is also kept within the range of 1 ≤ R < 2, ensuring a fine and uniform polygonal zinc-iron alloy phase on the surface, which is beneficial for guaranteeing the deep-drawing performance of the steel sheet.

[0115] By using the manufacturing methods of the above embodiments, through precisely controlling the air-fuel ratio in the preheating stage, the dew point in the soaking stage, and the alloying temperature in the alloying step during the continuous annealing process of the steel plate, it is beneficial for the low-silicon steel plate to be coated and control the surface phase structure in the coating, which is conducive to forming a polygonal zinc-iron alloy phase. At the same time, ensuring that the proportion of the polygonal zinc-iron alloy phase is above 95%, thereby improving the deep drawing performance of the steel plate. Moreover, for the steel plate manufactured by using the manufacturing method of the above embodiments of the present invention, the surface of the alloyed steel plate has a uniform polygonal zinc-iron alloy phase, reducing the surface friction coefficient of the coating. In addition, no Γ phase is formed on the interface between the steel plate coating and the steel plate substrate, and it has excellent anti-powdering performance, low cost, and good application prospects.

[0116] By using the manufacturing methods in the above embodiments for experiments, various different steel plates (including comparative examples and examples) can be obtained, and their tensile strength, elongation, and friction coefficient are measured. The specific data are shown in Table 2. Among them, the friction coefficient is evaluated by the aforementioned ball friction standard test method.

[0117] Referring to Table 2, take Example 1 and Comparative Example 1 as examples for illustration. Refer to Figure 2 and Figure 3 , Figure 2 is the SEM image of the coating surface morphology of Example 1 of the present invention, Figure 3 is the SEM image of the coating cross-section morphology of Example 1 of the present invention. In its coating surface phase structure, the proportion of the polygonal zinc-iron alloy phase is 100%, and the friction coefficient is only 0.15, indicating good deep drawing performance. While in the comparative example, Figure 4 shows the SEM image of the coating surface morphology of Comparative Example 1 of the present invention, Figure 5 shows the SEM image of the coating cross-section morphology of Comparative Example 1 of the present invention. It can be observed that the coating surface phase structure of the comparative example is uneven, with a mixture of fine granular structures and long strip structures, and the particle sizes are also uneven, resulting in poor deep drawing performance.

[0118] It can be seen from Examples 1-15 that in the manufacturing method of the present invention, the average Fe content in the alloyed hot-dip galvanized coating of the steel plate is within the range of 10-12% by mass, and it contains a polygonal zinc-iron alloy phase. The length-width ratio R of the polygonal zinc-iron alloy phase all satisfies: 1≤R<2. In the adopted manufacturing method, the steel plate substrate with a Si content within the range of 0-1.0% by mass is targeted. When the heating atmosphere in the preheating section is a mixed gas of air and gas, the air-fuel ratio λ is controlled within the range of 0.80≤λ≤0.90. When the heating atmosphere adopted in the preheating stage is an oxidizing atmosphere formed by a mixed gas of N2 and O2, the volume percentage V of oxygen is controlled within the range of 0<V≤0.01%; the dew point DP in the soaking stage is all within the range of -50~-30℃; the alloying temperature is all within the range of 500~550℃.

[0119] The manufacturing method of this invention, applied to low-silicon steel sheets and their coatings, ensures that the proportion of polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer exceeds 95% of the total area of ​​the alloy phases. The average grain size of these polygonal zinc-iron alloy phases is less than 5 μm, and the coefficient of friction is below 0.3, resulting in good deep-drawing performance. Some embodiments further satisfy the requirement that the average grain size of the polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer is in the range of 2–3 μm. In this case, the coefficient of friction can be further reduced to below 0.2, further improving the deep-drawing performance of the steel sheet.

[0120] Table 2 Comparison of experimental methods and measurement results between the examples and comparative examples

[0121]

[0122] In the examples and comparative examples where the air-fuel ratio λ is represented by " / ", the oxidizing atmosphere used in these examples or comparative examples is a mixture of N2 and O2 gas, and no fuel gas is used. In the examples and comparative examples where the oxygen volume percentage is represented by " / ", the mixture of air and fuel gas is used. Grain size represented by " / " indicates over-alloying or failure to obtain polygonal phases. Compositions A to K correspond to the chemical compositions of the steel plates in Table 3.

[0123] Regarding Comparative Example 1, with an air-fuel ratio above 0.90 in the preheating section and a dew point (DP) above -30℃ in the soaking section, the resulting coating had an excessively high Fe content, indicating over-alloying. The coating surface exhibited only a very small amount of polygonal zinc-iron alloy phase, approximately 5%. Although the coefficient of friction was low, the average thickness of the zinc-iron alloy phase (i.e., the interfacial Γ phase), with a Fe content exceeding 30% by mass at the interface between the coating and the substrate, was excessively thick, exceeding 1000 nm. A smaller thickness of the interfacial Γ phase results in better anti-powdering performance. Therefore, Comparative Example 1 exhibited poor anti-powdering performance, leading to severe powdering of the coating during stamping, which did not meet the application requirements.

[0124] Regarding Comparative Example 2, the air-fuel ratio in the preheating section was above 0.90, and the dew point DP in the soaking section was above -30℃. The obtained zinc-iron alloy phase had an aspect ratio greater than 2. The coating surface did not form a polygonal zinc-iron alloy phase with an aspect ratio R in the range of 1≤R<2. Furthermore, the Fe content in the obtained coating exceeded 12%, resulting in an excessively high coefficient of friction and poor deep-drawing performance.

[0125] Regarding Comparative Example 3, the air-fuel ratio in the preheating section was below 0.80, the alloying temperature was below 500℃, the coating was not fully alloyed, the coating surface did not form a polygonal zinc-iron alloy phase with an aspect ratio R in the range of 1≤R<2, the Fe content in the coating was below 10%, the friction coefficient was too high, and the deep drawing performance was poor.

[0126] Regarding Comparative Example 4, its soaking zone dew point DP is below -50℃, the aspect ratio of the generated alloy phase is too large, the polygonal zinc-iron alloy phase with aspect ratio R in the range of 1≤R<2 is not formed in the coating, the Fe content in the coating is too low, the friction coefficient is too large, and the deep drawing performance is poor.

[0127] Regarding Comparative Example 5, its alloying temperature was too high, exceeding 550℃, resulting in an excessively large aspect ratio of the generated alloy phase. The coating did not yield a polygonal zinc-iron alloy phase with an aspect ratio R within the range of 1≤R<2. The Fe content in the coating was too high, leading to over-alloying. Although the coefficient of friction was relatively low, the average thickness of the zinc-iron alloy phase (i.e., the interface Γ phase) with a Fe content exceeding 30% by mass at the interface between the coating and the substrate was too thick, exceeding 1000nm. Its anti-powdering performance was poor, and the coating powdered severely during stamping, failing to meet the usage requirements.

[0128] Regarding Comparative Example 6, the Si content in the steel plate substrate used was 1.35% by mass, which is high silicon steel. The air-fuel ratio in the preheating section of the process was above 0.90. The Fe content in the obtained coating was too high, resulting in over-alloying. The polygonal zinc-iron alloy phase with an aspect ratio R in the range of 1≤R<2 was not obtained, resulting in a large coefficient of friction and poor deep drawing performance.

[0129] Regarding Comparative Example 7, the alloying temperature during its alloying stage was below 500°C. Even using the process parameters of this invention, the Fe content in the obtained coating was still too low, failing to produce a polygonal zinc-iron alloy phase with an aspect ratio R in the range of 1 ≤ R < 2, resulting in under-alloying, an excessively high coefficient of friction, and poor deep-drawing performance.

[0130] In summary, the present invention provides a method for manufacturing alloyed hot-dip galvanized steel sheets. By controlling the atmosphere in the preheating and soaking stages of continuous annealing, specifically controlling the air-fuel ratio, dew point (DP), and hydrogen concentration, and also controlling the alloying temperature during the alloying stage, the method enables the alloyed hot-dip galvanized steel sheets to be manufactured using low-Si content steel sheets as the base material. This allows for control of the Fe content in the coating within a suitable range, as well as the surface phase structure of the coating and the proportion of polygonal zinc-iron alloy phases. Consequently, the method improves the friction coefficient of the steel sheet, enhances its deep-drawing performance, and facilitates wider applications.

[0131] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. An alloyed hot dip galvanized steel sheet characterized by, The alloyed hot-dip galvanized steel sheet has a substrate containing 0-1.0% by mass of Si, an average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is 10-12% by mass, the surface phase structure of the alloyed hot-dip galvanized layer contains polygonal zinc-iron alloy phases, and the length-width ratio R of the polygonal zinc-iron alloy phases satisfies 1≤R<2.

2. The galvannealed steel sheet according to claim 1, characterized in that, The polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer are δ phases.

3. The galvannealed steel sheet according to claim 1, characterized in that, The average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is in the range of 10.5-11.5% by mass.

4. The galvannealed steel sheet according to claim 1, characterized in that, The proportion of the polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer accounts for 95% or more of the total area of alloy phases in the alloyed hot-dip galvanized layer.

5. The galvannealed steel sheet according to claim 4, characterized in that, The proportion of the polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer accounts for 100% of the total area of alloy phases in the alloyed hot-dip galvanized layer.

6. The galvannealed steel sheet according to claim 1, characterized in that, The average grain size of the polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer is less than or equal to 5 μm.

7. The galvannealed steel sheet according to claim 6, characterized in that, The average grain size of the polygonal zinc-iron alloy phases in the alloyed hot-dip galvanized layer is in the range of 2-3 μm.

8. The galvannealed steel sheet according to claim 1, characterized in that, The average thickness of the zinc-iron alloy phases with Fe content exceeding 30% at the interface between the alloyed hot-dip galvanized layer and the substrate is in the range of 500-1000 nm.

9. The galvannealed steel sheet according to claim 1, characterized in that, The alloyed hot-dip galvanized layer further contains 0.01-1.0% by mass of Al.

10. The galvannealed steel sheet according to claim 1, characterized in that, The substrate of the alloyed hot-dip galvanized steel sheet further contains, by mass, C: 0.05-0.20%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01%, and the balance of Fe and inevitable impurities.

11. The galvannealed steel sheet according to claim 10, characterized in that, The substrate of the alloyed hot-dip galvanized steel sheet further contains, by mass, one or more of Al: 0.1-1.0%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, Cr: 0.01-0.5%, Mo: 0.05-0.2%, V: 0.005-0.2%, and B: 0.0001-0.10%.

12. Galvannealed steel sheet according to any one of claims 1 to 11, characterized in that, The alloyed hot-dip galvanized steel sheet has a tensile strength of ≥780 MPa, an elongation of ≥10%, and a friction coefficient of <0.

3.

13. A manufacturing method for manufacturing the galvannealed steel sheet according to any one of claims 1 to 11, characterized in that, The method comprises a continuous annealing step and an alloying step for a substrate of an alloyed hot-dip galvanized steel sheet containing 0-1.0% by mass of Si; the continuous annealing step comprises a preheating step and a soaking step, wherein, in the preheating step, the air-fuel ratio λ is controlled in the range of 0.80≤λ≤0.90 when air and fuel gas are used as the heating atmosphere, or the volume percentage V of O2 in the mixed gas of N2 and O2 is controlled in the range of 0<V≤0.01% when the mixed gas of N2 and O2 is used as the heating atmosphere; in the soaking step, the dew point DP is controlled in the range of -50 to -30 ℃; and in the alloying step, the alloying temperature is controlled in the range of 500-550 ℃.

14. The method of producing an alloyed hot-dip galvanized steel sheet according to claim 13, characterized by, The manufacturing method comprises the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing, and alloying; The continuous annealing step comprises: Preheating: the cold-rolled alloyed hot-dip galvanized steel sheet is heat treated in a direct-fired heating type heating furnace with direct-fired burners, the heating atmosphere of which is controlled to have an air-fuel ratio λ in the range of 0.80≤λ≤0.90 when air and fuel gas are used, and the alloyed hot-dip galvanized steel sheet is heated to a temperature in the range of 600-750℃; or the O2 volume percentage V in the mixed gas of N2 and O2 is controlled to be in the range of 0 Soaking: the preheated alloyed hot-dip galvanized steel sheet is heated and held at a soaking temperature in the range of 700-900℃ in an atmosphere in which the hydrogen concentration is limited to 3-25 vol%, the dew point DP is controlled to be in the range of -50 to -30℃, and the balance is N2 and unavoidable impurities; Hot-dip galvanizing step: the continuously annealed alloyed hot-dip galvanized steel sheet is cooled and then put into a zinc bath at a temperature in the range of 440-465℃ for hot-dip galvanizing; Alloying step: the hot-dip galvanized alloyed hot-dip galvanized steel sheet is put into an alloying furnace for alloying at a temperature in the range of 505-550℃, and then cooled and coiled to obtain the desired alloyed hot-dip galvanized steel sheet.

15. The method of producing an alloyed hot-dip galvanized steel sheet according to claim 13, characterized by, In the preheating step, when the oxidizing atmosphere used for heating is a mixed gas of N2 and O2, the O2 volume percentage is in the range of 0-0.005%.

16. The method of producing an alloyed hot-dipped galvanized steel sheet according to claim 13, characterized by, In the preheating step, the dew point DP is controlled to be ≥-40℃.

17. The method of producing an alloyed hot-dipped galvanized steel sheet according to claim 13, characterized by, In the soaking step, the dew point DP is controlled to be in the range of -40 to -30℃.

18. The method of producing an alloyed hot-dipped galvanized steel sheet according to claim 13, characterized by, In the soaking step, the hydrogen concentration is controlled to be in the range of 3-10%.

19. The method of producing an alloyed hot-dipped galvanized steel sheet according to claim 13, characterized by, In the alloying step, the alloying temperature is controlled to be in the range of 505-520℃.

20. The method of producing an alloyed hot-dipped galvanized steel sheet according to claim 13, characterized by, In the alloying step, the alloying time is controlled to be in the range of 10-30s.

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