Method for steel coating, hot-forming and hot-forming component

Abstract

The invention discloses a method for steel coating and a method for steel thermoforming and a thermoformed component. The coating method comprises a method for coating a steel plate, and the method comprises the following steps carried out in sequence: a) providing a tailor-welded steel plate formed by welding at least two bare steel plates, and the tailor-welded steel plate comprises a welding seam; and b) coating a coating on the surface of the tailor-welded steel plate to enable the coating to cover the welding seam, wherein the coating mode in the step b) is selected from roller coating or spray coating. The thermoforming tailor-welded component prepared through the coating method and the thermoforming method has high strength and good weather resistance at the weld joint.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of hot-formed steel. In particular, the present application relates to a method for steel coating, a method for hot forming of steel and a hot-formed component. BACKGROUND

[0002] The information provided in this section is for the purpose of generally presenting the context of the disclosure. The work of the presently named inventors, to the extent the descriptions are described in this section, as well as aspects of the descriptions that can not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0003] Hot forming technology is an important process for manufacturing high-strength lightweight steel components for automobiles. The high-temperature heating used in the production process can cause severe oxidation on the surface of the steel sheet. To solve this problem, the prior art mainly includes the following ways: the first is to carry out shot blasting treatment on the bare plate after hot forming, but this method has obvious defects, that is, the mechanical impact of shot blasting easily leads to deformation of the parts, and the treated parts lose surface protection and the rust prevention ability becomes poor; the other is to carry out pre-coating on the surface of the steel substrate, such as hot-dip coating of aluminum-silicon coating, zinc coating, or zinc-aluminum-magnesium alloy coating on the hot-rolled or cold-rolled coil by the steel plant. Although such pre-coated steel sheets can solve the oxidation problem during hot forming, the raw materials are subject to the upstream steel plant, and the edge and corner waste generated by the pre-coated steel sheet cannot be directly recycled and utilized, with low residual value and high production cost; and this mode cannot meet the flexible customization needs of parts manufacturers for steel materials of different shapes, strength grades or thicknesses.

[0004] More importantly, in the application scenario involving the tailor-welding process, direct use of pre-coated steel sheets for tailor-welding leads to mixing of aluminum elements in the coating into the weld at high welding temperature, which significantly reduces the strength of the weld and the heat-affected zone; in addition, due to the high-temperature oxidation effect of tailor-welding at the weld, part of the coating will fall off, not only affecting the appearance, but also causing the area to rust or corrode easily under the whole vehicle export transportation or harsh use environment. SUMMARY

[0005] The present application provides a method for steel coating, a method for hot forming of steel and a hot-formed component to solve one or more of the above problems and other aspects, or to provide an alternative technical solution to the prior art.

[0006] According to one aspect of the present application, a method for steel sheet coating is provided, which comprises the following steps performed in sequence: a) providing a tailor-welded steel sheet formed by welding at least two bare steel sheets, the tailor-welded steel sheet comprising a weld; and b) coating a coating on the surface of the tailor-welded steel sheet, so that the coating covers the weld; The coating in step b) is preferably performed by roll coating.

[0007] Preferably, in the method, the coating is performed by roll coating using a roll coating device comprising a steel roll and a rubber roll.

[0008] Preferably, in the method, the roll coating comprises double-sided roll coating, such that the upper and lower surfaces of the tailor-welded steel plate are coated simultaneously.

[0009] Preferably, in the method, the time t (in seconds) measured by a TAPPI viscosity cup and the plate thickness T (in millimeters) of the tailor-welded steel plate satisfy the following relationship: 90T ≤ t ≤ 120T.

[0010] Preferably, in the method, the gap (in millimeters) between the rubber roll and the tailor-welded steel plate is set to √T, where T is the plate thickness (in millimeters) of the tailor-welded steel plate.

[0011] Preferably, in the method, the roll coating speed is set to be less than or equal to 20 meters / minute, preferably less than or equal to 8 meters / minute, and more preferably 3-5 meters / minute.

[0012] Preferably, in the method, the following step is further included: c) drying the coated tailor-welded steel plate at a drying temperature of 100-800°C, preferably 200-400°C, for a drying time of 1-50 minutes, preferably 3-10 minutes.

[0013] Preferably, in the method, the steel plate is a high-strength steel plate.

[0014] According to another aspect of the present application, a method for coating a steel substrate is provided, comprising the following steps performed in sequence: a) providing a steel substrate selected from a single steel plate or a steel coil; and b) uniformly coating a coating on the surface of the steel substrate; The coating in step b) is preferably performed by roll coating or spraying.

[0015] According to another aspect of the present application, a method for steel hot forming is provided, characterized by comprising the following steps performed in sequence: a) providing a coated tailor-welded steel plate prepared by the method according to the present application or a coated steel substrate prepared by the method according to the present application; b) heating the coated steel to a temperature between Ac1 and 1000°C, and holding at a temperature higher than Ac3; and c) transferring the coated steel after the holding to a hot forming die, cooling and press forming; wherein the Ac1 is the temperature at which austenite starts to form in the steel during heating, and the Ac3 is the temperature at which the austenite transformation ends in the steel during heating.

[0016] Preferably, in the method, the heating time in step b) is 85T, wherein T is the thickness of the steel in millimeters; and / or wherein the speed of cooling in step c) is not less than 25°C / s.

[0017] Preferably, in the method, the coating applied on the surface of the steel is selected from organic resins and derivatives thereof, and the coating layer formed from the coating undergoes a ceramming transformation in step b).

[0018] According to another aspect of the present application, there is provided a hot formed component prepared by the method according to the present application.

[0019] Preferably, in the hot formed component, the hot formed component has a coating layer with a nominal thickness in the range of 3 μm to 33 μm.

[0020] Preferably, in the hot formed component, the hot formed component has a tensile strength of 1460 MPa-1480 MPa, a yield strength of 1020 MPa-1080 MPa, and an elongation after fracture of 5%-5.5%.

[0021] Compared with the prior art, the high-strength steel coating and hot forming method provided by the present application uniformly applies the coating to the surface of the steel by roll coating or spraying, can achieve a coating adhesion comparable to that of the hot plating in the prior art, effectively prevents the generation of scale on the surface of the steel component during the hot forming stage, and can achieve customized coating for various forms of steel, breaking the dependence on pre-plated coil stock of the steel plant, and significantly improving the utilization rate of the coating and reducing the production cost. Meanwhile, by adopting the new process sequence of first butt welding the bare steel plate and then coating the butt welded plate, on the one hand, the problem of the reduction of the strength of the weld area due to the mixing of the plated elements into the weld during the welding of the conventional plated steel plate is avoided, and on the other hand, the weld area is protected by a complete coating layer, significantly improving the weather resistance and rust resistance of the weld of the butt welded component. BRIEF DESCRIPTION OF DRAWINGS

[0022] The disclosure of the present application will become more readily apparent from the following description of the drawings. As those skilled in the art will appreciate, the drawings are not intended to limit the scope of the present application, but merely to illustrate it. In the drawings: Figure 1An exemplary process flow diagram showing a method for preparing a hot formed steel sheet for a door ring according to some embodiments of the present application is shown.

[0023] Figure 2A A photograph of a hot formed tailor welded door ring part prepared according to one embodiment of the present application is shown. Figure 2B A photograph of a door ring part formed from a commercially available aluminum silicon coated steel sheet is shown.

[0024] Figure 3A A photograph of the surface condition of a test sample from a hot formed component prepared according to one embodiment of the present application after 24 hours of weathering testing compared to a bare steel sheet test sample and an aluminum silicon coated steel sheet test sample is shown. Figure 3B A photograph of the surface condition of a test sample from a hot formed component prepared according to one embodiment of the present application after 48 hours of weathering testing compared to a bare steel sheet test sample and an aluminum silicon coated steel sheet test sample is shown. DETAILED DESCRIPTION

[0025] In order that the foregoing and other objects, features and advantages of the present application can be more readily understood, a detailed description will be described below with reference to the accompanying drawings. It is to be understood that all descriptions are illustrative and not restrictive, and that the scope of the present application is not to be limited to any embodiment described herein. For any single technical feature described or implied in the various embodiments described herein, the present application still allows for any combination or deletion of these technical features (or their equivalents) without any technical obstacles, and thus it is to be understood that more embodiments according to the present application are also included within the scope of the description herein.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for describing particular embodiments only and is not intended to be limiting of the application. In this application, the use of "about" for any particular

[0027] In this application, descriptions of "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or an indicated number of features. Thus, a feature defined with "first" or "second" can explicitly or implicitly include at least one of the features.

[0028] In this application, unless otherwise stated, the numerical ranges listed in this application are intended to include the endpoints, and all the numbers and sub-ranges within the ranges.

[0029] In this application, unless otherwise stated, all numbers expressing quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about."

[0030] As used in this application, the terms "comprising" and "including" encompass the case where the other elements mentioned are not present, as well as the case where they are present.

[0031] As used in this application, the term "nominal thickness" refers to the target thickness or average thickness of a coating or plating layer as set during the manufacturing process.

[0032] As used in this application, the term "base plate thickness" refers to the average thickness of the steel substrate to be coated, such as a tailor-welded blank, and is denoted herein by T, in millimeters (mm).

[0033] As used in this application, the term "wet film" refers to a liquid or semi-liquid film layer that contains solvent and / or uncrosslinked components when the coating is applied to the surface of the substrate and has not yet undergone drying or curing treatment.

[0034] In the application, unless otherwise specified, each reaction is carried out at room temperature and under normal pressure.

[0035] It is known in the art that hot forming processes are commonly used to manufacture high-strength steel components. In order to prevent the formation of scale on the surface of the steel plate during the high-temperature heating stage, the current main treatment method is to rely on the provision of pre-coated steel plates (such as aluminum-silicon plated steel plates) by large-scale steel plants. However, in practice, such a solution has limitations: first, it is heavily dependent on the supply of coils by steel plants, which not only makes the cost of raw materials uncontrollable and reduces the residual value of waste materials, but also cannot meet the flexible customization needs of component manufacturers for steel materials of various shapes, strengths or thicknesses or tailor-welded blanks; more importantly, for tailor-welded blanks, due to the high temperature of welding, oxidation inevitably occurs in the weld and the heat-affected zone, which is particularly prone to rust in the case of vehicle export or in harsh environments. In addition, direct use of pre-coated steel plates for tailor-welding also causes the aluminum element in the plating layer to enter the weld pool at high temperatures, generating, for example, brittle alloying compounds containing aluminum, which reduces the strength of the weld and the heat-affected zone.

[0036] Applicants have found, through in-depth research, that in the processing of high-strength steel components, combining welding technology with a specific roll coating process using an organic-based coating can surprisingly overcome both the strength reduction and rusting defects in the weld area. Specifically, the present application proposes a new process route in which the tailor-welding is carried out on bare plates, and the subsequent specific roll coating process achieves full coverage of the tailor-welded blank, including the weld area, so that the weld and the base material have the same weather resistance, while achieving the coordination and unity of the appearance of the component.

[0037] Furthermore, it should be noted that the coating preparation process for the hot-formed components involved in this application differs significantly from the traditional hot-dip galvanizing process in steel mills in terms of forming mechanism and processing method. The coating of the hot-formed components in this application undergoes roll coating and subsequent hot stamping. Its film formation process involves two curing processes: the first curing occurs during the drying stage after roll coating, during which the diluent in the coating evaporates, and the organic-based coating undergoes a cross-linking reaction with the steel substrate, achieving preliminary curing and forming a pre-coating; the second curing occurs during the high-temperature heating stage of hot forming, during which the coating undergoes a "ceramic-like" transformation, that is, the organic components, such as C and H elements in organic resins, volatilize or undergo graphitization, while the inorganic components react to form dense inorganic acid salts. Therefore, the coating prepared using the above process can effectively adhere to the steel substrate, ensuring the corrosion resistance of the welded steel components.

[0038] According to one aspect of this application, a method for coating steel plates is provided, the method comprising the following steps performed sequentially: a) Providing a welded steel plate formed by welding at least two bare steel plates, said welded steel plate including a weld seam; and b) Apply coating to the surface of the welded steel plates so that the coating covers the weld seam; In step b), the coating method is selected from roller coating or spray coating.

[0039] In this application, the steel substrate can be various types and strengths of steel, such as hot-formed steel. It is worth noting that, because the coating method of this application is highly adaptable to the shape of the steel substrate, it overcomes the equipment limitations of traditional steel mill pre-coating technology, which can only process standard steel coils. Therefore, the steel substrate can be of various shapes, including but not limited to uncut steel coils, or materials with specific contours cut from a plane, or welded steel plates with variations in thickness or strength distribution, such as single-layer composite steel plates. Typically, the thickness of the steel substrate is in the range of 0.5 mm to 3.5 mm.

[0040] Preferably, the steel plate according to this application can be a high-strength steel plate. As a non-limiting specific example, the steel matrix of the bare steel plate can be 22MnB5 steel.

[0041] In this application, the welding method for the welded steel plates is not particularly limited and can be any known welding technique in the art suitable for joining metal plates. For example, the welding includes, but is not limited to, laser welding, arc welding, electron beam welding, and any process that enables the high-strength steel plates to form welded steel plates including weld seams.

[0042] In this application, the coating can be applied by spraying or roller coating. Unlike existing hot-dip galvanizing processes that require continuous operation on the entire coil of steel, the roller coating or spraying process used in this application offers greater production flexibility. Traditional coil pre-plating processes generate a large amount of waste during subsequent blanking and stamping due to the cutting process, resulting in wasted coating on the waste material; however, the process in this application can coat only the effective areas of the product, improving coating utilization.

[0043] Preferably, the coating method can be roller coating. For hot-formed steel sheets requiring high surface flatness, roller coating can provide more uniform coating thickness control. The roller coating can be performed using a roller coating apparatus that includes a steel roller for metering and a rubber roller for coating.

[0044] In the coating method according to this application, the specific operation method of roller coating is known in the art. For example, the coating material can be pre-mixed evenly by stirring in a certain proportion, and then pumped into the middle of the steel roller and the rubber roller by a pump, so that the coating material is evenly distributed by the rotation of the rollers. Depending on the size of the sheet material to be coated, those skilled in the art can select an appropriate amount of coating material as needed. Before roller coating, the bare steel sheet can optionally be surface cleaned, for example, by removing rust and oil, to ensure that its surface is clean, free of contaminants, and does not contaminate the roller coating material.

[0045] Preferably, in some embodiments, the roll coating can be single-sided coating, that is, after coating one side of the steel plate, it is flipped over to coat the other side. In other embodiments, the roll coating can include double-sided roll coating, that is, the upper and lower surfaces of the steel plate are coated simultaneously. This can be achieved by a device including two sets of steel rollers and rubber rollers as described above, thereby enabling double-sided coating of the coated sheet as it passes through the rollers, improving production efficiency.

[0046] The applicant has discovered that the viscosity of the coating has a significant impact on the roller coating process, and that the viscosity requires tedious and repeated adjustments when coating sheets of different thicknesses. In view of this, this application proposes a specific relationship between coating viscosity and sheet thickness. In this application, the viscosity of the coating is measured using the Ford-4 viscosity cup method, which is well-known in the art. This measurement method involves conventional means of characterizing the kinematic viscosity of a fluid by recording the time (in seconds) required for a given volume of sample to flow out of a standard viscosity cup with a specific orifice. Unless otherwise stated, the viscosity described in this application is characterized by the outflow time measured using the above method at room temperature (approximately 25°C).

[0047] Preferably, the time t (in seconds) measured using a Coat-4 viscosity cup and the thickness T (in millimeters) of the plate coated on the welded steel plate satisfy the following relationship: 90T ≤ t ≤ 120T. This relationship allows for the selection of a coating with appropriate viscosity for different plate thicknesses, ensuring suitable flowability and adhesion of the coating and avoiding difficulties in coating or insufficient film thickness.

[0048] The applicant further discovered that, in order to obtain a coating with effective protection and appropriate thickness on welded plates of different thicknesses, the roller coating process parameters must be specially selected.

[0049] Preferably, the gap (in millimeters) between the rubber roller and the welded steel plate can be set to √T, where T is the thickness of the welded steel plate (in millimeters). The applicant has found that there is a non-linear optimal matching relationship between the gap setting and the plate thickness. If the gap is too large, it will lead to localized paint loss; if the gap is too small, the paint will be excessively compressed, resulting in uneven distribution. When the gap value is set to √T, the uniformity and integrity of the wet film can be ensured under different plate thicknesses.

[0050] Preferably, the roller coating speed can be set to less than or equal to 20 m / min, more preferably 8 m / min, and even more preferably 3 m / min to 5 m / min. Optionally, the parallelism between the steel roller and the rubber roller can also be controlled to obtain a more uniform coating.

[0051] Preferably, the coating method according to this application further includes a post-coating curing step. In some embodiments, the coated welded steel sheet is dried at a temperature of 100-800°C, preferably 200-400°C, for a time of 1-50 minutes, preferably 3-10 minutes. This step constitutes the first curing of the coating, designed to evaporate the solvent in the coating and allow the resin to undergo a preliminary cross-linking reaction with the substrate, transitioning from a wet film state to a touch-dry state. The drying can be carried out by any suitable heating method in the art, including but not limited to oven heating, infrared heating, induction heating, resistance wire heating, and gas burner heating. After drying, the sheet can be cooled as needed by blowing air or other cooling methods known in the art (e.g., a cooling rate of 50°C / m) until it cools to room temperature.

[0052] In other embodiments, the first curing can also be carried out by natural air drying. The natural drying time can be determined as needed based on the film formation on the substrate.

[0053] According to another aspect of this application, a method for hot forming of steel sheet is also provided, the method comprising the following steps performed in sequence: a) Provide a coated welded steel sheet prepared according to the method described in this application; b) Heat the coated steel sheet to a temperature between Ac1 and 1000°C, and hold it at a temperature above Ac3; and c) Transfer the heat-insulated welded steel plate into a hot forming mold, cool it, and stamp it into shape; Wherein, Ac1 is the temperature at which austenite begins to form in the welded steel plate during the heating process, and Ac3 is the temperature at which the austenite transformation of the welded steel plate ends during the heating process.

[0054] According to this application, in step b), the steel matrix is ​​fully austenitized by heating and holding. Ac1 and Ac3 can be determined according to the type of steel matrix; for example, for some high-strength hot-formed steels, Ac1 can be approximately 820°C and Ac3 approximately 850°C.

[0055] According to this application, preferably, in step b), the heating time can be 50-300 seconds. Preferably, the heating time (in seconds) in step b) can be 85T, where T is the thickness (in millimeters) of the welded steel plate. This time window ensures the phase transformation of the steel substrate and the subsequent transformation of the coating applied to the steel substrate.

[0056] Preferably, the coating applied to the surface of the welded steel plate may be selected from organic resins and their derivatives, which are commercially available and can form a coating that undergoes a ceramization-like transformation as described in step b) above. For the ceramization-like transformation, it should be understood that at high temperatures (e.g., 850-1000°C), the organic components in the original coating undergo thermal decomposition or graphitization, while the inorganic components react with oxygen to form inorganic acid salts or other dense composite oxides. This process constitutes the second curing of the coating, thereby forming a ceramization-like coating that can possess high-temperature oxidation resistance and corrosion resistance.

[0057] According to this application, preferably, in step c), the cooling rate within the mold is not less than 25°C / s to achieve the desired martensitic structure. This cooling can be achieved by various methods known in the art, including but not limited to using cooling water at temperatures below 20°C.

[0058] According to this application, in step c), the specific process parameters of compression molding can be optionally adjusted to optimize the thermoforming effect. For example, depending on the shape and size of the thermoformed part, the holding pressure of the press can be maintained at, for example, ≥800 tons, and the holding time can be controlled at, for example, ≥6 seconds.

[0059] According to another aspect of this application, a thermoformed welded component is provided, which is prepared by the method according to this application.

[0060] The thermoformed welded components prepared according to this application can obtain a thin and uniform coating by means of the preparation method of this application. Preferably, the nominal thickness of the coating can be in the range of 3 μm to 33 μm to obtain the desired coating adhesion and corrosion resistance.

[0061] Preferably, the hot-formed welded component prepared according to this application has the required high strength, particularly having a tensile strength of 1460MPa-1480MPa, a yield strength of 1020MPa-1080MPa, and an elongation at break of 5%-5.5% at the weld of the hot-formed welded component, and the fracture location is not located at the weld.

[0062] According to another aspect of this application, a method for coating a steel substrate is also provided, which may include the following steps performed in sequence: a) Provide a steel substrate, said steel substrate being selected from a single steel plate or steel coil; and b) Apply the coating evenly to the surface of the steel substrate; In step b), the coating method is selected from roller coating or spray coating.

[0063] As used in this application, "single steel plate" may include pre-cut sheet or blanked sheet; "steel coil" may include steel strip provided in a continuous winding form, which may be a continuous steel strip during the coating process, or a sheet formed by unwinding and cutting the coil.

[0064] It should be noted that the specific operating parameters of the roll coating process, the selection of the rheological properties of the coating, and the subsequent hot forming process have been described in detail above, specifically for welded steel plates. Although the above description pertains to welded steel plates with weld seam characteristics, those skilled in the art should understand that the roll coating process parameters, secondary curing process and conditions, and implementation details and principles of the hot forming process described above for welded steel plates are equally applicable to single steel plates or coils according to the embodiments of this aspect. In other words, when coating and hot forming the aforementioned single steel plates or coils, the relevant process steps and parameter ranges described above for welded steel plates can be directly referenced or consulted. For the sake of brevity, such process details will not be repeated here.

[0065] Unless otherwise stated, all properties and parameters involved in this application, such as the thickness of the steel sheet, the viscosity of the coating, and the mechanical properties of the components, can be measured and characterized by conventional testing methods known to those skilled in the art.

[0066] While similar or equivalent methods and materials described herein may be used in the practice or testing of this application, suitable embodiments are described below. These embodiments should be interpreted as addressing the technical problems of this application such that they are not limited to the explicitly stated combinations of features, but rather that the illustrated features can be rearranged without limitation. Example

[0067] The following will further illustrate the concept and technical effects of this application with reference to embodiments, so that those skilled in the art can fully understand the purpose, features, and effects of this application. Those skilled in the art will understand that the embodiments herein are for illustrative purposes only and are not intended to limit the scope of this application. Unless otherwise specified, the raw materials used in the embodiments are commercially available conventional analytical grade reagents, which are used directly without any purification.

[0068] For all samples in the embodiments of this application, the bare high-strength steel plate to be coated was a 22MnB5 steel substrate with a thickness of 1.2 mm. The steel substrate had the chemical composition (by weight percentage) shown in Table 1 below, with the balance being iron and unavoidable impurities generated during processing.

[0069] Table 1. Chemical composition of the steel substrate to be coated (by weight %) Element C Si Mn P S B wt. % 0.22 0.14 1.35 0.014 0.002 0.0018 Before use, all steel plates should undergo surface cleaning treatment, including rust removal and oil removal, to ensure that their surfaces are clean, dry, and free of contaminants.

[0070] Example 1 A sample of an organic resin-based coating according to this application was prepared in advance for coating a steel plate. The coating comprises a commercially available, heat-resistant organic resin known in the art, metal powder, and solvent. The proportions of the components in the coating were adjusted according to the thickness of the steel plate so that, at room temperature (25°C), the coating had a flow time of 200 s when measured using a Ford-4 viscosity cup according to GB / T 1723-1993.

[0071] In this embodiment, two bare 22MnB5 steel plates, after surface cleaning treatment, are first provided and welded together using laser welding to form a welded steel plate containing the weld seam. Simultaneously, a single bare 22MnB5 steel plate is also provided. Subsequently, a roll coating process is used to coat the welded steel plate and the single bare steel plate with a pre-prepared coating material. The roll coating apparatus includes a steel roller and a rubber roller, with the parallelism between the steel roller and the rubber roller controlled at 0.1. During the roll coating process, the coating material is pumped between the steel roller and the rubber roller, and an appropriate amount of coating material is drawn according to the plate size to control the coating thickness at approximately 30 μm. Uniform distribution of the coating material is achieved through the rotation of the rollers. For a 1.2 mm thick steel plate, the gap between the rubber roller and the welded steel plate is set to 1.10 mm. The welded steel plate passes through the roll coating equipment at a roll coating speed of 8 m / min. A single-sided coating method is used; after one side is coated, the other side is coated, thereby obtaining a coated welded steel plate covering the entire surface of the steel plate, including the weld seam.

[0072] After roll coating, the welded steel sheets and individual steel sheets enter an infrared heating furnace for the first curing process. The drying temperature is set at 300℃, and the drying time is 5 minutes. During this process, the thinner in the coating evaporates, causing the coating on the steel sheet to initially solidify from a liquid state. After baking, the resulting sheet material is cooled by a blower at a cooling rate of 50℃ / m until the sheet material cools to room temperature.

[0073] Subsequently, the coated and dried welded steel plates and individual steel plates are fed into a heating furnace for hot forming. The plates are heated to 950°C. The total heating and holding time is set to 102 seconds based on the thickness of the steel plate. During this heating and holding stage, the steel substrate is heated to complete austenitization; at this point, the resulting coating undergoes a second curing process to form a ceramic-like coating.

[0074] After the heat preservation period, the high-temperature sheet metal is transferred to a thermoforming die using an end effector for stamping. The cooling water temperature inside the die is maintained at 20°C, the press pressure is maintained at 800T, and the holding time is 6 seconds. The sheet metal is rapidly cooled within the die at a rate of 25°C / s to achieve martensitic transformation. After the sheet metal is formed and cooled to room temperature, the part is transferred to the receiving area using an end effector, resulting in thermoformed welded component 1-1 and thermoformed single-piece component 1-2.

[0075] The appearance of the resulting thermoformed components 1-1 and 1-2 is as follows Figure 2A As shown. Observation Figure 2A As can be seen, the surface of the coated hot-formed steel component sample in this embodiment is covered with a uniform and smooth coating, without the formation of oxide scale. The coating adheres well to the surface of the steel substrate, especially in the weld area, and no visible peeling or cracking was found.

[0076] Example 2 An organic resin-based coating sample was prepared in advance using the same components as in Example 1 and used to coat a steel plate. The proportions of the components in the coating were adjusted so that, at room temperature of 25°C, the coating had a flow time of 150 s when measured using a Forte 4 viscosity cup according to GB / T 1723-1993.

[0077] Using the same bare steel sheet material as in Example 1, the same welded steel sheet was obtained through laser welding. Subsequently, a double-sided roller coating process was employed to coat the welded steel sheet with the pre-prepared coating material described above. This double-sided roller coating process used a double-sided roller coating equipment equipped with two sets of steel rollers and two sets of rubber rollers, as well as a transmission device with a ratchet structure. Other parameters used in the roller coating process were the same as in Example 1, resulting in a double-sided coated welded steel sheet with the coating covering the entire surface of the steel sheet, including the weld seam.

[0078] Subsequently, the same subsequent preparation process as in Example 1 was used to obtain the hot-formed welded component 2. Visual inspection revealed that the double-sided coated hot-formed welded steel component 2 of this embodiment had no oxide scale on either side surface, and the coating adhered well to the surface of the rigid substrate, including the weld area, without any visible peeling or cracking.

[0079] Example 3 An organic resin-based coating sample was prepared in advance using a similar composition to that in Example 1, and was used to coat a steel plate. The proportions of the components in the coating were adjusted so that, at room temperature of 25°C, the coating had a flow time of 250 s when measured using a Forte-4 viscosity cup according to GB / T 1723-1993.

[0080] Using the same bare steel sheet material as in Example 1, the same welded steel sheet was obtained through laser welding. Subsequently, the pre-prepared coating was uniformly sprayed onto the surface of the welded steel sheet, completely covering the steel substrate and weld area. The wet film thickness after spraying was controlled at a level comparable to that used in the roller coating process in Example 1. The sprayed welded steel sheet was allowed to air dry for 24 hours or longer to obtain a coated welded steel sheet with the coating covering the entire surface of the steel sheet, including the weld.

[0081] Using the same subsequent preparation process as in Example 1, the hot-formed welded component 3 was obtained. Visual inspection revealed that no oxide scale was generated on the surface of the sprayed welded steel hot-formed component 3 in this embodiment, and the coating adhered well to the surface of the rigid substrate, including the weld area, without any visible peeling or cracking.

[0082] Comparative Example 1 Two commercially available hot-formed steel sheets with an aluminum-silicon (Al-Si) coating were selected. The steel substrate of the hot-formed steel sheets was 22MnB5 with a thickness of 1.2 mm. The Al-Si coating was deposited on the steel substrate using a hot-dip galvanizing process known in the art, and the coating thickness was approximately 30 μm. The two aluminum-silicon coated steel sheets were welded together using the same laser welding process as in Example 1 to obtain welded component 4.

[0083] The appearance of the resulting welded component 4 is as follows Figure 2B As shown. Observation Figure 2B As can be seen from visual observation, the aluminum-silicon coating of the welded components and its adjacent heat-affected zone is damaged, and obvious rust marks appear on the surface.

[0084] The welding strength and weather resistance of the thermoformed welded component 1-1 of Example 1 and the welded component 4 of Comparative Example 1 prepared in this application were further evaluated.

[0085] Weld strength testing of hot formed tailor welded blanks Tensile specimens were taken from near the weld of the welded components of Example 1 and Comparative Example 1 for tensile strength testing. The fracture locations of the three specimens from component 1-1 of Example 1, as well as the tensile strength, yield strength, and elongation after fracture of the component specimens, are recorded in Table 2.

[0086] Table 2. Tensile test data of tensile specimens from welded component 1-1 of Example 1 Sample No. Tensile strength (MPa) Yield strength (MPa) Elongation after fracture (%) Fracture location 1-1-1 1472 1042 5.5 Base metal 1-1-2 1474 1023 5 Base metal 1-1-3 1462 1075 5.5 Base metal In contrast, the welded component 3 of Comparative Example 1, which is made by directly welding aluminum-silicon coated steel plates, has lower tensile strength, yield strength and elongation after fracture than the welded component 1-1 in Example 1, and the fracture location is at the weld.

[0087] Table 3. Tensile test data of tensile specimens from welded aluminum-silicon coated steel plate component 3 of Comparative Example 1 Sample No. Tensile strength (MPa) Yield strength (MPa) Elongation after fracture (%) Fracture location 3-1 1185 910 3 Weld 3-2 1090 910 4.6 Weld 3-3 1359 1109 2.5 Weld The results show that during welding of the aluminum-silicon coated steel sheet in Comparative Example 1, the aluminum element in the coating can melt into the weld, leading to the formation of aluminum alloys in the weld and reducing the weld strength. However, Example 1 of this application uses a process of welding first and then roller coating, which can avoid the adverse effects of aluminum element on the weld strength and ensure the overall mechanical properties of the welded components.

[0088] Corrosion resistance testing of hot formed tailor welded blanks 1.2 mm thick 22MnB5 bare steel sheet was provided, and hot-formed bare steel components were obtained using the same hot-forming process as in Example 1. These components, along with the hot-formed components 1-2 from Example 1 and the hot-dip galvanized component from Comparative Example 1, underwent weathering resistance testing. According to GB / T 10125-2021 standard, samples of the same size were cut from the three samples and subjected to neutral salt spray testing (5% NaCl solution, 35°C). The samples were removed and observed after 20 and 48 hours of testing. The results showed… Figure 3A and Figure 3B Among them Figure 3A and Figure 3B From left to right, the samples are: a sample from a bare steel plate, a sample prepared according to Example 1, and an aluminum-silicon coated sample prepared according to Comparative Example 1.

[0089] from Figure 3A As can be seen, significant corrosion appeared on the surface of the bare steel plate sample within 24 hours of testing; after 48 hours ( Figure 3B The corrosion on the surface of the bare steel plate sample was further aggravated, and it was almost covered with yellowish-brown oxides; while during the test periods of 24 hours and 48 hours, no rust spots were observed on the surface of the sample prepared according to Example 1, which can achieve corrosion resistance comparable to that of the aluminum-silicon coated steel plate of Comparative Example 1.

[0090] Comparative Example 2 A coating sample for coating steel plates was prepared in advance using the same components as in Example 1. Different proportions of the components in the coating were adjusted compared to Example 1, so that the outflow times of the coating, measured using a Forte-4 viscosity cup according to GB / T 1723-1993, were 50 s, 100 s, 300 s, and 600 s at room temperature of 25°C.

[0091] Using the same bare steel sheet material, roller coating process, and equipment as in Example 1, the aforementioned coating samples were applied to the bare steel sheet. During the roller coating process, it was observed that the coating samples were difficult to distribute evenly between the rollers, resulting in uneven coating thickness.

[0092] Comparative Example 3 Paint samples for coating steel plates were prepared in advance using the same components and proportions as in Example 1. Using the same bare steel plate material as in Example 1, the above-mentioned paint samples were applied to the bare steel plate. The gap between the rubber roller and the steel plate in the roller coating equipment was tested at 0.8 mm, 1.1 mm, 1.3 mm, and 1.5 mm, respectively, while other roller coating parameters remained consistent with the example. The coating results at different gaps after roller coating are shown in Table 4 below.

[0093] Table 4. Coating results with different gaps (in millimeters) between rubber rollers and steel plates Gap (mm) 0.8 1.1 1.3 1.5 Result Excess paint squeezed out, uneven coating Uneven coating Partial loss after coating Failed to coat The results show that the samples coated using the gap between the rubber roller and the steel plate could not form an effective coating.

Claims

1. A method for coating steel plates, characterized in that, This includes the following steps performed in sequence: a) Providing a welded steel plate formed by welding at least two bare steel plates, said welded steel plate including a weld seam; and b) Apply coating to the surface of the welded steel plates so that the coating covers the weld seam; In step b), the coating method is selected from roller coating or spray coating.

2. The method according to claim 1, wherein the coating method is roller coating, which is performed using a roller coating apparatus comprising steel rollers and rubber rollers.

3. The method according to claim 2, wherein the roll coating includes double-sided roll coating, such that the upper and lower surfaces of the welded steel plate are coated simultaneously.

4. The method according to claim 2, wherein the time t (in seconds) measured by a Coat-4 viscosity cup and the thickness T (in millimeters) of the plate coated on the welded steel plate satisfy the following relationship: 90T ≤ t ≤ 120T.

5. The method according to claim 2, wherein the gap (in millimeters) between the rubber roller and the welded steel plate is set to √T, where T is the thickness (in millimeters) of the welded steel plate; and / or The roller coating speed is set to less than or equal to 20 m / min, preferably less than or equal to 8 m / min, and more preferably 3 m / min to 5 m / min.

6. The method according to claim 1, wherein the method further comprises the following step: c) Dry the coated welded steel plate at a temperature of 100-800℃, preferably 200-400℃, for a time of 1-50 minutes, preferably 3-10 minutes.

7. The method according to claim 1, wherein the steel plate is a high-strength steel plate.

8. A method for coating a steel substrate, characterized in that, This includes the following steps performed in sequence: a) Provide a steel substrate, said steel substrate being selected from a single steel plate or steel coil; and b) Apply the coating evenly to the surface of the steel substrate; In step b), the coating method is selected from roller coating or spray coating.

9. A method for hot forming of steel, characterized in that, This includes the following steps performed in sequence: a) Provide a coated welded steel plate prepared by the method according to any one of claims 1-7 or a coated steel substrate prepared by the method according to claim 8; b) Heat the coated steel to a temperature between Ac1 and 1000°C, and hold it at a temperature above Ac3; as well as c) Transfer the heat-insulated coated steel into a hot forming mold, cool it, and stamp it into shape; Wherein, Ac1 is the temperature at which austenite begins to form in the steel during the heating process, and Ac3 is the temperature at which the austenite transformation of the steel ends during the heating process.

10. The method according to claim 9, wherein the heating time (in seconds) in step b) is 85T, where T is the thickness of the steel (in millimeters); and / or The cooling rate in step c) is not less than 25°C / s.

11. The method of claim 9, wherein the coating applied to the steel surface is selected from organic resins and their derivatives, and the coating formed by the coating undergoes a ceramic-like transformation in step b).

12. A thermoformed component, characterized in that, The thermoformed component is prepared by the method described in any one of claims 9-11.

13. The thermoformed component according to claim 12, wherein the thermoformed component has a coating with a nominal thickness in the range of 3 μm to 33 μm.

14. The thermoformed component according to claim 12, wherein the thermoformed component has a tensile strength of 1460MPa-1480MPa, a yield strength of 1020MPa-1080MPa, and an elongation at break of 5%-5.5%.

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