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% of Si by mass, an average Fe content in an alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel plate is 7-10% by mass, the composition of a surface phase structure of the alloyed hot-dip galvanized layer comprises a columnar zinc-iron alloy phase, and the length-to-width ratio R of the columnar zinc-iron alloy phase satisfies: 2 ≤ R < 7. The alloyed hot-dip galvanized steel plate of the present invention has a powdering grade of less than or equal to 6 and good powdering resistance. Further disclosed in the present invention is a method for manufacturing the alloyed hot-dip galvanized steel plate. The method is applicable to a low-silicon steel plate. The alloy phase structure of a coating layer of the steel plate is controlled by controlling the air-to-fuel ratio in a preheating stage, the dew point (DP) in a soaking stage, and the alloying temperature in an alloying step in a continuous annealing step, so as to improve the powdering resistance 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 a steel sheet and a method for manufacturing the same, and more particularly, to an alloyed hot-dip galvanized steel sheet and a method for manufacturing the same. BACKGROUND

[0002] Automobile lightening is one of the key technologies to adapt to the modern automobile safety, energy saving and environmental protection trend. In order to balance high strength and good forming performance, a large number of ultra-high strength steels are applied to the automobile body-in-white. 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.

[0003] At the same time, improving the corrosion resistance of the vehicle body has become the goal pursued by users. The alloyed hot-dip galvanized sheet (referred to as GA steel sheet) is obtained by adding an alloying annealing process after the strip steel comes out of the zinc pot on the basis of the traditional hot-dip galvanizing production line, thereby obtaining a plated steel sheet with different Fe-Zn alloy phase structures. The plated layer contains different intermetallic compounds, and mainly includes Γ, Γ1, δ and ζ phases from the steel sheet substrate to the plated layer surface. Compared with ordinary galvanized sheets, the alloyed galvanized sheet has good corrosion resistance, heat resistance, painting property, weldability and rust prevention ability after painting. Due to the high hardness of the alloyed plated layer, it also has good scratch resistance.

[0004] The existing technology mainly focuses on high-silicon steel for the research of the plated layer of the steel sheet. Since high-silicon steel is not easy to be plated, the existing technology usually studies how to successfully plate the high-silicon steel without studying the plated layer structure, and the specific method for successful plating is to adopt high air-fuel ratio and high alloying temperature for plating. Among them, the air-fuel ratio needs to be above 1 (not including the end value 1) when the high-silicon steel is plated, and the alloying temperature also needs to be above 500℃.

[0005] For example, patent document JPH11279691 discloses an alloyed hot-dip galvanized steel sheet with high strength and excellent workability by optimizing the ratio of C, Si and Mn. However, in order to make the austenite remain, a certain amount of Si needs to be added in this invention, and the addition of Si element will deteriorate the platability of high-strength steel and hinder alloying. Moreover, in this invention, the hot-dip plated strip steel needs to be heated to a temperature range of above 500℃ and below 600℃ for alloying, which is not conducive to the performance control of high-strength steel.

[0006] For example, patent document CN102803540B proposes a high-strength alloyed hot-dip galvanized steel sheet with excellent workability and fatigue resistance. It is found that by properly adjusting the alloying elements, the hot-rolled sheet becomes a structure mainly composed of bainite and martensite, and by using this hot-rolled sheet as a raw material, after cold rolling, rapid heating is performed during the annealing process, and in the final structure, a proper amount of martensite is uniformly and finely dispersed, which is effective for improving the hole expansion property and fatigue resistance. After plating, alloying treatment of the plated layer is performed in the temperature range of 540-600°C, thereby generating a proper amount of pearlite and inhibiting the decrease in hole expansion performance caused by martensite.

[0007] In the prior art, there are also measures for high-silicon steel sheets, such as controlling the hot-rolled coiling temperature to be above 600°C, or pre-oxidizing the steel sheet at a temperature above 700°C during annealing, and increasing the dew point to above -20°C during annealing, to obtain an internal oxidation layer in the steel substrate, and during alloying, the plated layer can obtain a proportion of 2.0-15.0% by cross-sectional area ratio of the steel substrate, thereby improving the performance of the steel sheet. However, these measures can only be used when the plated layer Fe content is higher than 10% (not including 10%).

[0008] In addition, high-silicon steel is 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 maintaining low cost. However, the current prior art does not focus on the plated layer of low-silicon steel, and usually only describes its general manufacturing process, and usually only completes the manufacturing by high-temperature heating and alloying, with the alloying temperature set in the range of 540-560°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 plated layer of the manufactured low-silicon steel sheet.

[0009] In use, most of the automobile sheets are used under severe bending processing (flanging bending) and hole expanding processing. In the zinc-iron diffusion process, the zinc-iron alloy phase in the alloyed coating forms Γ, Γ1, δ and ζ phases according to the Fe content. However, the Zn-Fe alloy phases formed in the alloying process are harder and more brittle than pure zinc phase, and are easily damaged in the stamping process. The damage forms of the alloyed coating in the deformation process mainly include two types: flaky peeling called peeling, and granular peeling called powdering. The peeling is flaky particles formed due to the adhesion failure between the coating and the substrate interface, and the particle size is generally similar to or greater than the coating thickness, which is generally caused by the formation of an excessively thick ζ phase on the surface of the coating, which increases the surface friction of the coating. The powdering is granular particles formed due to the failure of the coating, and the particle size is generally less than the coating thickness, which is generally caused by the formation of a thick Γ phase at the interface between the substrate and the coating. Powdering can cause damage to the die during stamping and can also reduce the corrosion resistance of the coating. Therefore, how to ensure the performance of ultra-high strength steel and obtain an excellent coating structure in a suitable alloying process is a manufacturing difficulty of the alloyed hot-dip galvanized steel sheet.

[0010] Therefore, it is necessary to deeply study the coating structure of the low-silicon steel sheet, so that the low-silicon steel sheet with a relatively low cost has a tensile strength and elongation that meet the needs, and also has a good anti-powdering performance. SUMMARY

[0011] The present application provides a hot-dip galvanized steel sheet to solve the above technical problems.

[0012] In a first aspect, the embodiments of the present application disclose an alloyed hot-dip galvanized steel sheet, wherein the substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% of Si in terms of mass, the average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is 7-10% in terms of mass, and the surface phase structure of the alloyed hot-dip galvanized layer contains columnar zinc-iron alloy phases, and the length-width ratio R of the columnar zinc-iron alloy phases satisfies 2≤R<7.

[0013] By adopting the above technical scheme, the hot-dip galvanized layer of the hot-dip galvanized steel sheet of the present application has an average Fe content of less than 10% in terms of mass, and the hot-dip galvanized layer also contains columnar zinc-iron alloy phases, so that the anti-powdering performance is good.

[0014] Optionally, the columnar zinc-iron alloy phases in the alloyed hot-dip galvanized layer are ζ phases.

[0015] Optionally, the proportion of the columnar zinc-iron alloy phases 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.

[0016] Optionally, the proportion of the columnar 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 thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanizing layer is less than or equal to 1 / 5 of the total thickness of the alloyed hot-dip galvanizing layer.

[0018] Optionally, the thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanizing layer is 1 / 15-1 / 10 of the total thickness of the alloyed hot-dip galvanizing layer.

[0019] Optionally, the average thickness of the zinc-iron alloy phase with Fe content more than 30% by mass at the interface between the alloyed hot-dip galvanizing layer and the substrate is less than or equal to 500 nm.

[0020] Optionally, the average thickness of the zinc-iron alloy phase with Fe content more than 30% by mass at the interface between the alloyed hot-dip galvanizing layer and the substrate is in the range of 0-300 nm.

[0021] Optionally, the alloyed hot-dip galvanizing layer further contains 0.01-1.0% of Al by mass.

[0022] Optionally, the substrate of the alloyed hot-dip galvanizing steel plate contains, by mass, C: 0.05-0.20%, Mn: 1.5-2.8%, P≤0.02%, S≤0.01%, N≤0.01%, and the rest contains Fe and inevitable impurities.

[0023] Optionally, the substrate of the alloyed hot-dip galvanizing steel plate 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%.

[0024] Optionally, the alloyed hot-dip galvanizing steel plate has a tensile strength ≥780 MPa, an elongation ≥10%, and a powdering grade ≤6; wherein the powdering grade is determined by the reflectivity or reflectivity and the width of the powder adhered to the adhesive tape in the V-bending powdering test method.

[0025] In a second aspect, the present application also discloses a manufacturing method for manufacturing the galvannealed steel sheet, comprising a continuous annealing step and an alloying step of a base plate of the galvannealed 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, the air-fuel ratio is controlled to be within the range of 0.90-1.0 when the heating atmosphere adopts air and fuel gas, or the volume percentage of O2 is controlled to be within the range of 0.05-1% when the heating atmosphere adopts the mixed gas of N2 and O2; in the soaking step, the dew point DP is controlled to be within the range of-50--20℃; and in the alloying step, the alloying temperature is controlled to be within the range of 420-490℃.

[0026] 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 steel sheet manufacturing process, the surface phase structure in the coating of the low-silicon steel sheet is controlled, the columnar zinc-iron alloy phase is formed, and the powdering resistance of the steel sheet is improved.

[0027] Optionally, the manufacturing method of the galvannealed steel sheet comprises the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot galvanizing and alloying.

[0028] In the continuous annealing step, the following steps are included:

[0029] Preheating: the cold-rolled galvannealed steel sheet is heat-treated in a direct-fired heating furnace with a direct-fired burner, the air-fuel ratio is controlled to be within the range of 0.90-1.0 when the heating atmosphere adopts air and fuel gas, or the volume percentage of O2 is controlled to be within the range of 0.05-1% when the heating atmosphere adopts the mixed gas of N2 and O2, and the galvannealed steel sheet is heated to the range of 600-750℃.

[0030] Soaking: the preheated galvannealed steel sheet is heated and held at the soaking temperature of 750-850℃ in the atmosphere with the hydrogen concentration limited to 3-25% by volume, the dew point DP controlled to be within the range of-50--20℃, and the rest being N2 and inevitable impurities;

[0031] The hot galvanizing step comprises: cooling the continuously annealed galvannealed steel sheet and placing it into the zinc liquid within the range of 440-465℃ for hot dip plating;

[0032] The alloying step comprises: placing the hot-galvanizing galvannealed steel sheet into an alloying furnace for alloying, controlling the alloying temperature to be within the range of 420-490℃, and then cooling and coiling to obtain the required galvannealed steel sheet.

[0033] Optionally, the air-fuel ratio of the preheating step is controlled in the range of 0.90-0.95.

[0034] Optionally, the oxidizing atmosphere in the preheating step is a mixture of N2+O2, and the volume percentage of O2 in the mixture is in the range of 0.06-0.08%.

[0035] Optionally, the dew point DP in the preheating step is controlled to be ≥-40℃.

[0036] Optionally, the hydrogen concentration in the soaking step is in the range of 3-10% by volume.

[0037] Optionally, the dew point DP in the soaking step is controlled in the range of -30--20℃.

[0038] Optionally, the alloying temperature in the alloying step is controlled in the range of 440-460℃.

[0039] Optionally, the alloying time in the alloying step is controlled in the range of 10-30s. BRIEF DESCRIPTION OF DRAWINGS

[0040] Fig. 1 shows a schematic diagram of the cross-sectional morphology of the coating of the steel plate according to the present application; wherein 1 represents the steel plate substrate, 2 represents the interface layer between the coating and the substrate, 3 represents the main body of the coating, and 4 represents the surface layer of the coating;

[0041] Fig. 2 shows an SEM image of the surface morphology of the coating according to Example 2 of the present application;

[0042] Fig. 3 shows an SEM image of the cross-sectional morphology of the coating according to Example 2 of the present application;

[0043] Fig. 4 shows an SEM image of the surface morphology of the coating according to Comparative Example 1 of the present application;

[0044] Fig. 5 shows an SEM image of the cross-sectional morphology of the coating according to Comparative Example 1 of the present application;

[0045] Fig. 6 shows an SEM image of the surface morphology of the coating according to Comparative Example 5 of the present application;

[0046] Fig. 7 shows an SEM image of the cross-sectional morphology of the coating according to Comparative Example 5 of the present application;

[0047] Fig. 8 shows a schematic diagram of the process for determining the aspect ratio of the phase structure of the coating according to the present application; wherein the numbers 1-10 represent 10 randomly selected columnar zinc-iron alloy phases. DETAILED DESCRIPTION

[0048] The advantages and effects of the present application can be easily understood by those skilled in the art from the disclosure of the present specification. Although the present application will be described in connection with the preferred embodiments, this description is not intended to limit the features of the present application to those embodiments. On the contrary, the purpose of describing the application in connection with the embodiments is to cover other alternatives or modifications as can be possible in light of the application of the claims. In order to provide a thorough understanding of the present application, many specific details will be included in the following description. The present application can also be implemented without using these details. In addition, in order to avoid confusion or ambiguity of the present application, some specific details will be omitted in the description. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0049] It should be noted that in the present specification, similar reference numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0050] The terms "first", "second", and the like are only used for differentiation in description, and cannot be understood as indicating or implying relative importance.

[0051] In the description of the present embodiments, it should also be noted that unless otherwise explicitly specified and limited, the terms "provided", "connected", "connected" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be connected inside two elements. For those skilled in the art, the specific meaning of the above terms in the present embodiments can be understood according to the specific circumstances.

[0052] In order to make the purpose, technical solutions and advantages of the present application more clear, the embodiments of the present application will be further described in detail below with reference to the drawings.

[0053] According to the known prior art, it is not clear how to optimize the structure of the low-silicon steel plate coating to improve its anti-powdering performance. The inventors found through in-depth research that the anti-powdering performance of hot-dip galvanizing alloyed coating mainly depends on the iron content in the coating and the phase structure of the coating. The higher the iron content, the greater the powdering amount. However, the Fe content of the coating reflects the average value of the alloyed coating, and it is difficult to accurately reflect the forming performance of the coating on parts with high stamping requirements. Through a large number of experimental studies, it is found that the phase structure of the coating, especially the composition of the surface phase structure, is a key factor affecting the anti-powdering performance of the alloyed coating steel plate. The inventors have also found that when the surface phase structure of the coating contains a certain amount of columnar zinc-iron alloy phase, the anti-powdering performance of the low-silicon steel plate coating is better.

[0054] The present application provides a kind of alloyed hot-dip galvanized steel plate (GA steel plate), more specifically low silicon steel plate, the alloyed hot-dip galvanized steel plate has the Fe content of reasonable control alloyed hot-dip galvanizing layer (hereinafter referred to as "coating layer") and the phase structure of the coating layer, so that the steel plate has better anti-powdering performance. Figure 1 shows the schematic diagram of the cross-sectional morphology of the coating layer of the steel plate of the present application, including steel plate substrate 1, coating layer and substrate interface layer 2, coating layer main body 3, and coating layer surface layer 4. Figure 3 is a cross-sectional morphology diagram of the steel plate of the present embodiment.

[0055] The substrate of the alloyed hot-dip galvanized steel plate provided by the present application contains 0-1.0% Si (including end point values 0 and 1.0%) by mass, i.e. the steel plate can contain no Si element or not more than 1.0% Si element, which belongs to a low silicon steel. The present application optimizes the surface phase structure of the coating layer of the low silicon steel plate, improves the anti-powdering performance while avoiding increasing the cost.

[0056] In the alloyed hot-dip galvanizing layer of the alloyed hot-dip galvanized steel plate provided by the present application, it also contains 7-10% average Fe content (including end point values 7% and 10%) by mass. Among them, the average Fe content refers to the average Fe content in the coating layer, which is determined by EDS detection of the average value of more than 5 points.

[0057] When the average Fe content in the coating layer is in the range of 7-10%, the alloying on the surface of the coating layer is complete and it is not easy to adhere to the mold. At this time, the steel plate is stamped, the friction between the steel plate and the mold is moderate, the high frictional shear stress generated by the surface ζ phase will not cause strain concentration at the interface between the coating layer and the steel plate substrate, so it will not cause the coating layer to peel off at the interface, and it will not cause the stamping to crack, which is beneficial to the processing of the steel plate. In addition, Γ phase will not be formed on the interface between the steel plate substrate and the coating layer, which will not cause the steel plate to be hard and brittle, which is beneficial to control the phase structure of the coating layer and improve the anti-powdering performance of the coating layer. Preferably, when the Fe content in the coating layer does not exceed 10%, the possibility of forming Γ phase on the interface of the coating layer can be further reduced. More preferably, the average Fe content in the coating layer is in the range of 7-9% (including end point values 7% and 9%).

[0058] The surface phase structure of the alloyed hot-dip galvanized layer provided by the present application also contains columnar zinc-iron alloy phases (see FIG. 2), and the length-width ratio R of the columnar zinc-iron alloy phases satisfies 2≤R<7. Wherein, referring to FIG. 8, the columnar zinc-iron alloy phase indicates that the phase structure of the zinc-iron alloy phase is in the form of a long column, and it can be observed from the SEM (scanning electron microscope) image that the alloy phase has a longer side (length) and a shorter side (width), and the length-width ratio R is greater than or equal to 2 and less than 7. Further, the "columnar" in the columnar zinc-iron alloy phase can be in various forms of columnar such as cylinder, prism, etc., and can be regular columnar or irregular columnar, and those satisfying 2≤R<7 in length-width ratio can be considered as columnar zinc-iron alloy phases.

[0059] When the length-width ratio R of the columnar zinc-iron alloy phase satisfies 2≤R<7, it indicates that the Fe content of the zinc-iron alloy phase generated after alloying on the surface of the coating layer is moderate, which meets the requirements, and at this time, the alloying temperature in the alloying step during the manufacturing of the steel plate does not need to be higher to achieve a successful coating layer, and the alloying is sufficient, which is beneficial to improve the powdering resistance of the coating layer. In some embodiments, R satisfies 3≤R<7.

[0060] The determination method of the length-width ratio of the columnar zinc-iron alloy phase will be described below in combination with FIG. 8.

[0061] The alloyed hot-dip galvanized steel plate is observed under the SEM (scanning electron microscope), and a coating layer surface phase structure image as shown in FIG. 8 is directly taken, and the length and width of the columnar zinc-iron alloy phase are directly measured by the SEM (see the 1-10 marked in FIG. 8), and the ratio of the length and width is calculated.

[0062] Wherein, the range of the length-width ratio can be obtained by measuring the length-width ratio of each columnar zinc-iron alloy phase in a coating layer surface phase structure image taken. In the above embodiments, the length-width ratio R of the columnar zinc-iron alloy phase is in the range of 2≤R<7.

[0063] Further, the average grain size of the columnar zinc-iron alloy phase in the above embodiments is in the range of 2-10 μm (including the end point values 2 μm and 10 μm), and preferably in the range of 2-6 μm (including the end point values 2 μm and 6 μm). Wherein, referring to FIG. 8, the average grain size is measured by the longer side of the columnar zinc-iron alloy phase (i.e. the length in the above embodiments, which can also be considered as the long axis size of the columnar zinc-iron alloy phase), and the measurement method can be the same as the measurement method of the length in the above determination method of the length-width ratio of the columnar zinc-iron alloy phase. More specifically, the average grain size can be determined by measuring the length of a plurality of columnar zinc-iron alloy phases and calculating the average value, and at least 5 points are selected to obtain the average value.

[0064] In the alloyed hot-dip galvanized layer of the steel sheet in each of the above embodiments, the proportion of the columnar zinc-iron alloy phase on the surface of the plated layer is 95% or more of the total area of the alloy phase. The higher the proportion of the columnar zinc-iron alloy phase in the surface plated layer, the better the anti-powdering performance. Preferably, the proportion of the columnar zinc-iron alloy phase is 100% of the total area of the alloy phase, that is, the surface of the plated layer is covered with the columnar zinc-iron alloy phase. The proportion of the area of the columnar zinc-iron alloy phase on the surface of the plated layer can be determined by measuring the area of the columnar zinc-iron alloy phase on the surface of the plated layer using existing commercial software. More specifically, a picture of the surface phase structure can be taken using SEM, and the proportion of the area can be directly analyzed using commercial software. In order to ensure that the alloying is uniform at different positions of the steel sheet, at least three points are required to be measured at the edge and the middle of the steel sheet, respectively.

[0065] The proportion of the area of the zinc-iron alloy phase in the plated layer in the cross section of the steel sheet can also be detected, and can be determined by the following method: a 15*15 mm sample is cut from the alloyed hot-dip galvanized steel sheet along the rolling direction (the rolling direction of the finished product, that is, the length direction), or a 15*15 mm sample is cut from the alloyed hot-dip galvanized steel sheet along the transverse direction, and then inlaid with epoxy resin. After grinding and polishing, the sample is etched with 0.05% nitric acid alcohol solution for 3-5 s. Finally, backscattered electron image analysis of the cross section of the plated layer is performed using SEM, so that the proportion of the area of the zinc-iron alloy phase in the cross section of the plated layer can be obtained.

[0066] In addition, the thickness of the columnar zinc-iron alloy phase with an aspect ratio of 2≤R<7 in the alloyed hot-dip galvanized layer is 1 / 15-1 / 5 (including the end values 1 / 15 and 1 / 5) of the total thickness of the alloyed hot-dip galvanized layer. Specifically, the columnar zinc-iron alloy phase with an aspect ratio of 2≤R<7 is ζ phase. When the thickness of the columnar zinc-iron alloy phase with an aspect ratio of 2≤R<7 is not higher than 1 / 5 of the total thickness of the alloyed hot-dip galvanized layer and not lower than 1 / 15 of the total thickness of the alloyed hot-dip galvanized layer, it indicates that the Fe content in the plated layer is moderate, the alloying of the plated layer is complete, the friction coefficient of the surface of the plated layer is moderate, and the plated layer will not be easily peeled off. Preferably, the thickness of the columnar zinc-iron alloy phase with an aspect ratio of 2≤R<7 is 1 / 15-1 / 10 (including the end values 1 / 15 and 1 / 10) of the total thickness of the alloyed hot-dip galvanized layer, which can ensure that the friction coefficient of the surface of the plated layer is in the most appropriate range, further reduces the possibility of peeling of the plated layer, and thus further improves the anti-powdering performance.

[0067] Further, the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% at the interface between the alloyed hot-dip galvanized layer and the steel plate substrate is not more than 500 nm. Here, the zinc-iron alloy phase refers to Γ phase or Γ1 phase. Preferably, the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% at the interface between the alloyed hot-dip galvanized layer and the steel plate substrate is within 300 nm, i.e. within the range of 0-300 nm (including the end points 0 and 300 nm), i.e. the lower the proportion of hard and brittle Γ phase or Γ1 phase, the better the anti-powdering performance.

[0068] In the alloyed hot-dip galvanized layer, 0.01-1.0% of Al by mass is further contained. Specifically, the steel plate manufacturing involves an alloying process, which refers to adding other metal elements to the steel plate to make the steel plate into an alloy with expected performance, forming an alloyed plated layer. Diffusion usually occurs in the alloying process, and a Fe2Al5 barrier layer is formed at the interface between the substrate and the plated layer of the steel plate by hot-dip plating. In the subsequent heating process of 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 points 0.01% and 1.0%). When the Al content in the plated layer is within this range, it means that the degree of Zn-Fe alloying diffusion reaction in the plated layer is appropriate, and Al will not inhibit the speed of Zn-Fe alloying reaction in the plated layer, and the alloying is sufficient, which is beneficial to improve the anti-powdering performance.

[0069] In addition, the main component of the alloyed hot-dip galvanized layer in each embodiment of the present application is Zn, which is the main body of Zn, and the alloyed plated layer formed after zinc-iron diffusion. The plated layer contains not only Zn, Fe and Al, but also one or more of Si, Mn, Ti, Nb, Cr, Mo, V and B. These other elements in addition to Zn, Fe and Al generally do not affect the anti-powdering performance of the plated layer.

[0070] 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 to 0.20%, Si: 0 to 1.0%, Mn: 1.5 to 2.8%, P ≤ 0.02%, S ≤ 0.01%, N ≤ 0.01%, the balance including Fe and inevitable impurities. Preferably, the substrate of the alloyed hot-dip galvanized steel sheet of the present application can further add one or more of the following elements by mass: Al: 0.01 to 1.0%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, Cr: 0.01 to 0.5%, Mo: 0.05 to 0.2%, V: 0.005 to 0.2%, B: 0.0001 to 0.10%, taking into account the control of the properties and cost of the steel sheet. Further preferably, the substrate of the alloyed hot-dip galvanized steel sheet of the present application can further add one or more of the following elements by mass: Al: 0.1 to 1.0%, Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, Cr: 0.01 to 0.5%, Mo: 0.05 to 0.2%, V: 0.005 to 0.2%, B: 0.0001 to 0.10%.

[0071] The design principles of each chemical element in the alloyed hot-dip galvanized steel sheet of the present application are explained below.

[0072] 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 is controlled to be in the range of 0.05 to 0.20% by mass (including the end point values of 0.05% and 0.20%). At this time, the content of austenite formed when the steel sheet is annealed in the intercritical region 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.

[0073] 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 of the steel plate. 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 is <1.0%. Still further, the content of Si can be controlled in the range of 0.1-1.0% or 0.1-0.8% (including the end point values 0.1% and 0.8%). Still 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%). In some embodiments, the content of Si in the substrate is 0.25-1.0%. In some embodiments, the content of Si in the substrate is 0.25-0.89%. At this time, no coarse iron-based carbide is generated during the alloying process of the plated layer, the forming performance of the steel plate is not deteriorated, and the plateability of the substrate is also guaranteed. 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.

[0074] Mn can make the C curve (i.e. the isothermal transformation curve of undercooled austenite) of the steel plate right-shift, 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, its mechanical properties are between ferrite and cementite, it has high strength and moderate hardness, and has 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 the continuous casting process, preventing the welding performance of the steel plate from being affected.

[0075] 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 needs to be strictly controlled. The present application controls the content of P in the steel plate substrate 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.

[0076] 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 adversely affect 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.

[0077] 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.

[0078] 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, dispersedly distributed insoluble 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 the formation of a large amount of oxide inclusions 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 is 0.1 to 1.0%; in further embodiments, when present, the content of Al is 0.02 to 0.80%.

[0079] 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 deterioration of the formability of the steel sheet. In some embodiments, when present, the content of Ti is 0.005 to 0.05%. In some embodiments, when present, the content of Ti is 0.02 to 0.05%.

[0080] Nb and Ti are similar, Nb can be combined with C, N to form Nb(C, N), and can play a strengthening effect through precipitates, and inhibit ferrite grain growth, but also through fine grain strengthening (i.e. refine the grain size) to improve the strength of the steel plate. The steel plate substrate of the present application controls the content of Nb in the range of 0.005-0.1% by mass (including the end point values 0.005% and 0.1%), at this time the strengthening effect is obvious, and the amount of carbide precipitation is moderate, and the formability of the steel plate is better. In some embodiments, when contained, the content of Nb is 0.03-0.1%.

[0081] Cr is a ferrite forming element, can promote the diffusion of carbon elements to austenite, thereby improving the stability of austenite, but also can reduce the critical cooling rate during annealing. Therefore, Cr can replace part of C and / or Mn and be added to the chemical composition of the steel plate. The steel plate substrate of the present application controls the content of Cr in the range of 0.01-0.5% by mass (including the end point values 0.01% and 0.5%), which can ensure that the effect of Cr on the steel plate is obvious, and will not damage the ductility of the steel plate.

[0082] Mo has a similar effect to Cr. Mn can make the C curve of the steel plate (i.e. the supercooled austenite isothermal transformation curve) right shift, and improve the hardenability of the steel. The present application can add Mo element in an appropriate amount, and the steel plate substrate of the present application controls the content of Mo in the range of 0.05-0.2% by mass (including 0.05% and 0.2%), which can significantly improve the strength of the steel plate through Mo, and does not affect the surface quality of the plated layer of the steel plate, and can prevent excessive addition of Mo, and prevent excessive increase of the manufacturing cost of the steel plate. In some embodiments, when contained, the content of Mo can be 0.05-0.10%.

[0083] V mainly exists in the form of VC (vanadium monocarbide) in hot-dip galvanized dual-phase steel, which can have a pinning effect, refine the grain through pinning grain boundaries, and also disperse and precipitate in ferrite, thereby improving the strength and toughness of the steel plate. The steel plate substrate of the present application controls the content of V in the range of 0.005-0.2% by mass (including the end point values 0.005% and 0.2%), at this time, the strengthening effect of V on the steel plate is obvious, the amount of carbonitride (such as V(C, N)) is moderate, and the formability of the steel plate is better. In some embodiments, when contained, the content of V can be 0.04-0.15%.

[0084] The main role of B is to improve the hardenability of the steel. B is easy to segregate at the grain boundaries 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. In some embodiments, when B is included, the content of B can be 0.0001-0.005%.

[0085] In the alloyed hot-dip galvanized steel plate of each of the above embodiments, the tensile strength is ≥780 MPa, the fracture elongation is ≥10%, and the powdering grade is ≤6. In some embodiments, the tensile strength of the alloyed hot-dip galvanized steel plate is ≥810 MPa. In some embodiments, the tensile strength of the alloyed hot-dip galvanized steel plate is 780-1300 MPa, such as 820-1300 MPa. In some embodiments, the fracture elongation of the alloyed hot-dip galvanized steel plate is ≥15%. In some embodiments, the fracture elongation of the alloyed hot-dip galvanized steel plate is 10-23%. In some embodiments, the powdering grade of the alloyed hot-dip galvanized steel plate is ≤5, ≤4, or ≤3. In some embodiments, the powdering grade of the alloyed hot-dip galvanized steel plate is 1, 2, 3, 4, 5, or 6.

[0086] The powdering grade and the experimental method for determining the powdering grade are briefly introduced below. The specific experimental method of the present application distinguishes between steel plates of different thicknesses, and selects different test types and test methods for these steel plates, and also designs different evaluation methods to reasonably and accurately evaluate the powdering grade of the steel plate, so as to evaluate the powdering resistance performance thereof. The specific experimental method is shown in Table 1. Among them, the specific powdering grade determination experimental method can adopt the standard experimental method and steps in the art.

[0087] Table 1: Powdering grade experimental method

[0088] The following is described using a sample as an example. A 75 mm (width) * 200 mm (rolling direction) square sample is taken from the sample, and a V-bend test is performed. The V-bend test method includes the following steps. A V-bend angle and a V-bend die corresponding to the V-bend angle are selected according to the thickness of the square sample. The square sample is bent using the V-bend die by a V-bend press. Then, the square sample is turned over and flattened using a flat punch of a testing machine. A tape is attached to the bent portion (i.e., the peak of the bend) of the sample, and the tape is gently smoothed to firmly adhere zinc powder to the tape. Subsequently, the tape is slowly peeled off and attached to a whiteboard, and the reflectance or the reflectance and the width of the powder adhered to the tape are measured in the width direction of the tape. The powdering grade is evaluated based on the measured values.

[0089] In the above test method, the brand of the tape used is preferably 3M Scotch tape, or a tape having a quality equivalent to that of the 3M Scotch tape. The brand of the whiteboard used is preferably Sunloid Industrial Board A400, or a whiteboard having a quality equivalent to that of the Sunloid Industrial Board A400. The powdering grade of the steel sheet is determined by the reflectance of the powder adhered to the tape or the reflectance and the width of the powder.

[0090] The reflectance and the powdering width measured by the powdering test are used to determine the respective grades. When the powdering grade is 1, the amount of powdering is extremely small or the reflectance is large. When the powdering grade is 5, the amount of powdering is large or the reflectance is small. It is to be noted that when the reflectance is less than the lower limit of the reflectance range in which the grade can be directly evaluated, the powdering width at the widest portion of the tape is measured, and when the measured width is < 6 mm, the powdering grade is evaluated as 5. When the measured width is ≥ 6 mm, the powdering grade is evaluated according to the powdering evaluation grade table. In the case where the whiteboard used for reflectance measurement is a 12 mm * 7 mm whiteboard, Table 2 shows the powdering evaluation grade table.

[0091] Table 2: Powdering Evaluation Grade Table (12 mm * 7 mm Whiteboard)

[0092] In the table, " / " indicates that when the width is ≥ 6 mm, the powdering width is directly used for evaluation. When the width and the reflectance obtained by the test are in different grades, the powdering grade is evaluated based on the width. Referring to Table 2, for example, when the reflectance is ≥ 63%, the powdering grade is 1. When the reflectance is ≥ 48% and < 52%, the powdering grade is 4. When the reflectance is < 48%, the powdering width is measured, and when the measured powdering width is ≥ 6 mm and < 7 mm, the powdering grade is 6.

[0093] The prior art usually adopts a high air-fuel ratio, specifically an air-fuel ratio of more than 1 and less than 1, and a high dew point and a high alloying temperature of more than 500°C to manufacture a steel plate with good anti-powdering performance. The inventors have found through in-depth research that this manufacturing method applied to a low-silicon steel plate will cause the coating to be over-alloyed, the coating to be seriously powdered, the coating to be easily peeled off, and the anti-powdering performance of the steel plate to be extremely poor. The inventors have improved the manufacturing method of the low-silicon steel plate to control the phase structure of the coating surface of the low-silicon steel plate to improve the anti-powdering performance of the manufactured steel plate.

[0094] The present application provides a manufacturing method of an alloyed hot-dip galvanized steel plate, which is used to manufacture the alloyed hot-dip galvanized steel plate of each of the above embodiments, and includes the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing, and alloying. The inventors have mainly improved the steps of continuous annealing, hot-dip galvanizing, and alloying. Specifically, the air-fuel ratio is controlled to be in the range of 0.90-1.0 when air and fuel gas are used as the heating atmosphere in the preheating step, or the volume percentage of O2 in the mixed gas of N2 and O2 is controlled to be in the range of 0.05-1% when the mixed gas of N2 and O2 is used as the heating atmosphere in the preheating step; the dew point DP is controlled to be in the range of -50- -20°C in the soaking step; and the alloying temperature is controlled to be in the range of 420-490°C in the alloying step. The anti-powdering performance of the alloyed hot-dip galvanized steel plate manufactured by the above method is improved. The inventors have manufactured the alloyed hot-dip galvanized steel plate through in-depth research and found that the average Fe content in the coating is in the range of 7-10% by mass, and the surface layer of the coating contains columnar zinc-iron alloy phases, the proportion of the columnar zinc-iron alloy phases accounts for more than 90% of the total area of the alloy phases in the surface layer of the coating, and the length-width ratio R of the columnar zinc-iron alloy phases satisfies 2≤R<7. The inventors have also found through repeated experiments that when 2≤R<7 is satisfied, the anti-powdering performance of the steel plate is improved very obviously.

[0095] In some embodiments, the smelting, hot rolling, pickling, and cold rolling are as follows:

[0096] (1) Smelting: smelting according to the determined composition of the steel plate, and continuously casting the billet to produce the steel plate.

[0097] (2) Hot rolling: heating the billet to 1100-1250°C and controlling the rolling, with the opening rolling temperature being 950-1150°C, the final rolling temperature being 750-900°C, and the hot-rolled plate thickness being less than or equal to 20 mm.

[0098] (3) Pickling: pickling to remove the iron oxide scale generated in the hot rolling process.

[0099] (4) Cold rolling: the steel coil is cold rolled to a thickness of 2.0 mm or less, and the cold rolling reduction is ≥ 35%.

[0100] The present application improves the steel sheet manufacturing method, adjusts the process, controls the alloying of the plated layer and prevents over-alloying, controls the structure and proportion of the alloy phase on the plated layer surface, and improves the powdering resistance. The present application mainly improves the continuous annealing, hot galvanizing and alloying steps, and the steel sheet preferably has a Si content of 0.1 to 1.0% by mass. After the cold rolling is completed and the steel sheet substrate is obtained, the continuous annealing, hot galvanizing and alloying steps are performed in sequence.

[0101] (5) Continuous annealing: in the continuous annealing step, the steel strip is preferably passed through a preheating section and a soaking section, respectively, the annealing process is completed before reaching the plating section, and then the plating process is performed in the plating section.

[0102] (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 equipped with 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).

[0103] 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 above two options are mutually exclusive and cannot be used simultaneously. The two options are described below.

[0104] 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 0.90 to 1.0 (including the end point 0.90 and excluding the end point 1.0).

[0105] 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 (fuel 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 0.90 to 1.0, an effective thickness of the pre-oxidized layer can be formed on the surface of the steel sheet during the heating process in the preheating section. In the reducing atmosphere in the soaking section, the reduced iron layer on the surface of the steel sheet is not too thin, which is advantageous for improving the platability of the surface of the steel sheet, and Zn-Fe can also sufficiently diffuse during the subsequent alloying process. At the same time, the surface of the steel sheet does not form an excessive iron oxide layer, which can be sufficiently reduced during the subsequent reduction annealing process, and there is no residual iron oxide layer below the reduced iron layer, so the adhesion of the plated layer is good. Preferably, the air-fuel ratio can be further limited to the range of 0.90 to 0.95 (including the end values 0.90 and 0.95), which can further improve the platability of the surface of the steel sheet, improve the adhesion of the plated layer, and be advantageous for controlling the phase structure of the plated layer surface.

[0106] When the heating atmosphere in the heating furnace is an oxidizing atmosphere achieved by using a mixed gas of N2and O2, the manufacturing method of the present application controls the O2volume percentage to be in the range of 0.01 to 1.0% (including the end values 0.01% and 1.0%), which can sufficiently oxidize the steel sheet without over-oxidizing. Preferably, the O2volume percentage is in the range of 0.05 to 1.0% (including the end values 0.05% and 1.0%), which can prevent insufficient oxidation of the steel sheet due to a low oxygen content. More preferably, the O2volume percentage is in the range of 0.06 to 0.08% (including the end values 0.06% and 0.08%), which can further ensure the sufficiency of oxidation. At this time, the preheating section can also use an infrared radiation furnace for heating.

[0107] In addition, the heating temperature of the steel sheet in the preheating section is 600 to 750°C, which can form a pre-oxidized layer, so that the steel sheet can form an effective pure iron layer before galvanizing to improve its platability and alloying ability, and can also prevent the formed oxide layer from being too thick, prevent the formation of Si or Mn oxides from external oxidation, and avoid a decrease in the adhesion of the plated layer. In some embodiments, the heating temperature of the steel sheet in the preheating section is 600 to 705°C.

[0108] 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.

[0109] (b) Soaking: In the above embodiments, the continuous annealing step further includes a soaking section. Specifically, the steel sheet is heated and held (the holding time can be 5 to 30 s) at a soaking temperature of 750 to 850°C in an atmosphere in which the hydrogen concentration is 3 to 25% by volume and the dew point DP is in the range of -50 to -20°C, and the balance is N2and an unavoidable impurity atmosphere.

[0110] When the H2 concentration is in the range of 3 to 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 too high. Preferably, the H2 concentration is controlled in the range of 3 to 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.

[0111] The dew point DP of the soaking section is in the range of -50 to -20°C, and preferably in the range of -30 to -20°C (including the end values -30°C and -20°C). In order to obtain a columnar zinc-iron alloy coating with excellent powdering resistance, the present application has utilized the air-fuel ratio during the preheating process. The mechanism of pre-oxidation has formed a certain thickness of reduced iron layer on the surface of the steel sheet before galvanizing, thereby promoting Zn-Fe diffusion. At this time, the dew point DP of the soaking section is controlled in the range of -50 to -20°C, which can not only reduce the amount of Si, Mn and other oxides in the steel sheet matrix diffusing to the surface of the reduced iron, prevent the inhibition of Zn-Fe diffusion, and make the steel sheet easy to alloy, but also prevent Zn-Fe diffusion from being too sufficient, prevent the coating from being over-alloyed, and be beneficial to improving the powdering resistance. Preferably, the dew point DP of the soaking section is controlled in the range of -30 to -20°C, which is more conducive to controlling the degree of alloying, and can further improve the powdering resistance of the steel sheet.

[0112] In the continuous annealing step, by jointly controlling the air-fuel ratio of the preheating section and the dew point of the soaking section, the surface activity of the steel sheet can be controlled, which is conducive to forming an alloyed hot-dip galvanized coating with an average Fe content in the range of 7 to 10% by mass and a columnar zinc-iron alloy phase with an aspect ratio satisfying 2≤R<7 at an alloying temperature of 420 to 490°C in the subsequent alloying step.

[0113] (6) Hot-dip galvanizing: In each of the above embodiments, a hot-dip galvanizing step needs to be continued after the continuous annealing step. Specifically, the steel sheet needs to be cooled and then immersed in a zinc liquid with a temperature in the range of 440 to 465°C (including the end values 440°C and 465°C) for hot-dip galvanizing.

[0114] When the zinc liquid temperature is controlled in the range of 440 to 465°C (including the end values 440°C and 465°C), the viscosity of the zinc liquid is moderate, which is conducive to controlling the thickness of the coating, does not affect the appearance and alloying of the steel sheet, and the zinc liquid is not easy to evaporate, which can reduce the possibility of gaseous 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.

[0115] Preferably, the zinc liquid temperature can be in the range of 445 to 460°C, which is more conducive to controlling the thickness of the coating.

[0116] The zinc liquid can be a zinc liquid conventionally used in the art. In some embodiments, the Al content in the zinc liquid is in the range of 0.1-0.14%.

[0117] (7) Alloying: In each of the above embodiments, an alloying step is performed after the hot-dip galvanizing step. Specifically, the steel sheet is placed into an alloying furnace to perform alloying, the alloying temperature is controlled in the range of 420-490°C (including the end points 420°C and 490°C), and the alloying time is in the range of 10-30 s. Then, the steel sheet is cooled to 300°C by gas mist cooling, water mist cooling or other means, with an average cooling speed of not less than 15°C / s, preferably 20-40°C / s, and then air-cooled, water-quenched, leveled, straightened, post-processed, trimmed, oiled, divided, and coiled to obtain the desired alloyed hot-dip galvanized steel sheet having columnar zinc-iron alloy phase. At this time, the mutual diffusion of Zn and Fe can be ensured to obtain a qualified alloyed coating, and the proportion of the columnar zinc-iron alloy phase on the surface of the steel sheet can exceed 95%. At the same time, the thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanized layer of the manufactured steel sheet is not higher than 1 / 5 of the total thickness of the alloyed hot-dip galvanized layer, and the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% by mass at the interface between the alloyed hot-dip galvanized layer and the substrate is not more than 500 nm.

[0118] Preferably, the alloying temperature is controlled in the range of 440-460°C (including the end points 440°C and 460°C) according to the present application, which can make the proportion of the columnar zinc-iron alloy phase on the surface of the manufactured steel sheet reach 100%. At this time, the thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanized layer of the manufactured steel sheet is 1 / 15-1 / 10 of the total thickness of the alloyed hot-dip galvanized layer, and the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% by mass at the interface between the alloyed hot-dip galvanized layer and the substrate is in the range of 300 nm.

[0119] In addition, the alloying time is different on different continuous hot-dip galvanizing production lines, and the appropriate alloying time can be selected according to the length of the alloying furnace, the thickness of the strip steel, and the production speed of the strip steel. The manufacturing method according to the present application controls the alloying time in the range of 10-30 s (including the end points 10 s and 30 s), which can prevent over-alloying. Combined with the continuous annealing step and the hot-dip galvanizing step in any of the preceding embodiments, the alloyed hot-dip galvanized steel sheet with the proportion of the columnar zinc-iron alloy phase on the surface exceeding 95% can be obtained, and the aspect ratio R of the columnar zinc-iron alloy phase is also in the range of 2≤R<7, which is beneficial to ensure the anti-powdering performance of the steel sheet.

[0120] By precisely 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 steel plate manufacturing process, the manufacturing method of the above embodiments is conducive to coating and controlling the surface phase structure in the coating of the low-silicon steel plate, conducive to forming columnar zinc-iron alloy phases, thereby improving the anti-powdering performance of the steel plate. Moreover, the steel plate manufactured by the manufacturing method of the above embodiments of the present application has a fine and uniform columnar zinc-iron alloy phase on the surface of the steel plate after alloying, and no Γ phase is formed on the interface between the coating and the steel plate substrate, the anti-powdering performance is excellent, the cost is low, and it has good application prospects.

[0121] Embodiments

[0122] A steel plate substrate is manufactured by referring to the smelting, hot rolling, pickling, and cold rolling steps in CN117286396A. Table 3 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 embodiments and the comparative examples, and the rest is Fe. Among them, the Si content of J and K is greater than 1.0%, which is not the low-silicon steel described in the present application.

[0123] Table 3: Mass percentage of different embodiments and comparative examples of steel plate chemical composition

[0124] After the cold rolling is completed and the steel plate substrate is obtained, the continuous annealing, hot galvanizing, and alloying steps are sequentially performed as follows.

[0125] Continuous annealing: a continuous hot annealing galvanizing line with a preheating section, a soaking section, and a plating section is used, and the steel strip passes through the preheating section and the soaking section, respectively, and before reaching the plating section, the annealing process is completed, and then the plating process is performed in the plating section.

[0126] (a) Preheating: heat treating the hard-rolled plate in a direct-fired heating furnace with a direct-fired burner, controlling the air-fuel ratio in the heating furnace or controlling the oxygen volume content in the heating furnace, and heating the steel plate to a range of 600-750°C. Among them, the atmosphere in the heating furnace can choose to mix air and gas, at this time, the air-fuel ratio needs to be controlled in the range of 0.90-1.0; the atmosphere in the heating furnace can also choose to mix nitrogen and oxygen, at this time, the oxygen volume content needs to be controlled in the range of 0.01-1.0%.

[0127] (b) Soaking: heating and holding the steel plate at a soaking temperature of 750-850°C in an atmosphere of 3-25% by volume of hydrogen, a dew point DP of -50 to -20°C, and the rest being N2 and unavoidable impurities, for 5-30s.

[0128] Hot-dip galvanizing: after the steel sheet is cooled, it is put into a zinc bath with a temperature in the range of 440-465 °C, wherein the Al content in the zinc bath is in the range of 0.1-0.14%.

[0129] Alloying: the steel sheet is put into an alloying furnace for alloying, the alloying temperature is controlled in the range of 420-490 °C, and the alloying time is in the range of 10-30 s. Then the steel sheet is cooled to 300 °C by gas mist cooling, water mist cooling or other ways, with an average cooling speed no less than 15 °C / s, preferably 20-40 °C / s, and then air-cooled, water quenched, leveled, straightened, post-processed, trimmed, oiled, divided and coiled.

[0130] Table 4 shows the process parameters of annealing and alloying.

[0131] Table 4

[0132] The air-fuel ratio λ of the examples and comparative examples using “ / ” represents that the oxidizing atmosphere used in the examples or comparative examples is N2+O2 mixed gas, and no fuel gas is used. The oxygen gas volume percentage of the examples and comparative examples using “ / ” represents that the mixed gas used in the examples or comparative examples is a mixture of air and fuel gas. The air-fuel ratio and oxygen content of the comparative examples using “ / ” represent that no oxidizing atmosphere is used in the first preheating section of the comparative examples. Among them, no oxidizing atmosphere means that the steel sheet is preheated in an environment completely free of oxygen, for example, it can be preheated in an environment containing only fuel gas, or it can be preheated in an environment containing only nitrogen. Components A to I correspond to the chemical composition of the steel sheet in Table 3.

[0133] The Al content (%) in the alloying hot-dip plated layer of the steel sheet of each example and comparative example, the proportion (area ratio) of columnar zinc-iron alloy phase, the length-width ratio R of columnar zinc-iron alloy phase, the proportion of the thickness of columnar zinc-iron alloy phase in the total thickness of the alloying hot-dip galvanized layer, the Fe content in the plated layer, the tensile strength, the elongation at break, the average thickness of the interface Γ phase, and the powdering grade were detected, and the specific data are shown in Table 5. Among them, the proportion (area ratio) of columnar zinc-iron alloy phase, the length-width ratio R of columnar zinc-iron alloy phase, the Fe content in the plated layer and the differentiation grade were measured by the method described above. The Al content in the alloying hot-dip plated layer was measured by taking the average value of more than 5 points by EDS detection; the proportion of the thickness of columnar zinc-iron alloy phase in the total thickness of the alloying hot-dip galvanized layer and the average thickness of the interface Γ phase were measured 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, the elongation at break and other properties of the steel sheet were determined by GB / T 228.1-2021.

[0134] Table 5 Note: "NG" in the powdering grade means that the coating is not alloyed sufficiently. " / " in the length-width ratio of columnar phase means that there is no columnar phase in the coating.

[0135] With reference to Table 5, examples 2, comparative example 1 and comparative example 3 are taken as examples. With reference to Figures 2 and 3, Figure 2 is a SEM image of the surface morphology of the coating of example 2 of the present application, and Figure 3 is a SEM image of the cross-sectional morphology of the coating of example 2 of the present application. It can be seen from the figures that the proportion of columnar zinc-iron alloy phase in the surface phase structure of the coating is 100%, and the powdering grade is only 3, which indicates good powdering resistance.

[0136] In the comparative examples, Figure 4 shows a SEM image of the surface morphology of the coating of comparative example 1 of the present application, and Figure 5 shows a SEM image of the cross-sectional morphology of the coating of comparative example 1 of the present application; Figure 6 shows a SEM image of the surface morphology of the coating of comparative example 5 of the present application, and Figure 7 shows a SEM image of the cross-sectional morphology of the coating of comparative example 5 of the present application. It can be observed that the surface phase structure of the coating of the comparative examples is not uniform, with fine granular zinc-iron alloy phase and long strip-shaped zinc-iron alloy phase intermingled, and the powdering resistance is poor.

[0137] From examples 1-15, it can be seen that the average Fe content in the hot-dip galvanizing alloyed layer of the steel plate is in the range of 7-10% by mass, and contains columnar zinc-iron alloy phase, and the length-width ratio R of the columnar zinc-iron alloy phase satisfies 2≤R<7. In the manufacturing method adopted, the steel plate substrate has a Si content in the range of 0.1-1.0% by mass, the air-fuel ratio of the mixed gas of air and fuel gas used in the preheating stage is controlled in the range of 0.90-1.0, the heating atmosphere used in the preheating stage is an oxidizing atmosphere formed by the mixed gas of N2 and O2, and the oxygen volume percentage is controlled in the range of 0.05-1%; the soaking stage dew point DP is in the range of -50--20℃; and the alloying temperature is in the range of 420-490℃.

[0138] In the steel plate and its coating manufactured by the manufacturing method of the present application, the proportion of columnar zinc-iron alloy phase in the hot-dip galvanizing alloyed layer can satisfy more than 95% of the total area of the alloyed phase in the hot-dip galvanizing alloyed layer, and the powdering grade is not higher than 6. The greater the proportion of columnar zinc-iron alloy phase, the better the powdering resistance. When the proportion of columnar zinc-iron alloy phase reaches 100%, the powdering grade is further reduced, and the powdering grade is not higher than 5.

[0139] Some examples further satisfy that the thickness of the columnar zinc-iron alloy phase is 1 / 15-1 / 10 of the total thickness of the hot-dip galvanizing alloyed layer. At this time, the powdering grade of the steel plate can be further reduced, and is not higher than 5, and the powdering resistance is further improved.

[0140] Further, according to Table 5, in some embodiments, the interface between the coating and the substrate can not contain zinc-iron alloy phase with Fe content of more than 30% by mass, and in this case, the powdering level can reach level 1, and the anti-powdering performance is excellent.

[0141] Regarding Comparative Example 1, the air-fuel ratio in the preheating stage is 0.83, which is too low. Referring to FIG. 4, the steel sheet is not fully alloyed, and a large amount of pure zinc phase (η phase) exists in the coating. The area ratio of the columnar zinc-iron alloy phase in the coating is only 10%, and the thickness of the alloy phase shown in FIG. 4 is 1 / 2 of the total thickness of the alloyed hot-dip galvanized layer. The Fe content in the coating is 5.4% by mass, and the surface alloying of the coating is incomplete. For the alloyed steel sheet, the alloying state is marked as NG, i.e., not meeting the delivery requirements.

[0142] Regarding Comparative Example 2, no oxidizing atmosphere is used in the preheating stage (i.e., no oxygen is contained in the heating atmosphere), and pure fuel gas is used. The dew point DP, hydrogen concentration, and alloying temperature in the soaking stage and the alloying stage are controlled within a reasonable range, but the subsequent steps are similar to those of Comparative Example 1, and the surface alloying of the coating is incomplete. The area ratio of the columnar zinc-iron alloy phase is only 60%, and the Fe content in the coating is only 6.5% by mass. The alloying state of the steel sheet is also marked as NG, i.e., not meeting the delivery requirements.

[0143] Regarding Comparative Example 3, no oxidizing atmosphere is used in the preheating stage (i.e., no oxygen is contained in the heating atmosphere), and pure nitrogen gas is used. The air-fuel ratio is not controlled within a reasonable range, and only the dew point DP and the hydrogen concentration in the soaking stage are controlled within a reasonable range. However, the alloying temperature is higher than 500°C, and the Fe content in the obtained coating is too high. The aspect ratio of the columnar ζ phase is too low, and the thickness of the columnar ζ phase is too small. The Γ phase is formed at the interface between the steel sheet substrate and the coating, and there is no columnar zinc-iron alloy phase. The steel sheet is hard and brittle, and the powdering level is 8, i.e., the anti-powdering performance is poor.

[0144] Regarding Comparative Example 4, the steel sheet substrate used has a Si content of more than 1.0% by mass, which belongs to a high-silicon steel. The process used controls the air-fuel ratio in the preheating stage to be within the range of 0.90-1.0, the dew point DP in the soaking stage to be outside the range of -50- -20°C, and the alloying temperature to be within the range of 420-490°C. Due to the high Si content in the base component, the aspect ratio of the columnar ζ phase in the obtained coating is too low, the thickness is too large, there is no columnar zinc-iron alloy phase, the Fe content in the coating is too high, and the powdering level is 8, i.e., the anti-powdering performance is poor.

[0145] As for Comparative Example 5, the Si content in the steel sheet substrate is 1.6% by mass, which is a high-silicon steel. Since the Si content in the base component is too high, the alloying temperature in the process is controlled to be above 490°C. In addition, the air-fuel ratio is 1.1, which is also too high. The coating obtained in Comparative Example 5 has a low aspect ratio of alloy phase and a low thickness, and no columnar zinc-iron alloy phase exists. The Fe content in the coating is too high, and the powdering grade is 8, which is poor in powdering resistance.

[0146] In summary, the present application provides a method for manufacturing an alloyed hot-dip galvanized steel sheet. By controlling the atmosphere in the preheating and soaking stages of the continuous annealing process, specifically controlling the air-fuel ratio, the dew point DP, and the hydrogen concentration, and also controlling the alloying temperature in the alloying stage, the Fe content in the coating can be controlled within a lower range when a low-Si-content steel sheet is selected as the base material. The surface phase structure of the coating can be controlled, and the powdering resistance can be improved for wider subsequent applications.

[0147] Although the present application has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing is a description of the application in terms of preferred embodiments only and is not intended to limit the scope of the application, which is defined by the appended claims. Various changes in form and detail can be made thereto without departing from the spirit and scope of the application.

Claims

1. An alloyed hot-dip galvanized steel sheet, characterized in that, The substrate of the alloyed hot-dip galvanized steel sheet contains 0-1.0% Si by mass, the average Fe content in the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet is 7-10% by mass, and the surface phase structure of the alloyed hot-dip galvanized layer contains columnar zinc-iron alloy phase, the length-to-width ratio R of the columnar zinc-iron alloy phase satisfies: 2≤R<7.

2. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The columnar zinc-iron alloy phase in the alloyed hot-dip galvanized layer is a ζ phase; and / or the length-to-width ratio R of the columnar zinc-iron alloy phase satisfies: 3 ≤ R < 7.

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

4. The alloyed hot-dip galvanized steel sheet as described in claim 1, characterized in that, The thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanized layer is less than or equal to 1 / 5 of the total thickness of the alloyed hot-dip galvanized layer; preferably, the thickness of the columnar zinc-iron alloy phase in the alloyed hot-dip galvanized layer is 1 / 15 to 1 / 10 of the total thickness of the alloyed hot-dip galvanized layer.

5. 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 Fe content exceeding 30% by mass at the interface between the alloyed hot-dip galvanized layer and the substrate is less than or equal to 500 nm; preferably, the average thickness of the zinc-iron alloy phase with Fe content exceeding 30% by mass at the interface between the alloyed hot-dip galvanized layer and the substrate is in the range of 0 to 300 nm.

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

7. 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 contains, by weight, 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; preferably, the substrate of the alloyed hot-dip galvanized steel sheet also contains, by weight, 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%, and B: 0.0001-0.10%.

8. The alloyed hot-dip galvanized steel sheet according to any one of claims 1-7, characterized in that, The alloyed hot-dip galvanized steel sheet has a tensile strength ≥780 MPa, an elongation ≥10%, and a powdering grade ≤6; wherein, the powdering grade is determined by the reflectance or reflectance and width of the powder adhering to the tape in the V-bend powdering test method.

9. A method for manufacturing alloyed hot-dip galvanized steel sheet as described in any one of claims 1-8, characterized in that, The method includes a continuous annealing step and an alloying step for a substrate of alloyed hot-dip galvanized steel sheet containing 0-1.0% Si by mass; the continuous annealing step includes a preheating step and a homogenizing step, wherein, in the preheating step, when the heating atmosphere is air and fuel gas, the air-fuel ratio is controlled in the range of 0.90-1.0, or when the heating atmosphere is a mixture of N2 and O2, the volume percentage of O2 is controlled in the range of 0.05-1%; in the homogenizing step, the dew point DP is controlled in the range of -50℃ to -20℃; and in the alloying step, the alloying temperature is controlled in the range of 420-490℃.

10. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 9, characterized in that, The manufacturing method includes the following steps: smelting, casting, hot rolling, pickling, cold rolling, continuous annealing, hot-dip galvanizing, and alloying; The continuous annealing step includes: Preheating: The cold-rolled steel plate is heat-treated in a direct-fired heating furnace equipped with a direct-fired burner, and the steel plate is heated to a temperature range of 600-750°C; wherein, when the heating atmosphere is air and gas, the air-fuel ratio is controlled in the range of 0.90-1.0, or when the heating atmosphere is a mixture of N2 and O2, the volume percentage of O2 in the mixture is controlled in the range of 0.05-1%. Soaking: The preheated alloyed hot-dip galvanized steel sheet is heated and kept at a soaking temperature of 750-850°C in an atmosphere with hydrogen concentration limited to 3-25% by volume, dew point DP controlled within the range of -50°C to -20°C, and the balance being N2 and unavoidable impurities. The hot-dip galvanizing process includes: cooling the alloyed hot-dip galvanized steel sheet that has undergone continuous annealing, and then immersing it in a zinc bath at a temperature of 440–465°C for hot-dip galvanizing. The alloying process includes: placing the hot-dip galvanized alloyed hot-dip galvanized steel sheet into an alloying furnace for alloying, controlling the alloying temperature at 420-490℃, and then cooling and rolling it to obtain the desired alloyed hot-dip galvanized steel sheet.

11. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 9, characterized in that, The air-fuel ratio in the preheating step is controlled within the range of 0.90 to 0.95, or the O2 volume percentage is controlled within the range of 0.06 to 0.08%.

12. The method for manufacturing alloyed hot-dip galvanized steel sheet as described in claim 9, characterized in that, In the preheating step, the dew point DP is controlled to be ≥ -40℃.

13. The method for manufacturing hot-dip galvanized steel sheet as described in claim 9, characterized in that, The hydrogen concentration in the homogenization step is 3 to 10% by volume; and / or the dew point (DP) in the homogenization step is controlled within the range of -30 to -20°C.

14. The method for manufacturing hot-dip galvanized steel sheet as described in claim 9, characterized in that, The alloying temperature is controlled within the range of 440 to 460°C in the alloying step; and / or the alloying time is controlled within the range of 10 to 30 seconds in the alloying step.

Images