Pre-coated steel plate for hot forming and method of preparation thereof, and hot-formed steel member and its application.

Abstract

A pre-coated steel plate for hot forming and a method for preparing it, and a hot-formed steel member and an application thereof are disclosed in the present invention.In the pre-coated steel plate and in the method of preparing it, when the thickness of a pre-coating is 5-19 µm, the corresponding surface roughness of the pre-coating is defined, a hot-formed coated steel member can be obtained after hot forming and the coating on the hot-formed coated steel member has a high surface roughness of ra=1.80 µm; the steel member obtained has the advantages of light weight, high surface roughness and good paint adhesion and paint corrosion resistance, and a steel part made of the steel element of the present invention can be applied to a vehicle in order to reduce the vehicle's weight.

Description

1 / 35 “PRE-COATED STEEL PLATE FOR HOT FORMING AND METHOD OF PREPARATION THEREOF, AND HOT-FORMED STEEL MEMBER AND ITS APPLICATION” FIELD

[01] The present invention relates to a pre-coated steel plate for hot forming and method of preparing the same, and a hot-formed steel member and use thereof. STATE OF THE ART

[02] In recent years, as the automotive industry's requirements for safety, energy saving, and emissions have become increasingly stringent, vehicle lightness and improved safety have become the focus of the automotive industry. Generally, high-strength steel can be configured to achieve improvements in lightness and safety. However, when using a traditional cold stamping method, high-strength steel presents problems such as high rebound and difficulty in forming. In this case, hot stamping technology emerged. This technology separates forming and strengthening into two stages to produce very high-strength automotive parts, which offer advantages of very high strength, easy forming, and high forming precision.

[03] During the hot stamping process, in the case of using hot-formed steel, oxidation and decarburization can inevitably occur on the surface of the steel sheet, which affects the strength of the steel sheet. Furthermore, hot-formed parts need to be shot-blasted or pickled, which affects the dimensional accuracy of the product. In response to the above problems, coated hot-formed steel has become a conventional product in the current market. Currently, coated hot-formed steel products developed domestically and abroad mainly include aluminum-silicon (Al-Si) based products, zinc-based products (such as GI, GA), and similar products. Zinc-based coating is prone to embrittlement. Petition 870250085174, dated 09 / 21 / 2025, page 23 / 65 2 / 35 liquid zinc during high-temperature stamping, which leads to cracking in the stamping. The coating product currently being applied is primarily Al-Si. This product was first proposed by Arcelor Mittal and successfully applied in industrial experimental production, and has a typical coating composition of 87%Al-10%Si-3%Fe (percent by mass).

[04] Generally, to ensure a certain formability of a metal sheet, the surface of the material needs to have a certain surface roughness. The principle is that a reasonable microstructure on the surface of the material can better store lubricating oil, thus improving the formability of the material. Surface roughness refers to the small spacing and small irregularity between the peak and valley of the processed surface. Generally, the average surface roughness Ra is commonly set to characterize the surface roughness profile. The higher the surface roughness, the higher the Ra value.

[05] Products coated with Al-Si have good coating properties, such as phosphating and electrophoresis before heating. However, in the case where Al-Si coated products are used in the hot stamping process, due to mutual diffusion between the coating and the substrate during the heating process, Fe-Al and Fe-Al-Si alloy coatings are formed, which changes the coating properties and the phosphating performance of the coating is poor at this time. In order to ensure paint adhesion and subsequent corrosion resistance, such as electrophoresis, the surface roughness Ra of the coating after hot forming needs to be at a high value. Studies have shown that only when hot-formed steel with Al-Si coating exhibits a high surface roughness of the coating after hot forming can good paint adhesion and corrosion resistance be guaranteed.

[06] Arcelor Mittal's patent CN101583486B (hereinafter referred to as Patent 1) provides a coated steel strip and a product of Petition 870250085174, dated 09 / 21 / 2025, page 24 / 65 3 / 35 hot stamping prepared from it. The pre-coating of the coated steel strip according to Patent 1 has a typical composition of 8%-11% Si, 2%-4% Fe, and a balance of aluminum and inherent impurities, and a thickness of 20 μm to 33 μm, where 25 μm is generally used in the industry. Furthermore, the hot forming process window and the coating structure after hot forming are described in detail. The patent proposes that, if a certain thickness (not less than 20 μm) of the pre-coating is ensured, the surface roughness of the coating of an Al-Si product after hot forming is high. However, if the pre-coating thickness is small, less than 20 μm, the amount of coating liquefaction after heating is small.In this case, the liquefied coating manifests itself mainly as pitting, and a high surface roughness Ra cannot be guaranteed after heating, resulting in reduced adhesion of subsequent paints to the surface and reduced corrosion resistance.

[07] Patent CN108588612B (hereinafter referred to as Patent 2 provides a hot stamping element, a pre-coated steel plate for hot stamping, and a hot stamping process. The pre-coated steel plate provided by Patent 2 is pre-coated with an aluminum or aluminum alloy coating with a thickness of 3 μm to 19 μm. The hot forming process and the structure of the coating after hot forming are also specified. Patent 2 proposes that the surface roughness of the bonded layer is not determined by the thickness and structure of the bonded layer. As long as the pre-coating is liquefied during the heating process, this will result in significant surface roughness.However, the inventor of the present invention discovered that when the thickness of the pre-coating is small, such as the thickness range of the pre-coating in Patent 2, even if the coating is liquefied, the liquefied coating manifests itself mainly as well filling because of the amount of liquefaction of the coating after heating. Petition 870250085174, dated 09 / 21 / 2025, page 25 / 65 4 / 35 is small and large surface roughness Ra cannot be guaranteed after heating. Furthermore, Patent 2 proposes that the corrosion resistance performance of the coating after hot stamping is related only to layer α (interdiffusion layer). However, the investigation found that even if the formation of layer α is controlled, good ink adhesion and corrosion resistance cannot be guaranteed, which is related to the fact that it is not possible to guarantee a high surface roughness of the coating after heating.

[08] The small thickness of the aluminum or aluminum alloy pre-coating meets the current requirements for lightness in automobiles and is more economical and environmentally friendly. However, in a case where the pre-coating thickness is small, ensuring a high surface roughness of the coating after heating is crucial to improve paint adhesion and corrosion resistance of the aluminum or aluminum alloy pre-coating after hot forming. SUMMARY

[09] 1. Problems to be solved

[010] The present invention is based on the fact that existing pre-coated steel plates of aluminum or aluminum alloy cannot guarantee high surface roughness of the coating after heating when the pre-coating thickness is small. The objective of the present invention is to provide a pre-coated steel sheet for hot forming, a preparation method and process for controlling the pre-coated steel sheet, so that the coated steel member obtained from the hot forming of the pre-coated steel sheet has a high surface roughness of Ra > 1.80 μm. In addition, the steel member obtained has light weight, high surface roughness, good paint adhesion and paint corrosion resistance. Steel parts made from the steel member of the present invention can be configured in vehicles to make automobiles lighter. Petition 870250085174, dated 09 / 21 / 2025, page 26 / 65 5 / 35

[011] 2. Technical solutions

[012] To solve the above problems, the present invention adopts the following technical solutions.

[013] The inventor discovered through research that, to ensure good paint adhesion and corrosion resistance of the pre-coating (aluminum or aluminum alloy coating) after hot forming, the surface roughness of the coating after hot forming needs to be large. The thickness of the pre-coating of the present invention is greater than or equal to 5 μm and less than or equal to 19 μm. In the case where the thickness of the pre-coating is small, in order to ensure a large surface roughness requirement, the surface roughness of the coating after hot forming can be improved by increasing the surface roughness of the pre-coating. The thickness of the pre-coating in the present invention is greater than or equal to 5 μm. As if the thickness of the pre-coating is too thin (less than 5 μm), the coating thickness will be difficult to control during actual production and the surface quality of the coating will be poor.

[014] To ensure that the coating has Ra > 1.80 after hot forming, it is necessary to ensure a high surface roughness of the pre-coating. For example, in the case where the pre-coating thicknesses are 5 μm, 10 μm, 15 μm and 19 μm respectively, the Ra of the pre-coating is greater than or equal to 1.85, 2.00, 2.40 and 2.80 respectively. Furthermore, in the case where the surface roughness Ra of the pre-coating is very high, such as greater than 3.50, the surface roughness of the coating after hot forming is also very high, and the surface microstructure is very coarse, which can lead to a poor paint finish after painting. The pre-coating according to the present invention has a surface roughness Ra < 3.50.

[015] Specifically, the present invention provides a pre-coated steel plate, wherein a pre-coating is provided on at least one surface of a steel base, and the pre-coating has a thickness of Petition 870250085174, dated 09 / 21 / 2025, p. 27 / 65 6 / 35 μm to 19 μm, wherein in a case where the pre-coating has a thickness greater than or equal to 5 μm and less than or equal to 10 μm, the pre-coating thickness and surface roughness of the pre-coating are limited within the ABCD pattern, which comprises coordinates of the pre-coating thickness and surface roughness of the pre-coating defined by A (5 μm, 3.5 μm), B (10 μm, 3.5 μm), C (10 μm, 2.0 μm) and D (5 μm, 1.85 μm); In cases where the pre-coating thickness is greater than 10 μm and less than or equal to 15 μm, the pre-coating thickness and surface roughness of the pre-coating are defined within the BEFC standard, which comprises coordinates of the pre-coating thickness and surface roughness of the pre-coating defined by B (10 μm, 3.5 μm), E (15 μm, 3.5 μm), F (15 μm, 2.4 μm) and C (10 μm, 2.0 μm);In cases where the pre-coating thickness is greater than 15 μm and less than or equal to 19 μm, the pre-coating thickness and surface roughness are defined within the EGHF standard, which comprises coordinates for the pre-coating thickness and surface roughness defined by E (15 μm, 3.5 μm), G (19 μm, 3.5 μm), H (19 μm, 2.8 μm) and F (15 μm, 2.4 μm).

[016] In which, the surface roughness of the pre-coating is formed by the impression of the surface microgeometry of the rolling mill's work roll. In general, the surface roughness of the pre-coating can be adjusted by adjusting the rolling process parameters, such as the roll surface roughness, rolling force, and roll surface properties. It is worth noting that after the hot-dip galvanizing process, at least one surface of the substrate exhibits a pre-coating structure (aluminum or aluminum alloy coating). The pre-coating structure comprises, from the substrate side to the surface, an intermetallic compound layer (Fe2Al5, Fe2SiAl7) and an aluminum or aluminum alloy coating (unalloyed). The hardness of the intermetallic compound layer is significantly greater than that of the aluminum coating. In the case where the thickness of the pre-coating of the present Petition 870250085174, dated 09 / 21 / 2025, p. 28 / 65 7 / 35 The invention is small, less than or equal to 19 μm. As the thickness of the pre-coating decreases, the thickness of the aluminum coating becomes smaller, and the surface roughness of the pre-coating becomes smaller under the same conditions. High surface roughness can be achieved by increasing the surface roughness of the roll, increasing the rolling force, and the like. Therefore, a method is provided for preparing the aforementioned pre-coated steel plate, comprising steel production, continuous casting, hot rolling, pickling, and cold rolling, and a coating process. The coating process comprises a skin-passing process. In the skin-passing process, the surface roughness Ra of a roll is from 3.0 μm to 6.0 μm, and the rolling force is from 4,000 kN to 9,000 kN.

[017] In addition, in the coating process, a pre-coating is coated by a hot-dip coating method, vacuum evaporation or electroplating or other coating methods, provided that the method can achieve coating adhesion.

[018] Furthermore, in the coating process, a coating liquid comprises, by mass percent, 8-11% Si, 2-4% Fe and the remainder aluminum or aluminum alloy and unavoidable impurities.

[019] In addition, the coating process comprises a continuous annealing process, which has the following parameters: the annealing is conducted at a controlled temperature of 720°C to 850°C, and maintained at this temperature for 60 s to 120 s under a reducing atmosphere comprising 5 ~ 10% H2 + N2 by volume percentage (i.e., in terms of volume percentage of the reducing atmosphere, H2 accounts for 5 ~ 10% and the remainder is N2), the oxygen content is controlled below 20 ppm, and the dew point is controlled from -60°C to 0°C.

[020] A hot-formed coated steel member is provided, which is prepared from the aforementioned pre-coated steel plate through a hot forming process, in which the roughness of Petition 870250085174, dated 09 / 21 / 2025, page 29 / 65 8 / 35 steel member surface meets Ra > 1.80 μm.

[021] In addition, the hot-formed coated steel member is produced from steel plate pre-coated with aluminum or aluminum alloy coating by heating, hot stamping or other heat treatment operations, and the heating may be conducted by a resistance heating method, radiant heating or induction heating, which is not limited to the above method.

[022] Furthermore, the coating structure after hot forming consists, from the steel base to the surface, of only a first layer; wherein the first layer is located on the side of the steel base, i.e., the outermost side, and the first layer is an interdiffusion layer containing Al and Si (α-Fe and Fe3Al), comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight and the remainder Al.

[023] Furthermore, the coating structure after hot forming consists, from the steel base to the surface, of a first layer and a second layer sequentially:

[024] the first layer is located on the side of the steel base, that is, the innermost side, and the first layer is an interdiffusion layer containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight, and the remainder Al,

[025] the second layer is located outside the first layer, and is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight, and Al balance.

[026] Furthermore, the coating structure after hot forming consists, from the steel base to the surface, of a first layer, a second layer and a discontinuous surface layer sequentially:

[027] the first layer is located on the side of the steel base, that is, the innermost side, and the first layer is a layer of Petition 870250085174, dated 09 / 21 / 2025, p. 30 / 65 9 / 35 interdiffusion containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight, and the remainder Al,

[028] the second layer is located outside the first layer, and is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight, and Al in balance,

[029] the surface layer is located outside the second, discontinuous layer, and comprises a first intermetallic compound containing Al and Si and a second intermetallic compound containing Al and Si, wherein the first intermetallic compound layer comprises 55% to 79% Fe by weight, greater than or equal to 2% Si by weight and the remainder Al, and the second intermetallic compound comprises 30% to 50% Fe by weight, less than or equal to 2% Si by weight and the remainder Al.

[030] Furthermore, the coating structure after hot forming consists, from the steel base to the surface, of a first layer, a second layer and a third layer sequentially:

[031] the first layer is located on the side of the steel base, that is, the innermost side, and the first layer is an interdiffusion layer containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight, and the remainder Al,

[032] the second layer is located outside the first layer, and is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% Fe by weight, less than or equal to 2% Si by weight, and an equilibrium of Al,

[033] the third layer is located outside the second layer, and is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight, and Al balance.

[034] In addition, the coating structure after the Petition 870250085174, dated 09 / 21 / 2025, page 31 / 65 10 / 35 Hot forming consists, from the steel base to the surface, of a first layer, a second layer, a third layer, and a discontinuous surface layer sequentially:

[035] the first layer is located on the side of the steel base, that is, the innermost side, and the first layer is an interdiffusion layer containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight, and the remainder Al,

[036] the second layer is located outside the first layer, and is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% Fe by weight, less than or equal to 2% Si by weight, and an equilibrium of Al,

[037] the third layer is located outside the second layer, and is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight, and an equilibrium of Al,

[038] the surface layer is located outside the third, discontinuous layer, and comprises a first intermetallic compound containing Al and Si and a second intermetallic compound containing Al and Si, wherein the first intermetallic compound layer comprises 55% to 79% Fe by weight, greater than or equal to 2% Si by weight and the remainder Al, and the second intermetallic compound comprises 30% to 50% Fe by weight, less than or equal to 2% Si by weight and the remainder Al.

[039] Furthermore, the coating structure after hot forming consists, from the steel base to the surface, of a first layer, a second layer, a third layer and a fourth layer sequentially:

[040] the first layer is located on the side of the steel base, that is, the innermost side, and the first layer is an interdiffusion layer containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight, and the remainder Al, Petition 870250085174, dated 09 / 21 / 2025, p. 32 / 65 11 / 35

[041] the second layer is located outside the first layer, and is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% Fe by weight, less than or equal to 2% Si by weight, and Al in balance,

[042] the third layer is located outside the second layer, and is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight, and Al in balance,

[043] the fourth layer is located outside the third layer, and is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% Fe by weight, less than or equal to 2% Si by weight, and Al in balance.

[044] In addition, the hot-formed coated steel member has a total thickness of 0.4 mm to 3.0 mm.

[045] Furthermore, the chemical composition of the steel base is not limited, including low carbon steel, medium carbon steel, high carbon steel and the like. However, it should be noted that an excessively high content of carbon or alloying elements in the steel base causes problems such as difficulty in welding during production and poor coating quality.

[046] A steel product is supplied, comprising the aforementioned hot-formed coated steel member.

[047] A motorized land vehicle is provided, comprising the aforementioned coated hot-formed steel member.

[048]

[049] 3. Beneficial effects In comparison with existing pre-coated aluminum or aluminum alloy steel plates, where a thin pre-coating makes it impossible to guarantee high surface roughness after hot forming, the present invention provides a pre-coated steel plate and a method for preparing it. A member Petition 870250085174, dated 09 / 21 / 2025, p. 33 / 65 A 12 / 35 hot-formed coated steel member can be prepared from pre-coated steel plate prepared by controlling the method after hot forming. The coating on the hot-formed coated steel member has a high surface roughness Ra > 1.80 μm, and the resulting steel member has light weight, high surface roughness, good paint adhesion, and paint corrosion resistance. Steel parts prepared using the steel member of the present invention are applied in vehicles to make the vehicles lighter.

[050] After thorough research, the inventor discovered that the change in surface roughness of the aluminum or aluminum alloy coating after heating occurs mainly during the liquefaction and solidification process of the coating. The surface roughness of the coating after heating is mainly divided into two processes: one is that the liquefaction coating fills the pits (tiny spacing, “valley”); the other is that the liquefied coating becomes rough. In a case where the pre-coating thickness is large, such as not less than 20 μm, the amount of liquefaction of the coating after heating is large. In this case, the liquefied coating remains rough after filling the pits, and the surface roughness Ra is large. In the case where the pre-coating thickness is small, the amount of liquefaction of the coating after heating is small.In this case, the liquefied coating manifests itself mainly as filling pits, and a large surface roughness Ra cannot be guaranteed after heating.

[051] Therefore, the surface roughness Ra of the coating of Al-Si after heating is not only related to the thickness of the pre-coating, but also to the surface roughness Ra of the pre-coating. In a case where the pre-coating thickness is large, such as not less than 20 μm, the surface roughness Ra of the pre-coating has a relatively small impact on the surface roughness Ra of the coating after heating. In the case where the surface roughness Ra of the pre-coating is within a normal range between 1.00 and 2.00, the liquefied coating Petition 870250085174, dated 09 / 21 / 2025, p. 34 / 65 13 / 35 fills the holes and remains rough, in which case the surface roughness Ra is large. In the case where the pre-coating thickness is small, the surface roughness Ra of the pre-coating has a large impact on the surface roughness Ra of the coating after heating. The liquefied coating manifests itself mainly as pit filling. In this case, the surface roughness Ra of the pre-coating can be increased so that the liquefied coating incompletely fills the pits, or the surface roughness Ra of the pre-coating can be minimized so that the liquefied coating can remain rough after filling the pits to improve the surface roughness Ra of the coating after heating.Generally, there are two ways to ensure a high surface roughness Ra of the Al-Si coating after heating: one is to increase the pre-coating thickness, such as by at least 20 μm; the other is to increase the surface roughness Ra of the pre-coating or reduce the surface roughness Ra of the pre-coating in the case where the pre-coating thickness is small. However, studies have found that in the case where the surface roughness Ra of the pre-coating is small, less than 1.00, and the pre-coating thickness is less than 20 μm, the surface roughness Ra after heating cannot be guaranteed to be > 1.80. The results are shown in Table 1. Table 1 Surface roughness of pre-coatings with different thicknesses before and after heating (low surface roughness of the pre-coating) Pre-coating thicknesses (pm) 7 10 15 17 20 Surface roughness Ra before heating (pm) 0.94 0.99 0.44 0.69 0.99 Surface roughness Ra after heating (pm) 0.88 1.08 1.33 1.48 1.86

[052] Furthermore, the inventor discovered that the specific heating process has little effect on the surface roughness Ra of Petition 870250085174, dated 09 / 21 / 2025, page 35 / 65 14 / 35 coating after heating. The effects of common heating temperature and heating duration on the roughness of the pre-coating with thicknesses of 5 μm, 10 μm, 15 μm, 19 μm and 25 μm after heating are shown in Figure 1 and Figure 2, where the 5 μm thick pre-coating has an average roughness Ra of 1.69, the 10 μm thick pre-coating has an average roughness Ra of 1.67, the 15 μm thick pre-coating has an average roughness Ra of 1.69, the 19 μm thick pre-coating has an average roughness Ra of 1.72, and the 25 μm thick pre-coating has an average roughness Ra of 1.70.

[053] The inventor discovered through research that in the case where the thickness of the pre-coating is greater than or equal to 5 μm and less than or equal to 19 μm, as the heating temperature increases or the heating duration is prolonged, i.e., as the alloy grade of the coating increases, the coating structure changes regularly:

[054] (1) If the heating temperature is too low or the heating duration is too short, the coating is not fully bonded and aluminum or Fe2SiAl7 is usually present on the surface.

[055] (2) If the coating starts to be fully bonded, the coating structure has a four-layer structure from the steel base to the surface, which are an interdiffusion layer (α-Fe and Fe3Al), a second intermetallic compound layer (Fe-Al intermetallic compound layer), a first intermetallic compound layer (Fe-Al-Si intermetallic compound layer) and a second intermetallic compound layer (or surface layer, Fe-Al intermetallic compound);

[056] in which the interdiffusion layer comprises α-Fe and Fe3Al containing Al and Si, wherein the Fe content is greater than or equal to 80% by weight, the Si content is less than or equal to 5% by weight, and the remainder is Al;

[057] in the second layer of intermetallic compound (that is, the Fe-Al intermetallic compound layer and the surface layer), the Fe content is Petition 870250085174, dated 09 / 21 / 2025, page 36 / 65 15 / 35 of 30% to 50% by weight, the Si content is less than or equal to 2% by weight, and the rest is Al;

[058] in the first layer of intermetallic compound (Fe-Al-Si intermetallic compound layer), the Fe content is 55% to 79% by weight, the Si content is greater than or equal to 2% by weight, and the remainder is Al.

[059] (3) In the case where the alloy grade is further improved, the interdiffusion layer and the first intermetallic compound layer (Fe-Al-Si intermetallic compound layer) gradually become thicker. In the case where part of the first intermetallic compound layer extends to the surface, the surface layer of the coating is discontinuous and the coating structure is a three-layer structure and a discontinuous surface layer from the steel base to the surface, i.e., a diffusion layer (α-Fe and Fe3Al), a second intermetallic compound layer, a first intermetallic compound layer and a discontinuous surface layer. In this case, the discontinuous surface layer is a mixed structure containing the first intermetallic compound and the second intermetallic compound.In the case where the first layer of intermetallic compound completely occupies the surface, the coating structure is a three-layer structure from the steel base to the surface, namely, a diffusion layer (α-Fe and Fe3Al), a second intermetallic composite layer, and a first intermetallic composite layer.

[060] (4) In the case where the interdiffusion layer and the first intermetallic compound layer (Fe-Al-Si intermetallic compound layer) are further thickened, the second intermetallic compound layer between the interdiffusion layer and the first intermetallic compound layer is completely transformed into an interdiffusion layer or Fe-Al-Si intermetallic compound layer. In this case, the coating structure is a two-layer structure or more discontinuous surface layer from the steel base to the surface, i.e., an interdiffusion layer and a first intermetallic compound layer or more discontinuous surface layer. Petition 870250085174, dated 09 / 21 / 2025, p. 37 / 65 16 / 35 In the case of a discontinuous surface layer, the discontinuous surface layer is a mixed structure containing a first intermetallic compound and a second intermetallic compound. [061 ] (5) In the case where the coating continues to be bonded, the Fe content in the coating increases even further, and the first layer of intermetallic compound (Fe-Al-Si intermetallic compound layer) gradually transforms into Fe3Al. In this case, the coating consists of only one interdiffusion layer.

[062] Furthermore, the inventor discovered through research that the coating structure after heating has little impact on the surface roughness Ra of the coating. In the study, the pre-coating thickness is selected as 10 μm, the total thickness of the steel plate is selected as 1.4 mm, the average measured roughness Ra of the pre-coating is 1.67, the heating temperature is 930°C, and the heating duration is selected as 1 min, 3 min, 5 min, 7 min, 10 min, and 16 min. The roughness after heating was measured, and the structure and composition of the coating after heating were analyzed. The roughness after heating is shown in Table 2. The composition of each layer of the coating after heating is shown in Table 3. The coating structure is shown in Figure 3.

[063] From the point of view of the coating structure after heating, in the case where the heating duration is 1 min, the coating is not fully bonded and there is still an unbonded aluminum coating on the surface. In the case where the heating duration is 3 minutes, the coating is fully bonded and the coating structure is a four-layer structure from the steel base to the surface, including an interdiffusion layer (α-Fe and Fe3Al), a second intermetallic compound layer (Fe-Al intermetallic compound layer), a first intermetallic compound layer (Fe-Al-Si intermetallic compound layer) and a surface layer (Fe-Al intermetallic compound). In the case where the heating duration is 5 minutes, the coating transforms into three layers. Petition 870250085174, dated 09 / 21 / 2025, p. 38 / 65 17 / 35 and a discontinuous surface layer. The coating structure comprises, from the steel base to the surface, an interdiffusion layer (α-Fe and Fe3Al), a second intermetallic compound layer (Fe-Al intermetallic compound layer), a first intermetallic compound layer (Fe-Al / Si intermetallic compound layer), and a discontinuous surface layer. In the case where the heating duration is 7 minutes, the coating structure transforms into two layers, comprising, from the steel base to the surface, an interdiffusion layer (α-Fe and Fe3Al) and a first intermetallic compound layer (Fe-Al-Si intermetallic compound layer). In the case where the heating duration is 10 minutes, the coating still comprises two layers.The coating structure comprises, from the steel base to the surface, an interdiffusion layer (α-Fe and Fe3Al) and a first layer of intermetallic compound (Fe-Al-Si intermetallic compound layer). In the case where the heating duration is 16 minutes, the coating transforms into a single layer, and the coating consists of only one interdiffusion layer (α-Fe and Fe3Al).

[064] From the point of view of the surface roughness Ra of the coating after heating, in the case where the heating duration is 1 minute, the surface roughness Ra of the coating after heating is 1.62, which is slightly lower than the surface roughness Ra of 1.67 before heating. This is because there is still unalloyed aluminum coating in the coating, which leads to a small liquefaction of the coating and pit filling. In the case where the heating duration is greater than or equal to 3 minutes, the coating becomes fully bonded. In this case, as the heating duration increases, the coating structure transforms from four layers to three layers or more, a discontinuous surface layer, two layers, or one layer. In this case, the surface roughness Ra of the coating after heating changes little.Generally, the coating structure after heating has little impact on the surface roughness Ra of the coating after heating. Petition 870250085174, dated 09 / 21 / 2025, page 39 / 65 18 / 35 Table 2 Surface roughness Ra of the coating under different heating durations Heating time (min) 1 3 5 7 10 16 Surface roughness Ra after heating (μπ) 1.62 1.53 1.52 1.54 1.52 1.55 Table 3 Composition of each coating layer under different heating durations Heating time / min Analysis layer Element (% by weight) Phase Fe Al Si 1 Base side 32.9 56.6 10.5 Fe2SiAl7 Surface layer 5.6 92.6 1.8 Al 3 Interdiffusion layer 95.8 3.2 1.0 α-Fe+FesAl Fe-Al layer 47.0 51.4 1.6 FeaAls Fe-Al-Si layer 63.6 26.7 9.7 FeaSiAb Continuous surface layer 46.9 51.6 1.5 FeaAls 5 Interdiffusion layer 84.0 12.4 3.6 FesAl Fe-Al layer 48.2 50.5 1.3 FeaAls Fe-Al-Si layer 64.8 30.0 5.2 Fe6SiAl6 Discontinuous surface layer 47.6 51.5 0.9 FeaAls 7 Interdiffusion layer 92.0 6.3 1.7 α-Fe+FeaAl Fe-Al-Si layer 66.1 30.5 3.4 Fe10SiAl9 10 Interdiffusion layer 93.3 5.5 1.2 α-Fe+FeaAl Fe-Al-Si layer 68.4 29.3 2.3 Fe15SiAl13 16 Interdiffusion layer 85.0 12.4 2.6 Fe3Al

[065] Generally, for aluminum or aluminum alloy coatings, the heating process or the coating structure after heating has little impact on the surface roughness Ra of the coating after heating. The surface roughness Ra of the coating after heating is mainly related to the thickness of the pre-coating and the surface roughness of the pre-coating. In a case where the thickness of the pre-coating is large, the surface roughness Ra of the pre-coating has a relatively small impact on the surface roughness Ra of the coating after heating. The surface roughness Ra of the coating Petition 870250085174, dated 09 / 21 / 2025, p. 40 / 65 19 / 35 after heating is large. However, if the surface roughness Ra of the pre-coating is small, the surface roughness Ra of the pre-coating has a large impact on the surface roughness Ra of the coating after heating. If the surface roughness Ra of the pre-coating is large, the surface roughness Ra of the coating after heating can be guaranteed to be > 1.80, thus ensuring good paint adhesion and corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[066] Figure 1 shows the relationship of the surface roughness Ra of the coating after heating changing along the heating temperature with a pre-coating thickness of 5 μm, 10 μm, 15 μm, 19 μm and 25 μm.

[067] Figure 2 shows the relationship of the surface roughness Ra of the coating after heating changing over the heating duration with a pre-coating thickness of 5 μm, 10 μm, 15 μm, 19 μm and 25 μm.

[068] Figure 3 shows the relationship of the coating structure after heating to 930°C varying throughout the heating duration with a pre-coating thickness of 10 μm and a total steel sheet thickness of 1.4 mm.

[069] Figure 4 shows the coating structure before heating with pre-coating thicknesses of 5 μm, 10 μm, 15 μm, 19 μm and 25 μm.

[070] Figure 5 shows the typical surface morphology of the pre-coating.

[071] Figure 6 shows the surface morphology of the coating after heating with a pre-coating thickness of 5 μm, 10 μm, 15 μm, 19 μm and 25 μm.

[072] Figure 7 shows the limited range of pre-coating thickness and pre-coating surface roughness of the present Petition 870250085174, dated 09 / 21 / 2025, page 41 / 65 20 / 35 invention, wherein the pre-coating has a thickness greater than or equal to 5 μm and less than or equal to 19 μm. DETAILED DESCRIPTION OF THE MODALITIES

[073] The present invention can be described below with reference to specific examples and drawings. The following examples or experimental data are intended to illustrate the present invention. It should be clear to those skilled in the art that the present invention is not limited to these examples or experimental data.

[074] First, the steel plate pre-coated with aluminum or aluminum alloy coating of the present invention has a thickness of 0.4 mm to 3.0 mm, wherein the pre-coating has a thickness of 5 μm to 19 μm. This example provides steel plates with thicknesses of 0.8 mm, 1.4 mm, and 2.0 mm, respectively. The pre-coatings have thicknesses of 5 μm (in the present invention), 10 μm (in the present invention), 15 μm (in the present invention), 19 μm (in the present invention), and 25 μm (comparison), wherein the pre-coating is applied to the upper and lower surfaces of the steel plate.

[075] 22MnB5, commonly used in the market, is used as the steel base of the steel plate of the present invention. The pre-coating can be obtained by hot-dip galvanizing. A typical hot-dip galvanizing solution comprises (by mass percent) 8% to 11% Si, 2% to 4% Fe, and the remainder aluminum or aluminum alloy and unavoidable impurities. However, the present invention is not limited to this galvanizing method and galvanizing solution composition. Other galvanizing methods or other aluminum or aluminum alloy compositions may be used. The main function of the Si element in the galvanizing solution is to form an Fe-Al-Si inhibition layer on the surface of the steel plate, which can effectively prevent the formation of a brittle Fe2Al5 phase and improve the formability of the coating. In the case where the Si content exceeds 10%, this effect is significantly weakened.Experimental research shows that it is generally appropriate to control the Si content in liquid aluminum to between 8%. Petition 870250085174, dated 09 / 21 / 2025, page 42 / 65 21 / 35 and 11%. Fe has different solubility at different coating solution temperatures. The conventional temperature of an aluminum-silicon coating solution is 640°C to 680°C, in which case the solubility of Fe in the coating solution is 2% to 4%.

[076] By way of example, the steel base and pre-coating of the present invention have compositions shown in Table 4. Table 4 Composition of the steel base and pre-coating of the present invention Chemical component (% by weight) C Mn Pb Ti Cr Al Si Fe Steel base 0.23 1.27 0.015 0.0025 0.035 0.15 0.038 0.25 Bal. Pre-coating - - - - - - Bal. 9.0 2.2

[077] In which, Bal. represents the remainder in addition to other elements.

[078] The pre-coated steel sheet with aluminum or aluminum alloy coating of the present invention can be produced through the following process: steelmaking ^ continuous casting ^ hot rolling ^ pickling and cold rolling ^ coating process.

[079] (1) Steelmaking comprises pretreatment of liquid iron, converter casting, alloy fine-tuning and refining to ultimately obtain pure steel.

[080] (2) Continuous casting comprises injecting refined molten steel into a distributor, then distributing the molten steel into each crystallizer by the distributor, forming the casting, crystallizing, then removing the casting and cutting it into a plate of a given length.

[081] (3) Hot rolling comprises hot rolling of the plate above 1000°C to 1300°C, wherein the temperature of the plate exiting the heating furnace is controlled above 1100°C, and the final rolling temperature is controlled above 600°C to obtain a steel sheet. Petition 870250085174, dated 09 / 21 / 2025, page 43 / 65 22 / 35 hot-rolled; hot-rolled steel plate wound above, where the winding is conducted at a controlled temperature below 800°C.

[082] (4) Pickling and cold rolling comprise pickling and cold rolling of hot-rolled steel sheet to obtain a pickled and cold-rolled steel sheet.

[083] (5) The coating process comprises substrate cleaning ^ continuous annealing ^ hot dip coating ^ coating thickness control ^ cooling after coating ^ skin passing ^ straightening ^ lubrication ^ winding and unloading.

[084] (a) Substrate cleaning: To ensure good coating quality, the amount of residual oil on one side of the steel plate after cleaning is controlled below 20 mg / m2, and the amount of residual iron on one side of the steel plate is controlled below 10 mg / m2.

[085] (b) Continuous annealing: The annealing temperature and atmosphere are crucial for the quality, product structure and performance of the hot-dip coating. Annealing is conducted at a controlled temperature of 720°C to 850°C, and maintained at this temperature for 60 s to 120 s, a reducing atmosphere of 5% to 10% H2+N2 (by volume percent) is used, the oxygen content is controlled below 20 ppm and the dew point is controlled between -60°C and 0°C.

[086] (c) Hot-dip plating: The immersion coating is carried out at a controlled temperature of 640°C to 680°C for 2 s 8 s.

[087] (d) Coating thickness control: The coating thickness is controlled by blowing nitrogen with an air knife or by compressing air. The coating thickness of the present invention is controlled from 5 μm to 19 μm on one side.

[088] (e) Cooling after plating: Cooling after galvanizing is carried out by air cooling. The steel plate is cooled Petition 870250085174, dated 09 / 21 / 2025, page 44 / 65 23 / 35 below 300°C and then cooled with water to below 100°C.

[089] (f) Skin passing: According to the roughness requirements of the pre-coating, the parameters of the skin passing roll process are adjusted for production. This process also has the effect of improving the mechanical performance and flatness of the steel strip.

[090] (g) Straightening: After straightening the steel strip, the flatness of the steel strip is further improved. [091 ] (h) Lubrication: The surface of the steel strip is coated with anti-rust oil by means of an electrostatic lubricator.

[092] (i) Winding and unloading: The steel strip is wound and tensioned and finally transformed into a coil and unloaded.

[093] The structure of the pre-coating produced by the above process is shown in Figure 4. The thickness of the pre-coatings according to the present invention is, from left to right, 5 μm, 10 μm, 15 μm, 19 μm and 25 μm (comparison) respectively.

[094] After analysis, the base structure of each steel plate is a ferrite and pearlite structure. The pre-coating on the base side for the surface is respectively an intermetallic compound layer (Fe2Al5, Fe2SiAl7) and an aluminum coating. The thickness of the intermetallic compound layer of each pre-coating is approximately 4.5 μm (the thickness of the Fe2Al5 layer is less than 0.5 μm). The difference lies in the thickness of the aluminum coating. The thickness of the aluminum coating of each pre-coating is respectively 0.5 μm, 5.5 μm, 10.5 μm, 14.5 μm, and 20.5 μm.

[095] For the aforementioned steel plates with base plate thicknesses of 0.8 mm, 1.4 mm, and 2.0 mm and pre-coating thicknesses of 5 μm, 10 μm, 15 μm, 19 μm, and 25 μm, respectively, the surface-passing roller process parameters are adjusted to produce pre-coated steel plates with different surface roughness Ra values. It is worth noting that since the surface hardness of the aluminum or aluminum alloy pre-coating is soft and the Vickers hardness is less than 100, in Petition 870250085174, dated 09 / 21 / 2025, page 45 / 65 24 / 35 In the case of the aluminum or aluminum alloy pre-coated steel plate being finished, the roughness of the skin-passing roll can be partially transferred to the aluminum or aluminum alloy pre-coated steel plate. Generally, the transfer rate (roughness of the aluminum or aluminum alloy pre-coating / roughness of the skin-passing roll) does not exceed 80%. As the rolling force passing through the skin increases, the transfer rate increases. As the skin-passing roll wears, the transfer rate gradually decreases. The present invention produces a pre-coated steel plate with a high Ra value of surface roughness, which can be achieved by increasing the roughness of the skin-passing roll or by increasing the skin-passing rolling force. Furthermore, it should be noted that the thickness of the pre-coating of the present invention is small, less than or equal to 19 μm.As the thickness of the pre-coating decreases, the thickness of the aluminum coating decreases, improving surface roughness becomes more difficult to achieve, and the requirements for roller roughness and rolling strength become higher.

[096] To measure the surface roughness of pre-coated steel plate, the width direction of the steel plate (i.e., the width direction of the steel strip) is generally selected to be measured 3 times along the edge, middle, and edge, respectively, for a total of 9 times, and finally, the average value is obtained. The GB / T2523-2008 measurement method for surface roughness and peak number of cold-rolled metal sheets (strips) is adopted as the measurement standard, where the cutting wavelength is 0.8 mm, and the test distance is 4.8 mm. Pre-coatings with different surface roughness Ra values ​​have similar surface morphologies, all exhibiting a certain number of depressions on the surface, and the difference is that the higher the surface roughness Ra value of the pre-coating, the greater the depth of the pits. The typical surface morphology of the pre-coating is shown in Figure 5 (Example 24).

[097] A suitable heating process for the plate of Petition 870250085174, dated 09 / 21 / 2025, pp. 46 / 65 The aforementioned 25 / 35 pre-coated steel is selected to ensure that the pre-coating is fully bonded (there is no unalloyed aluminum coating). Generally, the heating temperature is not lower than 860°C and the heating duration is not lower than 3 minutes. If the thickness of the steel plate is large or the thickness of the pre-coating is large, the heating temperature or the heating duration should be increased accordingly. In the examples of the present invention, a resistance heating furnace is adopted. First, the width direction of the steel plate (i.e., the width direction of the steel strip) is selected to process the steel plate along the edge, middle, and border on a sample plate with a size of 150*300 mm, respectively.After placing the sample plate in the heating furnace for a specified period of time, the sample plate is removed, placed in a flat plate quenching mold and pressed, and the pressure is maintained for a specified period of time. Then, the surface roughness of the steel plate after heating is measured according to the same measurement method, and finally, the average value is obtained.

[098] The surface roughness Ra of the selected pre-coatings is similar (approximately 1.70) and the thickness of the pre-coatings is 5 μm, 10 μm, 15 μm, 19 μm and 25 μm, respectively. The surface roughness of the coatings before and after heating is shown in Table 5. The surface morphology of the coatings after heating is shown in Figure 6. Table 5 Surface roughness Ra of the coating before and after heating with the pre-coating having a thickness of 5 μm, 10 μm, 15 μm and 25 μm Pre-coating thickness (pm) 5 10 15 19 25 Ra before heating (pm) 1.69 1.67 1.69 1.72 1.70 Petition 870250085174, dated 09 / 21 / 2025, page 47 / 65 26 / 35 Ra after heating (Mm) 1.68 1.52 1.20 1.65 2.02

[099] It can be observed from the surface roughness of the coating before and after heating that, in the case where the pre-coating thickness is less than 20 μm, a large surface roughness Ra cannot be guaranteed after heating. In the case where the pre-coating thickness is 5 μm, 10 μm and 15 μm, Ra after heating decreases by 0.01, 0.15 and 0.49, respectively, compared to before heating, i.e., as the coating thickness increases, the downward trend of Ra after heating becomes more significant than before heating. In the case where the pre-coating thickness is 19 μm, Ra after heating is equivalent to before heating. In the case where the pre-coating thickness is 25 μm, Ra increases significantly after heating.It can be observed from the surface morphology of the coating after heating that, in the case where the pre-coating thickness is less than 20 μm, the coating does not become completely rough after heating. In this case, the coating exhibits a slight surface roughness. In the case where the pre-coating has a thickness of 5 μm and 10 μm, the coating after heating still exhibits a pitting morphology similar to that of the pre-coating. In the case where the pre-coating has a thickness of 15 μm, the coating after heating does not exhibit pitting morphology. In this case, the coating appears slightly rough; that is, in the case where the pre-coating thickness is less than or equal to 15 μm, the coating after heating manifests mainly as filling cavities. As the coating thickness increases, the decreasing trend of Ra after heating becomes more obvious than before heating.In the case where the pre-coating thickness is 19 μm, the degree of coating roughness after heating increases even further, which manifests itself in the fact that the coating roughness after heating begins to increase. Petition 870250085174, dated 09 / 21 / 2025, pp. 48 / 65 27 / 35 In the case where the pre-coating thickness is 25 μm, the coating is completely rough, meaning the coating exhibits significant surface roughness.

[0100] By combining the changes in surface roughness and surface morphology of the coating before and after heating, the inventor discovered that the change in surface roughness of the coating after heating the aluminum or aluminum alloy coating occurs mainly during the liquefaction and solidification processes of the coating. The surface roughness of the coating after heating is mainly divided into two processes: one is that the liquefied coating fills holes (small gaps, “valleys”), and the other is the roughness of the liquefied coating. In a case where the thickness of the pre-coating is large, such as not less than 20 μm, the amount of liquefaction of the coating after heating is large. In this case, the liquefied coating continues to be rough after filling the pits, and the surface roughness Ra is large.If the pre-coating thickness is small, the amount of coating liquefaction after heating is also small. In this case, the liquefied coating mainly fills pits, and a high surface roughness Ra after heating cannot be guaranteed. Therefore, the surface roughness Ra of the coating after heating can be increased by increasing the surface roughness Ra of the pre-coating.

[0101] Next, the aforementioned heated steel plate is coated by a selected appropriate process (phosphating, electrophoresis), and the coated surface is subjected to a scratch corrosion test to assess paint adhesion and corrosion resistance (in the case where the maximum corrosion expansion in width does not exceed 4 mm, it meets the requirements).

[0102] After hot forming, the samples are phosphated with the phosphating agents and test parameters shown in Table 6. Then, the resulting phosphated plate is subjected to electrophoresis (ink model). Petition 870250085174, dated 09 / 21 / 2025, page 49 / 65 28 / 35 electrophoresis: Kansai HT-8000C). The dry film thickness of the electrophoresis is approximately 18 μm. A cyclic corrosion method is then performed, in which a single cycle includes 8 hours of normal temperature maintenance (25±3°C, during which the saline solution is sprayed 4 times for 3 minutes each time, the saline solution composition is: 0.9% by weight NaCl, 0.1% by weight CaCl2, 0.0750.9% by weight NaHCO3), then 8 hours of moist heating (49±2°C, 100% RH) and finally 8 hours of drying (60±2°C, <30% RH), for a total of 26 cycles to evaluate corrosion resistance. Table 6 Phosphating process parameters Step Process Model of phosphating agent Concentration (pt) Hour / S Temperature / °C 1 Degreasing (immersion) FC-L5000A FCE2021SBB FAL 9-15 120 50 2 Washing with deionized water 60 TR 3 Surface adjustment PL-X 2g / L 30 TR 4 Phosphating PB-L3035SM AD-4813 AD-4856 NT-4055 AC-131 FA 0.9 AT 20.0 CA 3.0 120 35 5 Washing with deionized water 60 TR

[0103] Table 7 shows the main production processes, the surface roughness of the pre-coating, the surface roughness of the coating after heating, and the results of the scratch corrosion test corresponding to the examples. Petition 870250085174, dated 09 / 21 / 2025, pages 50 / 65 29 / 35 Table 7: Main production processes, surface roughness of the pre-coating, surface roughness of the coating after heating, and results of the scratch corrosion test corresponding to the examples. Example Base plate thickness (mm) Pre-coating thickness (mm) Annealing temperature (°C) Plating solution temperature (°C) Key parameters of the skin passage process Pre-coating Ra (pm) Coating Ra after heating (pm) Scratch corrosion test results Cylinder Ra (pm) Rolling force (kN) Maximum corrosion expansion width (mm) Determination Example 1 0.85 730 650 4.50 8206 1.85 1.83 3.2 Meets the requirement Example 2 (comparison) 0.85 752 655 3.50 3305 1.32 1.28 5.0 Does not meet the requirement Example 3 0.8 10 753 650 4.00 6760 2.00 1.85 3.2 Meets the requirement Example 4 (comparison) 0.8 10 750 651 3.00 4150 1.20 0.92 5.5 Does not meet the requirement Example 5 0.8 15 760 640 4.50 8080 2.50 1.90 3.2 Meets the requirement Example 6 (comparison) 0.8 15 755 655 3.00 4688 1.59 1.30 5.0 Does not meet the requirement Example 7 0.8 19 750 650 4.50 6800 2.80 1.85 3.2 Meets the requirement Petition 870250085174, dated 09 / 21 / 2025, pages 51 / 65 30 / 35 Example 8 (comparison) 0.8 19 750 655 3.00 5688 1.79 1.20 5.0 Does not meet the requirement Example 9 (comparison) 0.8 25 770 650 3.00 5400 2.00 2.23 2.9 Meets the requirement Example 10 1.4 5 739 650 4.50 7800 1.87 1.85 3.2 Meets the requirement Example 11 (comparison) 1.4 5 760 655 3.50 5500 1.42 1.38 4.9 Does not meet the requirement Example 12 1.4 10 790 650 4.00 8080 2.20 2.00 3.0 Meets the requirement Example 13 (comparison) 1.4 10 750 652 3.50 7150 1.90 1.70 4.3 Does not meet the requirement Example 14 1.4 15 760 640 4.50 6660 2.40 1.85 3.2 Meets the requirement Example 15 (comparison) 1.4 15 755 655 4.00 5000 2.30 1.65 4.6 Does not meet the requirement Example 16 1.4 19 780 650 4.50 7600 2.90 2.05 3.0 Meets the requirement Example 17 (comparison) 1.4 19 750 655 4.00 5600 2.40 1.58 4.4 Does not meet the requirement Example 18 (comparison) 1.4 25 770 650 3.00 2180 1.70 2.02 2.9 Meets the requirement Petition 870250085174, dated 09 / 21 / 2025, pages 52 / 65 31 / 35 Example 19 2.0 5 790 650 4.50 7850 1.90 1.86 3.2 Meets the requirement Example 20 (comparison) 2.0 5 820 665 4.00 5500 1.69 1.68 4.9 Does not meet the requirement Example 21 2.0 10 850 650 4.00 7890 2.10 2.00 3.0 Meets the requirement Example 22 (comparison) 2.0 10 750 652 3.00 4700 1.67 1.52 4.6 Does not meet the requirement Example 23 2.0 15 760 680 5.00 6000 2.70 2.15 2.8 Meets the requirement Example 24 (comparison) 2.0 15 755 655 3.00 5500 1.69 1.20 5.0 Does not meet the requirement Example 25 2.0 19 780 650 5.50 7800 3.50 2.80 3.0 Meets the requirement Example 26 (comparison) 2.0 19 750 655 3.00 3500 1.72 1.65 4.2 Does not meet the requirement Example 27 (comparison) 2.0 25 750 650 5.50 6500 3.50 1.60 4.5 Does not meet the requirement Petition 870250085174, dated 09 / 21 / 2025, pages 53 / 65 32 / 35

[0104] In the examples above, the pre-coating thicknesses of 5 μm, 10 μm, 15 μm and 19 μm are the thickness range of the pre-coating of the present invention. The pre-coating prepared by the method of the present invention contrasts with the pre-coating prepared without the method of the present invention.

[0105] Specifically, with reference to Figure 7, the pre-coating thickness is 5 μm. The pre-coatings prepared by the method of the present invention include Examples 1, 10, and 19, and the comparative examples include Examples 2, 11, and 20. It can be observed that in the case where the pre-coating thickness is 5 μm (greater than or equal to 5 μm and less than or equal to 10 μm), using Examples 1, 10, and 19 of the present invention, the surface roughness Ra of the pre-coating is > 1.85. The thickness and surface roughness of the pre-coating are limited within the ABCD standard. The surface roughness Ra of the coating after heating is > 1.80. In this case, the paint has good adhesion and corrosion resistance, which meets the requirements of the standard.In comparative examples, the surface roughness Ra of the pre-coating is less than 1.85, the thickness and surface roughness of the pre-coating are not within the ABCD standard, and the surface roughness Ra of the coating after heating is <1.80. In this case, the paint exhibits low adhesion and corrosion resistance, which does not meet the requirements of the standard.

[0106] In a case where the pre-coating thickness is 10 μm, the pre-coatings prepared by the method of the present invention include Examples 3, 12, and 21, and the comparative examples include Examples 4, 13, and 22. It can be observed that in the case where the pre-coating thickness is 10 μm (greater than or equal to 5 μm and less than or equal to 10 μm), using Examples 3, 12, and 21 of the present invention, the surface roughness Ra of the pre-coating is > 2.00. The thickness and surface roughness of the pre-coating are limited within the ABCD standard. The surface roughness Ra of the coating after heating is > 1.80. In this case, the paint has good adhesion and corrosion resistance, which meets the requirements of the standard. In Petition 870250085174, dated 09 / 21 / 2025, pp. 54 / 65 33 / 35 comparative examples, the surface roughness Ra of the pre-coating is <2.00, the thickness and surface roughness of the pre-coating are not within the ABCD standard, and the surface roughness Ra of the coating after heating is <1.80. In this case, the paint exhibits low adhesion and corrosion resistance, which does not meet the requirements of the standard.

[0107] In a case where the pre-coating thickness is 15 μm, the pre-coatings prepared by the method of the present invention include Examples 5, 14, and 23, and the comparative examples include Examples 6, 15, and 24. It can be observed that in the case where the pre-coating thickness is 15 μm (greater than 10 μm and less than or equal to 15 μm), using Examples 5, 14, and 23 of the present invention, the surface roughness Ra of the pre-coating is > 2.40. The thickness and surface roughness of the pre-coating are limited within the BEFC standard. The surface roughness Ra of the coating after heating is > 1.80. In this case, the paint has good adhesion and corrosion resistance, which meets the requirements of the standard. In comparative examples, the surface roughness Ra of the pre-coating is <2.40, the thickness and surface roughness of the pre-coating are not within the BEFC standard, and the surface roughness Ra of the coating after heating is <1.80.In this case, the paint has low adhesion and corrosion resistance, which does not meet the requirements of the standard.

[0108] In a case where the pre-coating thickness is 19 μm, the pre-coatings prepared by the method of the present invention include Examples 7, 16, and 25, and the comparative examples include Examples 8, 17, and 26. It can be observed that in the case where the pre-coating thickness is 19 μm (greater than 15 μm and less than 20 μm), using Examples 7, 16, and 25 of the present invention, the surface roughness Ra of the pre-coating is > 2.80. The thickness and surface roughness of the pre-coating are limited within the EGHF standard. The surface roughness Ra of the coating after heating is > 1.80. In this case, the paint has good adhesion and corrosion resistance, which meets the requirements of the standard. In Petition 870250085174, dated 09 / 21 / 2025, pp. 55 / 65 34 / 35 comparative examples, the surface roughness Ra of the pre-coating is <2.80, the thickness and surface roughness of the pre-coating are not within the EGHF standard, and the surface roughness Ra of the coating after heating is <1.80. In this case, the paint exhibits low adhesion and corrosion resistance, which does not meet the requirements of the standard.

[0109] Furthermore, the 25 μm thick pre-coating commonly used by Arcelor Mittal is selected for comparison, in particular, Examples 9, 18 and 27. It can be observed that in the case where the surface roughness of the pre-coating is not greater than 2.00 (Examples 9 and 18), the surface roughness Ra of the coating after heating is > 1.80. In this case, the paint has good adhesion and corrosion resistance, and the maximum width of corrosion expansion is 2.9 mm, which meets the requirements of the standard. However, in the case where the surface roughness of the pre-coating is very large (Example 27, Ra is 3.50), the surface roughness Ra of the coating after heating is <1.80.In this case, the paint exhibits low adhesion and corrosion resistance, which does not meet the requirements of the standard, indicating that even if the pre-coating thickness is large, if the surface roughness of the pre-coating is high enough, high surface roughness cannot be guaranteed after heating.

[0110] Generally, the pre-coatings of the present invention prepared by the method of the present invention in a thickness range of 5 μm, 10 μm, 15 μm and 19 μm, in particular, Examples 1, 3, 5, 7, 10, 12, 14, 16, 19, 21, 23 and 25, have a maximum paint corrosion expansion width of 2.8 mm to 3.2 mm. However, pre-coatings prepared without using the present invention, in particular, Examples 2, 4, 6, 8, 11, 13, 15, 17, 20, 22, 24 and 26, have a maximum paint corrosion expansion width greater than 4.0 mm, which does not meet the requirements. The maximum expansion width due to corrosion of the 25 μm thick pre-coating is 2.9 mm (Examples 9 and 18), meaning that the paint adhesion and corrosion resistance of the pre-coating prepared by the present invention are equivalent to those of the pre-coating prepared by Petition 870250085174, dated 09 / 21 / 2025, pp. 56 / 65 35 / 35 present invention. the pre-coating with thickness commonly used by Arcelor Mittal. However, the pre-coating prepared without the use of the present invention exhibits poor paint adhesion and corrosion resistance.

[0111] In cases where the pre-coating thickness is small, ensuring a high surface roughness of the coating after hot forming is particularly important to improve paint adhesion and corrosion resistance of the aluminum alloy coating after hot forming.

[0112] According to the examples above, for pre-coating aluminum or aluminum alloy coating, in a case where the thickness of the pre-coating is small (less than 20 μm), after hot forming of the pre-coating prepared by the present invention, the coating has high roughness, which can ensure good paint adhesion and corrosion resistance.

[0113] The above embodiments have elaborated on the purpose and implementation effects of the present invention. It should be understood that the above embodiments are merely descriptions of preferred embodiments of the present invention and do not limit the concept and scope of the present invention. Within the spirit and principles of the present invention, and without departing from the design ideas of the present invention, various modifications, equivalent substitutions, improvements and the like made by those skilled in the art or by adopting the technical concepts and technical solutions of the present invention are within the scope of protection of the present invention. Petition 870250085174, dated 09 / 21 / 2025, pp. 57 / 65

Claims

1 / 5 CLAIMS 1. Pre-coated steel plate for hot forming, characterized in that a pre-coating is provided on at least one surface of a base steel, and the pre-coating has a thickness of 5 μm to 19 μm, wherein in the case where the pre-coating has a thickness greater than or equal to 5 μm and less than or equal to 10 μm, the thickness of the pre-coating and the surface roughness of the pre-coating are limited within the ABCD pattern, which comprises coordinates of the thickness of the pre-coating and the surface roughness of the pre-coating defined by A (5 μm, 3.5 μm), B (10 μm, 3.5 μm), C (10 μm, 2.0 μm) and D (5 μm, 1.85 μm); In cases where the pre-coating thickness is greater than 10 µm and less than or equal to 15 µm, the pre-coating thickness and surface roughness of the pre-coating are defined within the BEFC standard.which includes coordinates for the pre-coating thickness and the pre-coating surface roughness defined by B (10 pm, 3.5 pm), E (15 pm, 3.5 pm), F (15 pm, 2.4 pm) and C (10 pm, 2.0 pm); In cases where the pre-coating thickness is greater than 15 µm and less than or equal to 19 µm, the pre-coating thickness and surface roughness are defined within the EGHF standard, which comprises coordinates for the pre-coating thickness and surface roughness defined by E (15 µm, 3.5 µm), G (19 µm, 3.5 µm), H (19 µm, 2.8 µm) and F (15 µm, 2.4 µm). In a pre-coating coating process, a coating liquid comprises, by mass percentage, 8-11% Si, 2-4% Fe, and the remainder aluminum and unavoidable impurities, wherein the pre-coated steel sheet is suitable for preparing a hot-formed coated steel member with a surface roughness Ra^1.80 pm by means of a hot forming process, in which the surface roughness is measured according to the GB / T2523-2008 measurement method for surface roughness and peak number of cold-rolled metal sheets.

2. Pre-coated steel plate for hot forming according to claim 1, characterized in that the coating of the hot-formed coated steel member consists of only a first layer of the base steel up to the surface, and the first layer is an interdiffusion layer containing Al and Si, comprising greater than or equal to 80% Fe by weight, less than or equal to 5% Si by weight and the remainder Al.

3. Pre-coated steel plate for hot forming, according to claim 1, characterized in that the coating of the hot-formed coated steel member consists, from the base steel to the surface, of a first layer and a second layer sequentially: the first layer is an interdiffusion layer containing Al and Si, comprising 80% or more of Fe by weight, 5% or less of Si by weight and the remainder of Al; the second layer is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% of Fe by weight, 2% or more of Si by weight and the remainder of Al.

4. Pre-coated steel plate for hot forming, according to claim 1, characterized in that the coating of the hot-formed coated steel member consists, from the base steel to the surface, of a first layer, a second layer and a discontinuous surface layer, sequentially: the first layer is an interdiffusion layer containing Al and Si, comprising 80% or more of Fe by weight, 5% or less of Si by weight and the remainder Al; the second layer is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% of Fe by weight, 2% or more of Si by weight and the remainder Al; the surface layer is discontinuous and comprises a first intermetallic compound containing Al and Si and a second intermetallic compound. Petition 870250085174, dated 09 / 21 / 2025, p.19 / 65 3 / 5 containing Al and Si, wherein the first intermetallic compound comprises 55% to 79% Fe by weight, greater than or equal to 2% Si by weight and the remainder Al, and the second intermetallic compound comprises 30% to 50% Fe by weight, less than or equal to 2% Si by weight and the remainder Al.

5. Pre-coated steel plate for hot forming, according to claim 1, characterized in that the coating of the hot-formed coated steel member consists, from the base steel to the surface, of a first layer, a second layer and a third layer, sequentially: the first layer is an interdiffusion layer containing Al and Si, comprising 80% or more of Fe by weight, 5% or less of Si by weight and the remainder of Al; the second layer is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% of Fe by weight, 2% or less of Si by weight and the remainder of Al; the third layer is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% of Fe by weight, 2% or more of Si by weight and the remainder of Al.

6. Pre-coated steel plate for hot forming, according to claim 1, characterized in that the coating of the hot-formed coated steel member consists, from the base steel to the surface, of a first layer, a second layer, a third layer and a discontinuous surface layer, sequentially: the first layer is an interdiffusion layer containing Al and Si, comprising 80% or more of Fe by weight, 5% or less of Si by weight and the remainder Al; the second layer is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% of Fe by weight, 2% or less of Si by weight and the remainder Al; the third layer is a first layer of intermetallic compound. Petition 870250085174, dated 09 / 21 / 2025, p.20 / 65 4 / 5 containing Al and Si, comprising 55% to 79% Fe by weight, greater than or equal to 2% Si by weight and the remainder Al, the surface layer is discontinuous and comprises a first intermetallic compound containing Al and Si and a second intermetallic compound containing Al and Si, wherein the first intermetallic compound comprises 55% to 79% Fe by weight, greater than or equal to 2% Si by weight and the remainder Al, and the second intermetallic compound comprises 30% to 50% Fe by weight, less than or equal to 2% Si by weight and the remainder Al.

7. Pre-coated steel plate for hot forming, according to claim 1, characterized in that the coating of the hot-formed coated steel member consists, from the base steel to the surface, of a first layer, a second layer, a third layer and a fourth layer, sequentially: the first layer is an interdiffusion layer containing Al and Si, comprising 80% or more of Fe by weight, 5% or less of Si by weight and the remainder of Al; the second layer is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% of Fe by weight, 2% or less of Si by weight and the remainder of Al; the third layer is a first layer of intermetallic compound containing Al and Si, comprising 55% to 79% of Fe by weight, 2% or more of Si by weight and the remainder of Al;The fourth layer is a second layer of intermetallic compound containing Al and Si, comprising 30% to 50% Fe by weight, less than or equal to 2% Si by weight, and the remainder Al.

8. Pre-coated steel plate for hot forming according to claim 1, characterized in that the total thickness of the coated hot-formed steel member is from 0.4 mm to 3.0 mm.

9. Method for preparing the pre-coated steel plate defined according to claim 1, characterized in that it comprises Petition 870250085174, dated 09 / 21 / 2025, page. 21 / 65 5 / 5 steel production ^ continuous casting ^ hot rolling ^ pickling and cold rolling ^ coating process, wherein the coating process comprises a film-passing process, and in the film-passing process, the surface roughness Ra of a roll is from 3.0 μm to 6.0 μm and the rolling force is from 4,000 kN to 9,000 kN, in a pre-coating coating process, the coating liquid comprises, by mass percentage, 8-11% Si, 2-4% Fe and the remainder aluminum and unavoidable impurities, wherein the surface roughness is measured according to the GB / T2523-2008 method for measuring surface roughness and peak number of cold-rolled metal plates.

10. Method according to claim 9, characterized in that the coating process comprises a continuous annealing process, which has the following parameters: the annealing is conducted at a controlled temperature of 720°C to 850°C, and maintained at this temperature for 60 s to 120 s under a reducing atmosphere comprising 5 ~ 10% H2 + N2 by volume percentage (i.e., in terms of volume percentage of the reducing atmosphere, H2 accounts for 5 ~ 10% and the remainder is N2), the oxygen content is controlled below 20 ppm and the dew point is controlled between -60°C and 0°C. Petition 870250085174, dated 21 / 09 / 2025, p. 22 / 65

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