Alloyed hot-dip galvanized steel plate and manufacturing method therefor

Abstract

Disclosed in the present invention is an alloyed hot-dip galvanized steel plate. A substrate of the alloyed hot-dip galvanized steel plate 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 plate is 10-12% by mass. A surface phase structure composition of the alloyed hot-dip galvanized layer contains a polygonal zinc-iron alloy phase, and the length-to-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1≤R<2. The alloyed hot-dip galvanized steel plate of the present invention has good deep-drawing performance. Further disclosed in the present invention is a manufacturing method for the alloyed hot-dip galvanized steel plate, which method is applicable to a low-silicon steel plate. By controlling the air-fuel ratio in a preheating stage and the dew point (DP) in a uniform heating stage in a continuous annealing step and the alloying temperature in an alloying step, the alloy phase structure of the steel plate coating is controlled, so as to improve the deep-drawing performance of the steel plate.

Description

Alloyed hot-dip galvanized steel sheet and method for manufacturing alloyed hot-dip galvanized steel sheet TECHNICAL FIELD

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

[0002] The automobile industry requires the use of higher strength steel sheets for the purpose of weight reduction and safety. High-strength dual-phase steel has good mechanical properties and use properties, and can be effectively applied to the production and manufacture 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 manufacture 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 sheets 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 sheet 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 needs to be matched with a high alloying temperature of 500℃ or above.

[0004] For example, patent document CN104040001B discloses an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance and paint thickness adhesion. The steel sheet has an alloyed hot-dip galvanized layer containing 7-15% Fe by mass, and contains at least 0.015% by mass of precipitates containing one or more than two selected from Nb, V, Mo and W 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 sheet 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 to 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 to 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 generally describes its conventional 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 the increasing requirement of corrosion resistance of automobiles, alloyed hot-dip galvanized steel sheets are widely used. After hot-dip galvanizing, the coating is heated to an alloying temperature, so that Fe of the substrate diffuses into the coating to form different zinc-iron alloy phases. In the zinc-iron diffusion of the alloyed coating, the zinc-iron alloy phases in the coating form Γ, Γ1, δ and ζ phases according to the Fe content. Compared with pure zinc coating, the zinc-iron alloy phases have better appearance and corrosion resistance. In the use of automobile sheets, most of them are used under the conditions of severe bending processing (flanging) and hole expansion processing, but the Zn-Fe alloy phases generated in the alloying process are harder and more brittle than pure zinc phases, and are easy to be damaged in the stamping process. There are mainly two forms of damage of the alloyed coating in the deformation process: the granular peeling is called powdering, and the flaky peeling is called peeling. Powdering is due to the internal failure of the coating to form granular particles, and the particle size is generally smaller than the thickness of the coating, which is generally caused by the formation of thick Γ phase at the interface between the substrate and the coating. Peeling is due to the failure of the adhesion between the coating and the substrate interface to form flaky particles, and the particle size is generally similar to or greater than the thickness of the coating, which is generally caused by the formation of too thick ζ phase on the surface of the coating, which increases the friction of the coating surface. Peeling causes the coating to peel off from the substrate, reduces the corrosion resistance of the coating, and causes the base material to corrode and rust early.

[0009] Therefore, it is necessary to deeply study the coating structure of low-silicon steel sheets, so that the low-silicon steel sheets with lower cost have the required tensile strength and elongation, and also have good deep drawing performance. SUMMARY

[0010] The present application provides an alloyed hot-dip galvanized steel sheet to solve the above technical problems.

[0011] In a first aspect, embodiments of the present application disclose an alloyed hot-dip galvanized steel sheet, the substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% of Si by mass, the 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.

[0012] By adopting the above technical scheme, the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet of the present application has an average Fe content of 10-12% by mass, and the alloyed hot-dip galvanized layer also contains polygonal zinc-iron alloy phases, so that the deep drawing performance is good.

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

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

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

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

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

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

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

[0020] Optionally, the alloyed hot-dip galvanizing layer further contains 0.01-1.0% of Al by mass.

[0021] Optionally, the substrate of the alloyed hot-dip galvanizing steel sheet further contains C: 0.05-0.20%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01% by mass, and the balance contains Fe and inevitable impurities.

[0022] Optionally, the substrate of the alloyed hot-dip galvanizing steel sheet further contains 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%, B: 0.0001-0.10% by mass.

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

[0024] In a second aspect, the present application also discloses a manufacturing method of an alloyed hot-dip galvanized steel sheet, comprising a continuous annealing step and an alloying step of a substrate of the alloyed hot-dip galvanized steel sheet containing 0-1.0% of Si by mass; the continuous annealing step comprises a preheating step and a soaking step, wherein, in the preheating step, when the heating atmosphere adopts air and fuel gas, the air-fuel ratio λ is controlled to be in the range of 0.80≤λ≤0.90, or in the preheating step, when the heating atmosphere adopts a mixed gas of N2 and O2, the volume percentage V of O2 in the mixed gas is controlled to be in the range of 0

[0025] 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 in the continuous annealing step in the manufacturing process of the steel sheet, the surface phase structure in the plated layer of the low-silicon steel sheet is controlled, and the polygonal zinc-iron alloy phase is formed, so that the deep drawing performance of the steel sheet is improved.

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

[0027] The continuous annealing step comprises:

[0028] Preheating: the alloyed hot-dip galvanized steel sheet after cold rolling is heat-treated in a direct-fired heating furnace with a direct-fired burner, when the heating atmosphere in the direct-fired heating furnace adopts air and fuel gas, the air-fuel ratio λ is controlled to be in the range of 0.80≤λ≤0.90, and the alloyed hot-dip galvanized steel sheet is heated to a temperature in the range of 600-750℃; or when the heating atmosphere in the direct-fired heating furnace adopts a mixed gas of N2 and O2, the volume percentage V of O2 in the mixed gas is controlled to be in the range of 0

[0029] Soaking: the alloyed hot-dip galvanized steel sheet after preheating is heated and kept at a soaking temperature of 700-900℃ in a hydrogen atmosphere with a hydrogen concentration limited to 3-25% by volume and a dew point DP controlled to be in the range of -50--30℃, and the rest is N2 and an unavoidable impurity atmosphere, and the keeping time can be 5-30s;

[0030] The hot-dip galvanizing step comprises: cooling the alloyed hot-dip galvanized steel sheet after the continuous annealing, and then putting it into a zinc liquid in the range of 440-465℃ for hot-dip plating;

[0031] The alloying step comprises: putting the hot-dip galvanizing alloyed hot-dip galvanizing steel plate into an alloying furnace for alloying, controlling the alloying temperature in the range of 505-550℃, and then cooling and coiling to obtain the required alloyed hot-dip galvanizing steel plate.

[0032] Optionally, the oxidizing atmosphere heated in the preheating step is a mixed gas of N2+O2, wherein the volume percentage of O2 is in the range of 0-0.005%.

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

[0034] Optionally, the dew point DP in the preheating step is controlled to be ≥-40℃.

[0035] Optionally, the hydrogen concentration in the preheating step is controlled in the range of 3-10%.

[0036] Optionally, the alloying temperature in the alloying step is controlled in the range of 505-520℃.

[0037] Optionally, the alloying time in the alloying step is controlled in the range of 10-30s. BRIEF DESCRIPTION OF DRAWINGS

[0038] Fig. 1 shows a schematic diagram of the cross-sectional morphology of the coating of the steel plate of the present application; in the figure, 1 represents the steel plate substrate, 2 represents the coating main body, and 3 represents the coating surface layer.

[0039] Fig. 2 shows an SEM image of the surface morphology of the coating of Example 1 of the present application.

[0040] Fig. 3 shows an SEM image of the cross-sectional morphology of the coating of Example 1 of the present application.

[0041] Fig. 4 shows an SEM image of the surface morphology of the coating of Comparative Example 1 of the present application.

[0042] Fig. 5 shows an SEM image of the cross-sectional morphology of the coating of Comparative Example 1 of the present application.

[0043] Fig. 6 shows a schematic diagram of the process of determining the aspect ratio of the phase structure of the coating. DETAILED DESCRIPTION

[0044] 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.

[0045] 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.

[0046] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0047] 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.

[0048] 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.

[0049] According to the known prior art, it is not clear how to optimize the structure of the low silicon steel sheet coating to improve its deep drawing performance. The inventors have found through intensive research that the deep drawing performance (i.e. stamping performance) of a hot-dip galvanizing alloyed coating mainly depends on the surface phase structure of the coating. For complex deep drawing parts, poor control of the surface phase structure of the alloyed coating will not only cause the coating to peel off, but also cause the steel sheet to crack during stamping. The inventors have found through extensive experimental research that if the deep drawing performance of the alloyed coating is to be improved, the key point is to control the formation of a fine granular full delta phase structure, i.e. polygonal zinc-iron alloy phase, on the surface of the coating 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 better. The delta phase (i.e. polygonal zinc-iron alloy phase) has a higher hardness and can reduce the friction coefficient of the coating during stamping, thereby improving the stamping performance of the steel sheet.

[0050] The present application provides a kind of alloyed hot-dip galvanized steel sheet (GA steel sheet), more specifically low silicon steel sheet, which has a reasonably controlled Fe content of the alloyed hot-dip galvanized layer (hereinafter referred to as "coating") and the phase structure of the coating, so that the steel sheet has better deep drawing performance. Figure 1 shows a schematic diagram of the cross-sectional morphology of the coating of the steel sheet of the present application, which includes a steel sheet substrate 1, a coating main body 2, and a coating surface layer 3. Figure 3 is a cross-sectional morphology diagram of the actual coating of the steel sheet of the present embodiment.

[0051] The substrate of the alloyed hot-dip galvanized steel sheet provided by the present application contains 0-1.0% Si (including the end point values 0 and 1.0%) by mass, i.e. the steel sheet can contain no Si element or contain no more than 1.0% Si element, which belongs to a low silicon steel. The present application optimizes the surface phase structure of the coating of the low silicon steel sheet, improves its deep drawing performance while avoiding cost increase.

[0052] In the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet provided by the present application, it also contains an average Fe content of 10-12% by mass (including the end point values 10% and 12%), preferably an average Fe content of 10.5-11.5% by mass (including the end point values 10.5% and 11.5%). The average Fe content is the average Fe content in the coating, and the specific determination method of the average Fe content can use EDS to take the average of more than 5 points.

[0053] When the average Fe content in the coating is in the range of 10-12%, the coating surface is not prone to form thick columnar ζ phase, the Fe content in the coating is moderate, and the coating usually does not adhere to the mold. At this time, when the steel plate is stamped, the friction between the steel plate and the mold is moderate, and the coating at the interface will not be easily peeled off, which is beneficial to the processing of the steel plate. In addition, thick Γ phase will not be formed at the interface between the steel plate substrate and the coating, which will not cause the steel plate to be hard and brittle, which is beneficial to the control of the phase structure of the coating, and will not cause serious powdering during stamping, which will not have a large impact on the performance of the steel plate. Preferably, when the average Fe content in the coating is in the range of 10.5-11.5%, the fine polygonal zinc-iron alloy phase in the coating can be further ensured, the possibility of forming thick Γ phase at the interface between the coating and the substrate is reduced, the stamping performance of the steel plate is improved, and the anti-powdering performance is also good.

[0054] The surface phase structure of the alloyed hot-dip galvanized layer provided by the present application also contains a polygonal zinc-iron alloy phase (see Figure 2), and the length-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1≤R<2. In Figure 2, the polygonal zinc-iron alloy phase represents that the phase structure of the zinc-iron alloy phase is in the form of relatively fine particles, and it can be observed from the SEM (scanning electron microscope) image that the particles in the alloy phase have a relatively long side (length) and a relatively short side (width). Specifically, the longest distance between the end faces of the two ends of the polygonal zinc-iron alloy phase is the length, and the longest distance of the polygonal zinc-iron alloy phase perpendicular to the length direction is the width. Here, the polygonal shape refers to the cross section obtained by assuming the δ phase as a two-dimensional morphology parallel to the plate surface direction, and the polygonal shape can include quadrilaterals, pentagons, hexagons and other shapes, and can also be a polygon with more sides. The length-width ratio R of the polygonal zinc-iron alloy phase is greater than or equal to 1 and less than 2.

[0055] When the length-width 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 produced after alloying on the surface of the coating is moderate, which meets the requirements, and the coating will not be easily peeled off, which is beneficial to the improvement of the deep drawing performance of the coating. In some embodiments, R satisfies: 1≤R<1.7.

[0056] The determination method of the length-width ratio of the polygonal zinc-iron alloy phase and the area ratio determination method will be described below in conjunction with Figure 6.

[0057] The alloyed hot-dip galvanized steel plate is placed under the SEM (scanning electron microscope) for observation, and a coating surface phase structure image as shown in Figure 6 is obtained by direct shooting. The length and width of the polygonal zinc-iron alloy phase are directly marked and measured in the SEM image (see 1-10 marked in Figure 6), and the ratio of the length and the width is calculated.

[0058] The range of the aspect ratio of each polygonal zinc-iron alloy phase can be obtained by measuring the aspect ratio of each polygonal zinc-iron alloy phase in one photograph of the surface phase structure of the plated layer. In each of the above embodiments, the aspect ratio R of the polygonal zinc-iron alloy phase is in the range of 1≤R<2.

[0059] In the alloyed hot-dip galvanized layer of the steel sheet in each of the above embodiments, the proportion of the polygonal zinc-iron alloy phase accounts for more than 95% of the total area of the alloy phases. When the polygonal zinc-iron alloy phase with 1≤R<2 accounts for more than 95% of the total area of the alloy phases on the surface of the plated layer, the friction coefficient of the surface of the plated layer is moderate, which is beneficial to improving the deep drawing performance of the steel sheet. The higher the proportion of the polygonal zinc-iron alloy phase in the plated layer, the more the proportion of the polygonal zinc-iron alloy phase in the plated layer can be ensured to be sufficient to reduce the friction coefficient of the surface of the plated layer, and the better the stamping performance. Preferably, the proportion of the polygonal zinc-iron alloy phase accounts for 100% of the total area of the alloy phases, that is, the surface of the plated layer is fully covered with the polygonal zinc-iron alloy phase. The area proportion of the polygonal zinc-iron alloy phase can be determined by measuring the area of the polygonal zinc-iron alloy phase on the surface of the plated layer using existing commercial software to analyze the area proportion of the surface phase structure of the plated layer. More specifically, the surface phase structure picture can be taken by SEM, and the area proportion can be directly analyzed by using commercial software. In order to ensure the uniformity of alloying at different positions of the steel sheet, at least three points are required at the edge and the middle of the steel sheet to determine the area proportion.

[0060] The area proportion of the zinc-iron alloy phase in the plated layer on the cross section of the steel sheet can also be detected, which can be determined by the following method: a 15*15mm sample is cut from the alloyed hot-dip galvanized steel sheet along the rolling direction (the rolling direction of the finished product, i.e. the length direction), or a 15*15mm sample is cut from the alloyed hot-dip galvanized steel sheet along the transverse direction, and then inlaid with epoxy resin, and after grinding and polishing, the sample is etched with 0.05% nitric acid alcohol solution for 3-5s. Finally, the cross section of the plated layer is analyzed by backscattered electron image analysis using SEM, so that the area proportion of the zinc-iron alloy phase in the cross section of the plated layer can be obtained.

[0061] In addition, in order to make the steel sheet and the plated layer thereof have excellent deep drawability, in addition to obtaining a surface phase structure of nearly full polygonal δ phase, the grain size of the polygonal δ phase also affects the surface roughness and the friction coefficient of the plated layer. When the size of the polygonal zinc-iron alloy phase is less than or equal to 5 μm, the roughness of the plated layer surface will not be too high to affect the formability thereof. Preferably, when the size of the polygonal zinc-iron alloy phase is in the range of 2-3 μm (including the end point values 2 μm and 3 μm), the friction coefficient of the plated layer surface can be guaranteed to reach a lower level, which is more conducive to improving the formability of the steel sheet and further improving the deep drawability of the steel sheet. The grain size refers to the average grain size. Referring to FIG. 6, the average grain size is measured according to the measurement value of the longer side of the polygonal zinc-iron alloy phase (i.e. the length of the polygonal zinc-iron alloy phase in the foregoing embodiments, which can also be considered as the long axis size of the polygonal zinc-iron alloy phase). The measurement method can be the same as the measurement method of the length in the determination method of the length-width ratio of the polygonal zinc-iron alloy phase. More specifically, the average grain size can be determined by measuring the length of a plurality of polygonal zinc-iron alloy phases and calculating the average value.

[0062] Further, at the interface between the alloyed hot-dip galvanized layer and the steel sheet substrate, the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% is in the range of 500 nm-1000 nm (including the end point values 500 nm and 1000 nm). The zinc-iron alloy phase here is Γ phase. By making the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% at the interface between the plated layer and the steel sheet substrate in the range of 500 nm-1000 nm, the plated layer can have good deep drawability and also good anti-powdering performance. In some embodiments, the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% is in the range of 500 nm-800 nm.

[0063] In the alloyed hot-dip galvanized layer, 0.01-1.0% of Al by mass is further contained. Specifically, the alloying process is involved in the manufacture of the steel sheet, which means that other metal elements are added to the steel sheet to make the steel sheet into an alloy with desired properties, forming the plated layer. Diffusion usually occurs in the alloying process, and an Fe2Al5 barrier layer is formed at the interface between the substrate of the steel sheet and the plated layer by hot-dip plating. In the subsequent heating process of the alloying, the Fe2Al5 barrier layer will decompose and diffuse into the plated layer. Therefore, the alloyed hot-dip galvanized layer contains not only Fe and Zn, but also a certain amount of Al, and the Al content is generally 0.01-1.0% by mass (including the end point values 0.01% and 1.0%). When the Al content in the plated layer is in this range, it indicates that the degree of Zn-Fe alloying diffusion reaction in the plated layer is appropriate, and the Al will not inhibit the speed of the Zn-Fe alloying reaction in the plated layer, and the alloying is sufficient, which is conducive to improving the deep drawability.

[0064] In addition, the main component of the alloyed hot-dip galvanizing layer in each of the above embodiments of the present application is Zn, and the alloyed layer formed after diffusion of zinc and iron contains not only Zn, Fe and Al, but also one or more of Si, Mn, Ti, Nb, Cr, Mo, V and B, and these elements other than Zn, Fe and Al do not generally affect the deep drawing performance of the plated layer.

[0065] Further, in the substrate of the alloyed hot-dip galvanized steel sheet of each of the above embodiments, the following elements are contained by mass: C: 0.05-0.20%, Si: 0-1.0%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01%, and the balance contains Fe and inevitable impurities. Preferably, in consideration of the control of the performance and cost of the steel sheet, the substrate of the alloyed hot-dip galvanized steel sheet of the present application can further contain one or more of the following elements by mass: Al: 0.01-1.0%, 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%. Preferably, in consideration of the control of the performance and cost of the steel sheet, the substrate of the alloyed hot-dip galvanized steel sheet of the present application can further contain one or more of the following elements by mass: 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%.

[0066] The design principles of each chemical element in the alloyed hot-dip galvanized steel sheet of the present application are explained in detail below.

[0067] C is the most basic strengthening element in steel and is also an austenite stabilizing element. The content of C in the substrate of the steel sheet of the present application is controlled to be in the range of 0.05-0.20% by mass (including the end point values of 0.05% and 0.20%). At this time, the content of austenite formed during annealing of the critical zone of the steel sheet is sufficient, the stability of austenite and the hardenability of martensite are improved, and the strength and plasticity of the material and the welding performance are both good.

[0068] Si is a substitutional solid solution alloying element, which can promote the enrichment of C in austenite, increase the stability of austenite, improve the strength of the steel plate, and to some extent, improve the toughness. The present application comprehensively considers the mechanical properties of the steel plate and the surface quality control of the plated layer, and controls the content of Si in the range of 0-1.0% (including the end point values 0 and 1.0%). Further, the content of Si can be controlled in the range of 0.1-1.0% (including the end point values 0.1% and 1.0%), or in the range of 0.1-0.8% (including the end point values 0.1% and 0.8%). Further, the content of Si in the above embodiments of the present application is controlled in the range of 0.1-0.45% (including the end point values 0.1% and 0.45%). At this time, no coarse iron carbide will be produced during the alloying process of the plated layer, the forming performance of the steel plate will not be deteriorated, and the plateability of the substrate can be ensured. Among them, the plateability is used to indicate whether the steel plate is easy to be plated; the higher the plateability, the easier the steel plate is to be plated; the lower the plateability, the less likely the steel plate is to be plated.

[0069] Mn can make the C curve (i.e. the isothermal transformation curve of undercooled austenite) of the steel plate right shift, and improve the hardenability of the steel plate. Mn can expand the austenite phase region, reduce Ac3 and Ac1, delay the pearlite phase change, and also can improve the stability of austenite. Among them, Ac3 refers to the actual temperature of phase change when carbon steel is heated; Ac1 refers to the temperature at which austenite begins to form when the steel is heated. Pearlite is a mechanical mixture composed of ferrite and cementite, and its mechanical properties are between ferrite and cementite, with high strength, moderate hardness and certain plasticity. The present application controls the content of Mn in the steel plate substrate to be more than 1.5% by mass (including the end point value 1.5%) and less than 2.8% by mass (including the end point value 2.8%). At this time, the steel plate has good hardenability, strong strengthening effect, high tensile strength, and can reduce the possibility of cracks appearing on the billet during continuous casting process, and prevent affecting the welding performance of the steel plate.

[0070] P in steel can significantly reduce the plasticity and toughness of the steel, especially in low temperature environment, which can cause cold brittleness phenomenon to occur, so the content of P must be strictly controlled. The content of P in the steel plate substrate is controlled to be less than 0.02% by mass (including 0.02%). At this time, the steel plate is not prone to cold brittleness phenomenon at low temperature.

[0071] S is an impurity element in the steel sheet, and needs to be maintained at a low level in the steel sheet, and the lower the content of S, the better. S can have a bad effect on the weldability of the steel sheet and the manufacturability during casting and hot rolling. Specifically, S can react with Fe in the steel sheet to form FeS, which in turn causes a hot shortness problem. In addition, S can also form coarse MnS in combination with Mn, which reduces the ductility of the steel sheet. The present invention controls the content of S in the steel sheet substrate to be 0.01% or less (including 0.01%) by mass to ensure that the ductility of the steel sheet is good.

[0072] N can easily form coarse nitrides in the steel sheet, thereby deteriorating the ductility of the steel sheet. The present invention controls the content of N in the steel sheet substrate to be 0.0100% or less (including 0.0100%) by mass to prevent the ductility of the steel sheet from being severely damaged.

[0073] Al, when present in solid solution, can increase the stacking fault energy of the steel sheet, thereby suppressing the precipitation of cementite and the transformation of the γ phase to martensite, and improving the stability of austenite. In addition, Al can form fine and dispersed solute points with C and N to pin grain boundaries, which can refine the grains. The present invention controls the content of Al in the steel sheet substrate to be in the range of 0.01 to 1.0% (including the end point values of 0.01% and 1.0%), which can both provide the stability of austenite and prevent a large amount of oxide inclusions from being formed in the steel sheet, preventing the impact of steelmaking and continuous casting, and reducing the possibility of nozzle clogging during continuous casting. In some embodiments, when present, the content of Al in the substrate is 0.1 to 1.0%. In some embodiments, when present, the content of Al in the substrate is 0.02 to 0.90%.

[0074] Ti can play a strengthening role by forming Ti(C,N), TiN and TiC precipitates in combination with C and N, and can improve the strength of the steel sheet by inhibiting the growth of ferrite grains. The present invention controls the content of Ti in the steel sheet substrate to be in the range of 0.005 to 0.1% by mass (including the end point values of 0.005% and 0.1%), which can both play a refining and strengthening role, and prevent the content of the above-mentioned precipitates from being too high and the size from being too large, thereby preventing the forming performance of the steel sheet from being deteriorated. In some embodiments, when present, the content of Ti in the substrate is 0.01 to 0.05%. In some embodiments, when present, the content of Al in the substrate is 0.02 to 0.05%.

[0075] Similar to Ti, Nb can form Nb(C,N) by combining with C and N, and can play a strengthening role through precipitates, and inhibit ferrite grain growth, and also can improve the strength of the steel sheet by fine-grain strengthening (i.e., refining the grain size). The steel sheet substrate of the present application controls the content of Nb to be in the range of 0.005 to 0.1% by mass (including the end point values of 0.005% and 0.1%), at which time the strengthening effect is obvious, and the amount of carbide precipitation is moderate, and the formability of the steel sheet is good. In some embodiments, when included, the content of Nb in the substrate is 0.01 to 0.1%, such as 0.04 to 0.09%.

[0076] Cr is a ferrite-forming element, and can promote the diffusion of carbon elements to austenite, thereby improving the stability of austenite, and can also reduce the critical cooling rate during annealing. Thus, Cr can replace part of C and / or Mn and be added to the chemical composition of the steel sheet. The steel sheet substrate of the present application controls the content of Cr to be in the range of 0.01 to 0.5% by mass (including the end point values of 0.01% and 0.5%), which can ensure that the effect of Cr on the steel sheet is obvious, and also will not destroy the ductility of the steel sheet. In some embodiments, when included, the content of Cr in the substrate is 0.01 to 0.1%. In some embodiments, when included, the content of Cr in the substrate is 0.01 to 0.05%.

[0077] Mo has a similar effect as Cr. Mn can shift the C curve (i.e., the isothermal transformation curve of supercooled austenite) of the steel sheet to the right, and improve the hardenability of the steel. The present application can add Mo in an appropriate amount, and the steel sheet substrate of the present application controls the content of Mo to be in the range of 0.05 to 0.2% by mass (including 0.05% and 0.2%), which can significantly improve the strength of the steel sheet through Mo, and also will not affect the surface quality of the plated layer of the steel sheet, and can also prevent excessive addition of Mo, and prevent excessive increase in the manufacturing cost of the steel sheet. In some embodiments, when included, the content of Mo in the substrate is 0.05 to 0.1%.

[0078] V mainly exists in the form of VC (vanadium monocarbide) in hot-dip galvanized dual-phase steel, and can have a pinning effect, refine the grains by pinning the grain boundaries, and also can be dispersed and precipitated in ferrite, thereby improving the strength and toughness of the steel sheet. The steel sheet substrate of the present application controls the content of V to be in the range of 0.005 to 0.2% by mass (including the end point values of 0.005% and 0.2%), at which time the strengthening effect of V on the steel sheet is obvious, the amount of carbonitride (e.g., V(C,N)) precipitation is moderate, and the formability of the steel sheet is good. In some embodiments, when included, the content of V in the substrate is 0.01 to 0.15%. In some embodiments, when included, the content of V in the substrate is 0.04 to 0.15%.

[0079] The main role of B is to improve the hardenability of the steel. B is easy to segregate at the grain boundary of austenite, delaying the transformation of austenite to ferrite. A small amount of B added to the steel plate can have a more obvious effect. The content of B in the steel plate substrate of the present application is controlled to be less than or equal to 0.10% by mass (including 0.10%), and further preferably less than or equal to 0.005% by mass (including 0.005%), while being controlled to be greater than or equal to 0.0001% by mass (including 0.0001%), so that B can play a role, and at the same time prevent the strength of the steel plate from being too high, which is beneficial to obtaining good plasticity and improving the hardenability of the steel plate.

[0080] Further, the alloyed hot-dip galvanized steel plate of the present application has a tensile strength of ≥780 MPa, an elongation of ≥10%, and a friction coefficient of <0.3. The friction coefficient can be used to evaluate the deep drawability of the steel plate, and the smaller the friction coefficient, the better the deep drawability. Generally, when the friction coefficient is not higher than 0.3, the deep drawability is considered to be good. In some embodiments, the friction coefficient of the alloyed hot-dip galvanized steel plate of the present application is in the range of 0.1-0.3, such as in the range of 0.11-0.28.

[0081] The friction coefficient and the experimental method for determining it are described below. The friction coefficient can be determined by a standard ball friction test method. The standard ball friction test method is described in detail below.

[0082] The friction coefficient is measured using an FPR2200 tester, and 60mm x 60mm samples are taken from the edge and middle of the steel strip, which are coated with 550HN lubricating oil by immersion, and then placed for 1h or more after coating. The ball friction uses a bearing steel precision steel ball with a diameter of 4.76mm and a hardness value of HRC63. The test load is 30N, the test speed is 1rpm, the test temperature is 60℃, and at least 10 cycles of motion trajectory are used. The friction coefficient value is obtained as follows: first, take the average value of each cycle of 2-N cycles (N is the actual number of motion cycles), and take the maximum value of the average value of the selected cycles as the friction coefficient value of the material.

[0083] The prior art generally uses a high air-fuel ratio, specifically an air-fuel ratio of greater than 1 but not including 1, and a high dew point to manufacture a steel plate with good deep drawability. The inventors have found through in-depth research that this manufacturing method applied to low-silicon steel plates will cause the coating to be over-alloyed, and the coating is extremely prone to peeling, resulting in extremely poor deep drawability. The inventors have improved the manufacturing method of low-silicon steel to control the phase structure of the coating surface of the low-silicon steel plate, thereby improving the deep drawability of the manufactured steel plate.

[0084] The present application provides a manufacturing method of an alloyed hot-dip galvanized steel sheet, for manufacturing the alloyed hot-dip galvanized steel sheet of each of the above embodiments, comprising the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing and alloying. The present inventors mainly improve the steps of continuous annealing, hot-dip galvanizing and alloying. Specifically, in the preheating step, when the heating atmosphere adopts air and fuel gas, the air-fuel ratio λ is controlled in the range of 0.80≤λ≤0.90, or in the preheating step, when the heating atmosphere adopts a mixed gas of N2 and O2, the volume percentage V of O2 in the mixed gas is controlled in the range of 0<V≤0.01%; in the soaking step, the dew point DP is controlled in the range of -50~-30℃; and in the alloying step, the alloying temperature is controlled in the range of 500~550℃.

[0085] The deep drawing performance of the alloyed hot-dip galvanized steel sheet obtained by the above manufacturing method is improved. The present inventors have found, through in-depth research on the alloyed hot-dip galvanized layer (hereinafter referred to as "plating layer") of the alloyed hot-dip galvanized steel sheet manufactured, that the average Fe content in the plating layer is in the range of 10~12% by mass, and the plating layer further contains polygonal zinc-iron alloy phases, the proportion of the polygonal zinc-iron alloy phases accounts for more than 95% of the total area of the alloy phases in the plating layer, and the length-width ratio R of the polygonal zinc-iron alloy phases satisfies: 1≤R<2. The present inventors have also found through repeated experiments that when 1≤R<2 is satisfied, the improvement of the deep drawing performance of the steel sheet is very obvious.

[0086] The smelting, casting, hot rolling, pickling and cold rolling methods well known in the art can be used to implement the present application to prepare the substrate of the present application. In some embodiments, the steel sheet substrate of the present application can be manufactured using the manufacturing method and steps in CN117286396A. Exemplary smelting, hot rolling, pickling and cold rolling can include the following steps:

[0087] (1) Smelting: smelting according to the determined composition of the steel sheet, continuous casting to produce a casting blank, and casting the steel sheet.

[0088] (2) Hot rolling: heating the casting blank to 1100~1250℃ and controlling the rolling, the opening rolling temperature is 950~1150℃, the final rolling temperature is 750~900℃, and the hot rolled plate thickness is less than or equal to 20mm.

[0089] (3) Pickling: pickling to remove the iron oxide scale generated during hot rolling.

[0090] (4) Cold rolling: cold rolling the steel coil to a thickness of less than or equal to 2.0mm, and the cold rolling reduction is ≥35%.

[0091] The present invention improves the deep drawability of a steel sheet by improving the manufacturing method of the steel sheet, adjusting the process, and controlling the structure and proportion of the plated alloy phase. The present invention is mainly an improvement in the continuous annealing, hot-dip galvanizing, and alloying steps, and the steel sheet preferably has a Si content of 0 to 1.0% by mass. After the cold rolling is completed and a steel sheet substrate is obtained, the continuous annealing, hot-dip galvanizing, and alloying steps are performed in this order.

[0092] (5) Continuous annealing: In the continuous annealing step, preferably, a continuous hot annealing galvanizing line having a preheating section, a soaking section, and a plating section is used, and the steel strip is passed through the preheating section and the soaking section, and the annealing process is completed before reaching the plating section, and then the plating process is performed in the plating section.

[0093] (a) Preheating: The preheating section is included in the continuous annealing, and specifically, the work-hardened sheet is heat-treated in a direct-fired heating furnace having a direct-fired burner, the air-fuel ratio in the heating furnace is controlled or the oxygen volume content in the heating furnace is controlled, and the steel sheet is heated to a range of 600 to 750°C (including the end points 600°C and 750°C).

[0094] In the heating furnace, the atmosphere can be selected to be a mixture of air and fuel gas, in which case the air-fuel ratio needs to be controlled, or the atmosphere can be selected to be a mixture of nitrogen and oxygen, in which case the oxygen volume content needs to be controlled. The two options above are mutually exclusive and cannot be used at the same time. The two options are described below.

[0095] When the atmosphere in the heating furnace is air and fuel gas, the air-fuel ratio in the heating furnace is controlled to be in a range of 0.80 to 0.90 (including the end points 0.80 and 0.90).

[0096] Specifically, the air-fuel ratio refers to the volume of air contained in a unit volume of the mixed gas (m 3) to the volume of air theoretically required for complete combustion of the fuel gas (gas) contained in a unit volume of the mixed gas. The fuel gas can be, for example, natural gas, such as methane, and the like. When the air-fuel ratio is in the range of 0.80 to 0.90, a pre-oxidized layer of an appropriate thickness can be formed on the surface of the steel sheet during heating in the preheating section. In the reducing atmosphere in the soaking section, the pre-oxidized layer forms a reduced iron layer, which is advantageous for improving the platability of the surface of the steel sheet, while ensuring sufficient diffusion of Zn-Fe during subsequent alloying. Also, since a high temperature is required for the formation of the δ phase, in order to obtain a volume percentage of the δ phase of more than 95%, a high temperature is required for the alloying temperature. If the air-fuel ratio is too large, at the subsequent high alloying temperature, the ζ phase formed at the interface during the hot-dip plating process can diffuse too much at the high alloying temperature to form the Γ phase, resulting in over-alloying of the plated layer. Therefore, the manufacturing method of the present application requires a weak pre-oxidized atmosphere in the preheating section, so that the partial outer oxide formed at the interface plays a role in inhibiting the diffusion of Zn-Fe to form the ζ phase at the initial stage of the hot-dip plating, and thus the upper limit of the air-fuel ratio is controlled to be 0.90.

[0097] When the atmosphere in the heating furnace is an oxidizing atmosphere achieved by using a mixed gas of N2and O2, as described above, since the manufacturing method of the present application requires a weak oxidizing atmosphere in the preheating section, the volume percentage of O2is controlled to be in the range of 0 to 0.01% (not including the end value 0, including the end value 0.01%). More preferably, the volume percentage of O2is in the range of 0 to 0.005% (not including the end value 0, including the end value 0.005%), which can further ensure that the partial outer oxide formed at the interface is within an appropriate range. In some embodiments, the volume percentage of O2is controlled to be in the range of more than 0 to 0.006%. In some embodiments, the volume percentage of O2is controlled to be in the range of 0.003 to 0.006%.

[0098] Further, the heating temperature of the steel sheet in the preheating section is 600 to 750°C, which can form a pre-oxidized layer of an appropriate thickness, so that the steel sheet can form an effective pure iron layer before galvanizing to improve the platability and alloying ability thereof, and can prevent the formation of an excessively thick oxidized layer, prevent the formation of an outer oxide of Si or Mn, and avoid a decrease in the adhesion of the plated layer. In some embodiments, the heating temperature in the preheating section is controlled to be in the range of 600 to 710°C.

[0099] Further, the dew point of the preheating section is controlled to be ≥ -40°C, which is advantageous for forming the desired thickness of the pre-oxidized layer on the surface of the steel sheet.

[0100] (b) soaking: in each of the above embodiments, the continuous annealing step further comprises a soaking section. Specifically, the steel sheet is heated and held (the holding time can be 5-30 s) at a soaking temperature of 700-900 °C in an atmosphere of 3-25 vol% H2, a dew point DP of -50 to -30 °C, and the balance N2 and unavoidable impurities. In some embodiments, the soaking temperature is 750-850 °C.

[0101] When the H2 concentration is in the range of 3-25 vol% (including the end values 3 vol% and 25 vol%), the hydrogen is sufficient to inhibit the amount of Fe oxidation, and the cost is not excessively high. Preferably, the H2 content is controlled in the range of 3-10 vol% (including the end values 3 vol% and 10 vol%), and the hydrogen concentration in this range is sufficient to inhibit the amount of Fe oxidation.

[0102] The dew point DP of the soaking section is in the range of -50 to -30 °C, and preferably -40 to -30 °C (including the end values -40 °C and -30 °C). In order to obtain an alloyed coating layer with more than 95% of the surface being δ phase, it is necessary to utilize the partial external oxide on the interface between the steel sheet and the coating layer to inhibit the generation of ζ phase at the initial stage of immersion plating. When the dew point DP is controlled in the range of -50 to -30 °C, it is possible to satisfy the inhibition of the generation of ζ phase at the initial stage of immersion plating by utilizing the partial external oxide on the interface between the reduced iron layer and the steel sheet. When the dew point DP is controlled in the range of -40 to -30 °C, it is possible to further inhibit the generation of ζ phase at the initial stage of immersion plating on the basis of good platability, which is more conducive to the control of the alloy phase of the coating layer to improve the deep drawing performance.

[0103] By comprehensively controlling the air-fuel ratio of the preheating section and the dew point of the soaking section in the continuous annealing stage, the activity of the surface of the steel sheet can be regulated, which is conducive to the generation of fine and uniform polygonal zinc-iron alloy phases, and is conducive to the formation of an alloyed hot-dip galvanized coating layer with an average Fe content in the range of 10-12% by mass and a length-width ratio satisfying 1≤R<2 at an alloying temperature of 500-550 °C in the subsequent alloying step.

[0104] (6) hot-dip galvanizing: in each of the above embodiments, the continuous annealing step is followed by a hot-dip galvanizing step. Specifically, the steel sheet is cooled and then immersed in a zinc bath at a temperature in the range of 440-465 °C (including the end values 440 °C and 465 °C) for hot-dip plating.

[0105] When the temperature of the zinc liquid is controlled in the range of 440-465°C (including the end point values 440°C and 465°C), the viscosity of the zinc liquid is moderate, which is favorable for controlling the thickness of the plated layer, does not affect the appearance and alloying of the steel plate, and the zinc liquid is not easy to evaporate, which can reduce the possibility of the evaporated Zn adhering to the grate, thereby avoiding the zinc ash falling as much as possible, and then preventing the zinc ash from adhering to the surface of the steel strip after falling, and preventing the steel strip from forming defects.

[0106] Hot-dip plating can be performed using a zinc liquid well known in the art. In some embodiments, the Al content in the zinc liquid used is in the range of 0.1-0.14%.

[0107] (7) Alloying: In each of the above embodiments, an alloying step is performed after the hot-dip galvanizing step is completed. Specifically, the steel plate is placed in an alloying furnace for alloying, the alloying temperature is controlled in the range of 500-550°C (including the end point values 500°C and 550°C), the alloying time is in the range of 10-30 s, and then cooling is performed using air mist cooling, water mist cooling or other methods, the average cooling speed is not less than 15°C / s, preferably 20-40°C / s, and then air cooling is performed after cooling to about 200°C.

[0108] At this time, the Zn and Fe can be ensured to diffuse with each other to obtain an alloyed plated layer of δ phase of more than 95%, i.e., the proportion of the polygonal zinc-iron alloy phase accounts for more than 95% of the total area of the alloy phase in the alloyed hot-dip galvanized layer, so that the plated layer is fully alloyed and does not over-alloy. The area ratio of the polygonal phase is measured by using SEM to take a picture of the surface phase structure, and a commercial software can be directly analyzed to obtain the area ratio. In order to ensure the uniform alloying of the steel plate at different positions, at least 3 points are required at the edge and the middle of the steel strip to determine the area ratio.

[0109] Preferably, the alloying temperature is controlled in the range of 505-550°C according to the present application. More preferably, the alloying temperature is controlled in the range of 505-520°C (including the end point values 505°C and 520°C) according to the present application. At this time, it is helpful 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 favorable for improving the deep drawing performance.

[0110] In addition, the alloying time is different in different continuous hot galvanizing production lines, and the alloying time can be selected according to the alloying furnace length of the production line, the thickness of the strip steel and the production speed of the strip steel.

[0111] By using the manufacturing method of each of the above embodiments, by accurately controlling the preheating phase air-fuel ratio in the continuous annealing step, the dew point in the soaking phase and the alloying temperature in the alloying step during the manufacturing process of the steel plate, it is beneficial to the coating of the low-silicon steel plate and the control of the surface phase structure in the coating, and it is beneficial to the formation of polygonal zinc-iron alloy phase while ensuring that the proportion of the polygonal zinc-iron alloy phase is more than 95%, thereby improving the deep drawing performance of the steel plate. And the steel plate manufactured by using the manufacturing method of the above embodiments of the present application has uniform polygonal zinc-iron alloy phase on the surface of the steel plate after alloying, which reduces the surface friction coefficient of the coating. In addition, Γ phase will not be formed on the interface between the coating and the steel plate substrate, the anti-powdering performance is also excellent, and the cost is low, which has good application prospect.

[0112] Examples and comparative examples

[0113] The steel plate substrate of each example and comparative example was prepared by referring to the smelting, hot rolling, pickling and cold rolling steps in CN117286396A. Table 1 lists the chemical composition of the steel plate cast by the present application, including the mass percentage (%) of the steel plate chemical composition of the examples and comparative examples, and the rest is Fe.

[0114] Table 1: Mass percentage of chemical composition of steel plate in different examples and comparative examples

[0115] After the cold rolling was completed and the steel plate substrate was obtained, the continuous annealing, hot galvanizing and alloying steps were performed in sequence as follows.

[0116] (5) Continuous annealing: a continuous hot annealing and galvanizing line with a preheating section, a soaking section and a plating section was used, and the steel strip passed through the preheating section and the soaking section, respectively, and before reaching the plating section, the annealing process was completed, and then the plating process was carried out in the plating section.

[0117] (a) preheating: the hard-rolled steel plate is heated in a direct-fired heating furnace with a direct-fired burner, the air-fuel ratio in the furnace is controlled or the oxygen volume content in the furnace is controlled, and the steel plate is heated to a range of 600-750°C. When the atmosphere in the furnace is air and fuel gas, the air-fuel ratio in the furnace is controlled to be in a range of 0.80-0.90; when the atmosphere in the furnace is an oxidizing atmosphere achieved by using a mixed gas of N2 and O2, the volume percentage of O2 is controlled to be in a range of 0-0.01% (not including the end point value 0, including the end point value 0.01%).

[0118] (b) soaking: the steel plate is heated at a soaking temperature of 700-900°C in an atmosphere with a hydrogen concentration of 3-25% by volume and a dew point DP of -50 to -30°C, and the rest being N2 and inevitable impurities, and the temperature is maintained for 5-30s.

[0119] (6) hot-dip galvanizing: after the continuous annealing step, a hot-dip galvanizing step is performed. Specifically, the steel plate is cooled and then immersed in a zinc bath with a temperature in a range of 440-465°C for hot-dip galvanizing, and the Al content in the zinc bath is in a range of 0.1-0.14%.

[0120] (7) alloying: after the hot-dip galvanizing step, an alloying step is performed. Specifically, the steel plate is placed in an alloying furnace for alloying, and the alloying temperature is controlled in a range of 500-550°C, and the alloying time is in a range of 10-30s.

[0121] After alloying, cooling is performed at an average cooling rate of not less than 15°C / s, and then air cooling is performed after cooling to 200°C, water quenching, flattening, tension leveling, post-treatment, edge trimming, oiling, roll division, and coiling are performed to obtain the desired hot-dip galvanizing steel plate with polygonal zinc-iron alloy phase.

[0122] Table 2 shows the process parameters of annealing, hot-dip galvanizing, and alloying.

[0123] Table 2

[0124] In the examples and comparative examples in which the air-fuel ratio λ is represented by " / ", the oxidizing atmosphere used in the examples or comparative examples is a mixed gas of N2 and O2, and no fuel gas is used. In the examples and comparative examples in which the oxygen volume percentage is represented by " / ", the mixed gas used is a mixture of air and fuel gas. In the grain size, " / " indicates over-alloying or failure to obtain a polygonal size phase, and components A to K correspond to the chemical composition of the steel plate in Table 1.

[0125] The content of Al (%) in the alloyed hot-dip plated layer of the steel sheet of each embodiment and the comparative example, the proportion (area ratio) of the polygonal phase, the aspect ratio R of the polygonal phase, the average grain size of the polygonal phase, the content of Fe in the plated layer, the average thickness of the interface Γ phase, the tensile strength, the elongation at break, and the friction coefficient were detected, and the specific data are shown in Table 3. Among them, the proportion (area ratio) of the polygonal phase, the aspect ratio R of the polygonal phase, the average grain size of the polygonal phase, the content of Fe in the plated layer, and the friction coefficient were determined by the method described above. The content of Al in the alloyed hot-dip plated layer was determined by EDS detection of the average value of more than 5 points; the proportion of the thickness of the columnar zinc-iron alloy phase in the total thickness of the alloyed hot-dip galvanized layer, and the average thickness of the interface Γ phase were determined by preparing a cross-section metallographic sample, and directly measuring the average value of more than 5 points under the field of view of 2000 times SEM; the tensile strength, elongation at break, and other properties of the steel sheet were determined by GB / T 228.1-2021.

[0126] Table 3: Comparison table of experimental methods and determination results of embodiments and comparative examples

[0127] Experiments were carried out using the manufacturing methods in each of the above embodiments, and a variety of different steel sheets (including comparative examples and embodiments) were obtained, and the tensile strength, elongation, and friction coefficient were determined, and the specific data are shown in Table 3. Among them, the friction coefficient was evaluated by the aforementioned ball friction standard test method.

[0128] With reference to Table 3, examples 1 and comparative example 1 are taken as examples for illustration. With reference to FIGS. 2 and 3, FIG. 2 is an SEM image of the plated layer surface morphology of the embodiment 1 of the application, and FIG. 3 is an SEM image of the cross-section morphology of the plated layer of the embodiment 1 of the application. In the phase structure of the plated layer surface thereof, the proportion of the polygonal zinc-iron alloy phase is 100%, and the friction coefficient is only 0.15, indicating good deep drawing performance. In the comparative example, FIG. 4 shows an SEM image of the plated layer surface morphology of the comparative example 1 of the application, and FIG. 5 shows an SEM image of the cross-section morphology of the plated layer of the comparative example 1 of the application. It can be observed that the phase structure of the plated layer surface of the comparative example is not uniform, and the fine granular structure and the long strip-shaped structure are intermingled, and the particle size is also not uniform, and the deep drawing performance is poor.

[0129] As can be seen from Examples 1-15, in the manufacturing method of the present application, the average Fe content in the galvannealed layer of the steel sheet is in the range of 10-12% by mass, and the polygonal Zn-Fe alloy phase is contained, and the length-width ratio R of the polygonal Zn-Fe alloy phase satisfies 1≤R<2. In the manufacturing method, the steel sheet substrate with the Si content in the range of 0.1-1.0% by mass is used, the mixed gas of air and fuel gas is used as the heating atmosphere in the preheating section to control the air-fuel ratio λ in the range of 0.80≤λ≤0.90, and when the oxidizing atmosphere formed by the mixed gas of N2 and O2 is used in the preheating section, the oxygen volume percentage V is controlled in the range of 0<V≤0.01%; the dew point DP in the soaking section is in the range of -50 to -30°C; and the alloying temperature is in the range of 500-550°C.

[0130] In the steel sheet and the plated layer manufactured by the manufacturing method of the present application, the proportion of the polygonal Zn-Fe alloy phase in the total area of the alloy phase in the galvannealed layer is more than 95%, the average grain size of the polygonal Zn-Fe alloy phase is less than 5 μm, and the friction coefficient is less than 0.3, so that the deep drawing performance is good. In some examples, the average grain size of the polygonal Zn-Fe alloy phase in the galvannealed layer is in the range of 2-3 μm. In this case, the friction coefficient can be further less than 0.2, and the deep drawing performance of the steel sheet is further improved.

[0131] In Comparative Example 1, the air-fuel ratio in the preheating section is more than 0.90, and the dew point DP in the soaking section is more than -30°C. The Fe content in the obtained plated layer is too high, and the plated layer is over-alloyed. Only a small amount of polygonal Zn-Fe alloy phase, only 5%, is formed on the surface of the plated layer. Although the friction coefficient is small, the average thickness of the Zn-Fe alloy phase with the Fe content more than 30% by mass at the interface between the plated layer and the substrate (i.e., the interface Γ phase) is too thick, more than 1000 nm. The smaller the thickness of the interface Γ phase, the better the powder resistance. Therefore, the powder resistance of Comparative Example 1 is poor, and the plated layer is seriously powdered during stamping, which does not meet the use requirements.

[0132] In Comparative Example 2, the air-fuel ratio in the preheating section is more than 0.90, and the dew point DP in the soaking section is more than -30°C. The length-width ratio of the obtained Zn-Fe alloy phase is more than 2, and the polygonal Zn-Fe alloy phase with the length-width ratio R in the range of 1≤R<2 is not formed on the surface of the plated layer. The Fe content in the obtained plated layer is more than 12%, the friction coefficient is too large, and the deep drawing performance is poor.

[0133] In Comparative Example 3, the air-fuel ratio in the preheating section is less than 0.80, and the alloying temperature is less than 500°C. The plated layer is not fully alloyed, and the polygonal Zn-Fe alloy phase with the length-width ratio R in the range of 1≤R<2 is not formed on the surface of the plated layer. The Fe content in the plated layer is less than 10%, the friction coefficient is too large, and the deep drawing performance is poor.

[0134] As to Comparative Example 4, the dew point DP of the soaking section is lower than -50℃, the generated alloy phase has too large aspect ratio, the polygonal zinc-iron alloy phase with the 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 not good.

[0135] As to Comparative Example 5, the alloying temperature is too high, higher than 550℃, the generated alloy phase has too large aspect ratio, the polygonal zinc-iron alloy phase with the aspect ratio R in the range of 1≤R<2 is not obtained in the coating, the Fe content in the coating is too high, over-alloying occurs, although the friction coefficient is small, the average thickness of the zinc-iron alloy phase with the Fe content of more than 30% by mass at the interface between the coating and the substrate (i.e. the interface Γ phase) is too thick, more than 1000 nm, the anti-powdering performance is not good, the coating is seriously powdered during stamping, and the use requirement is not met.

[0136] As to Comparative Example 6, the Si content in the steel plate substrate is 1.35% by mass, which belongs to high-silicon steel, the air-fuel ratio in the preheating section is more than 0.90 in the process, the Fe content in the obtained coating is too high, over-alloying occurs, the polygonal zinc-iron alloy phase with the aspect ratio R in the range of 1≤R<2 is not obtained, the friction coefficient is large, and the deep drawing performance is poor.

[0137] As to Comparative Example 7, the alloying temperature in the alloying stage is lower than 500℃. Even if the process parameters of the application are used, the Fe content in the obtained coating is still too low, the polygonal zinc-iron alloy phase with the aspect ratio R in the range of 1≤R<2 is not generated, under-alloying occurs, the friction coefficient is too large, and the deep drawing performance is not good.

[0138] In summary, the application provides a manufacturing method of an alloyed hot-dip galvanized steel plate. By controlling the atmosphere of the preheating section and the soaking section in the continuous annealing, specifically controlling the air-fuel ratio, the dew point DP and the hydrogen concentration, and also controlling the alloying temperature in the alloying stage, when the low-Si-content steel plate is selected as the base material of the alloyed hot-dip galvanized steel plate, the Fe content in the coating can be controlled in a proper range, the surface phase structure of the coating and the proportion of the polygonal zinc-iron alloy phase can be controlled, thereby improving the friction coefficient of the steel plate and enhancing the deep drawing performance, so as to be more widely applied subsequently.

[0139] Although the application has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by the skilled in the art that the foregoing is a further detailed description of the application in connection with specific embodiments, and the application is not limited to these descriptions. The skilled in the art can make various changes in form and details without departing from the spirit and scope of the application, including making a number of simple deductions or substitutions.

Claims

1. An alloyed hot-dip galvanized steel sheet, characterized in that, The substrate of the alloyed hot-dip galvanized steel sheet contains 0 to 1.0% of 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. The surface phase structure composition of the alloyed hot-dip galvanized layer contains a polygonal zinc-iron alloy phase, and the length-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1 ≤ R < 2.

2. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The polygonal zinc-iron alloy phase of the alloyed hot-dip galvanized layer is a δ phase; and / or, the length-width ratio R of the polygonal zinc-iron alloy phase satisfies: 1 ≤ R ≤ 1.

7.

3. The alloyed hot-dip galvanized steel sheet as described in 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 to 11.5% by mass.

4. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The proportion of the polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer accounts for 95% or more of the total area of the alloy phases in the alloyed hot-dip galvanized layer; preferably, the proportion of the polygonal zinc-iron alloy phase in the alloyed hot-dip galvanized layer accounts for 100% of the total area of the alloy phases in the alloyed hot-dip galvanized layer.

5. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, 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; preferably, 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.

6. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, 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 to 1000 nm.

7. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The alloyed hot-dip galvanized layer further contains 0.01 - 1.0% of Al by mass.

8. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The substrate of the alloyed hot-dip galvanized steel sheet, by mass, further contains C: 0.05 - 0.20%, Mn: 1.5 - 2.8%, P ≤ 0.02%, S ≤ 0.01%, N ≤ 0.01%, and the remaining amount contains Fe and inevitable impurities; preferably, the substrate of the alloyed hot-dip galvanized steel sheet, by mass, further contains one or more of Al: 0.01 - 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%.

9. The alloyed hot-dip galvanized steel sheet according to any one of claims 1-8, characterized in that, The alloyed hot-dip galvanized steel sheet has a tensile strength ≥ 780 Mpa, an elongation rate ≥ 10%, and a friction coefficient < 0.

3.

10. A method for manufacturing an alloyed hot-dip galvanized steel sheet as described in any one of claims 1-9, characterized in that, It includes a continuous annealing step and an alloying step for the substrate of the alloyed hot-dip galvanized steel sheet containing 0 to 1.0% of 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 using 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.

11. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 13, characterized in that, The manufacturing method includes the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing, and alloying; Among them, the continuous annealing step includes: Preheating: Heat-treat the cold-rolled steel plate in a direct-fired heating furnace equipped with a direct-fired burner. In the direct-fired heating furnace, when the combustion atmosphere is air and gas, the air-fuel ratio λ is controlled within the range of 0.80 ≤ λ ≤ 0.90, and the steel plate is heated to the range of 600 - 750°C; or when the combustion atmosphere in the direct-fired heating furnace is a mixed gas of N2 and O2, control the volume percentage V of O2 therein within the range of 0 < V ≤ 0.01%, and heat the steel plate to the range of 600 - 750°C; Soaking: Heat and hold the preheated steel plate at a soaking temperature of 700 - 900°C in an atmosphere where the hydrogen concentration is limited to 3 - 25 vol%, the dew point DP is controlled within the range of -50 - -30°C, and the balance is N2 and inevitable impurity atmosphere; The hot-dip galvanizing step includes: Cool the steel plate after continuous annealing and dip it in molten zinc within the range of 440 - 465°C for hot-dip galvanizing; The alloying step includes: Put the hot-dip galvanized steel plate into an alloying furnace for alloying, control the alloying temperature within the range of 500 - 550°C, and then cool and coil it to obtain the required alloyed hot-dip galvanized steel plate.

12. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 11, characterized in that, When the oxidizing atmosphere heated in the preheating step is a mixed gas of N2 + O2, the volume percentage of O2 therein is within the range of 0 - 0.005%; and / or, control the dew point DP ≥ -40°C in the preheating step.

13. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 11, characterized in that, Control the dew point DP in the soaking step within the range of -40 - -30°C; and / or, control the hydrogen concentration in the soaking step within the range of 3 - 10%.

14. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 11, characterized in that, Control the alloying temperature within the range of 505 - 520°C in the alloying step.

15. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 11, characterized in that, Control the alloying time within the range of 10 - 30 s in the alloying step.

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