Coated steel sheets for hot stamping and aqueous surface treatment liquids applicable thereto.

Abstract

The present invention relates to a coated steel sheet for hot stamping and an aqueous surface treatment liquid applicable thereto. A composite coating layer is formed on the surface of the coated steel sheet by using the improved aqueous surface treatment liquid, so that the obtained coated steel sheet for hot stamping has good high-temperature lubricity during the hot stamping process, the processability of the coated steel sheet for hot stamping is improved, the wear of the surface of the hot stamping die is reduced, the frequency of die repair is reduced, and the production efficiency is improved.

Description

[Technical Field]

[0001] This invention belongs to the field of metal surface treatment, and more particularly to coated steel sheets for hot stamping and aqueous surface treatment solutions applicable thereto. [Background technology]

[0002] Hot-stamped ultra-high-strength steel can reduce the weight of a vehicle while providing crash safety performance for the vehicle body. It is one of the effective ways to achieve energy savings and emission reductions in automobiles. Therefore, hot-stamped parts and the hot-stamping process are increasingly used in the automotive industry and are now widely used in the manufacture of automotive parts such as A-pillars, B-pillars, door rings, center channels, and bumpers.

[0003] Among ordinary carbon steels, only steel grades such as boron-based alloy steels can achieve a complete transformation to a martensitic microstructure during the cooling process following hot stamping formation. Among these, 22MnB5 is the most commonly used hot-stamped steel. Before hot forming, this material has a ferrite-pearlite microstructure with a yield strength of approximately 550 MPa. After complete martensitic transformation, the yield strength can reach approximately 1500 MPa. To achieve a complete transformation to a martensitic microstructure, the material must undergo sufficient austenitization at high temperatures, followed by hot forming and quenching to complete the phase transformation process. The austenitization treatment is carried out in a heating furnace, where the sheet is heated to a temperature above Ac3 (typically 900-950°C) and held for 3 to 10 minutes, depending on the thickness of the sheet. After complete austenitization, the material is transferred from the heating furnace to a hot forming press. This process is carried out in air, and the cooling rate must exceed 27 K / sec before reaching 400°C to ensure a non-diffusion phase transformation to the martensite phase and ultimately achieve the corresponding mechanical strength.

[0004] However, when the hot stamping method is applied to boron-based alloy steel sheets, the surface of the steel sheet will oxidize, leading to decarburization and scale formation. Decarburization reduces the surface strength of the steel sheet, while scale increases friction between the steel sheet and the mold, reducing the service life of the mold. Furthermore, the scale that adheres to the mold surface must be removed during production, which reduces production efficiency. Hot-stamped parts require shot blasting as a post-processing step. For parts with reduced sheet thickness, shot blasting can easily induce residual stress on the surface of the part, leading to deformation. In addition, for parts requiring corrosion resistance, post-processing steps such as surface cleaning, surface pre-treatment, and application of rust-preventive coatings are necessary, which reduces production efficiency.

[0005] To prevent oxidation and decarburization on the surface of steel sheets and to improve the high-temperature resistance and corrosion resistance of hot-stamped steel sheets, high-temperature oxidation prevention technologies applicable to hot-stamped steel sheets have been developed. These mainly include molten aluminum plating with aluminum-silicon (Al-10 wt%Si), pure zinc (GI), alloy zinc-iron (GA), and electroplated zinc-nickel (Zn-11 wt%Ni) coatings, as well as multifunctional protective coatings. However, the heating temperature during hot pressing (700-1000°C) is higher than the melting and boiling points of Zn metal and alloys, leading to embrittlement induced by the liquid metal (Zn or Zn-Fe) (liquid metal-induced embrittlement), which limits the use of GI and GA coatings. Zn-Ni coatings have some potential applications, but they are expensive and have low productivity. Antioxidant oils, protective coatings, etc., can prevent high-temperature oxidation of steel sheet substrates to a certain extent, but they suffer from poor surface adhesion and uniformity when exposed to high temperatures. This can lead to coating powdering and peeling, lack of corrosion resistance, and narrow weld windows.

[0006] Aluminum silicon (AlSi) coatings have several advantages, including no oxide scale peeling during heating, no need for sandblasting after stamping, high formation accuracy, no nitrogen protection requirement, and large weld windows after heating. Therefore, they are now widely used in hot stamping processes. Furthermore, AlSi coatings also have a certain rust-preventive effect, which improves corrosion resistance. However, during the heating process of a steel sheet with an AlSi coating on its surface, elemental Fe from the steel sheet diffuses into the AlSi coating, creating various Al-Fe-Si layers and Al-Fe alloy layers in the bulk phase and on the surface of the AlSi coating. Among these, the Al-Fe alloy layer has extremely high hardness, which can lead to surface damage and wear problems on both the processed part and the mold due to contact and friction between the AlSi-coated steel sheet and the mold during the hot stamping process.

[0007] Due to the high surface hardness of the Al-Fe alloy layer, it is difficult to achieve relative sliding at high temperatures, resulting in poor lubricity. During the hot stamping process, it is prone to cracking, and defects such as cracks perpendicular to the interface between the coating and the iron substrate may appear in areas with significant deformation, and the surface may pulverize, leading to reduced formability of the steel sheet. Furthermore, the detached Al-Fe metal compound adheres to the mold, which further worsens the friction between the coated steel sheet and the mold, thereby degrading the quality of the final part and causing wear on the mold surface. As a result, more frequent mold repairs are required to remove the Al-Fe metal compound adhered to the mold surface, leading to decreased production efficiency and increased production costs for the finished parts.

[0008] Therefore, in order to prevent damage to the surface of parts and molds during the hot stamping process of steel sheets coated with an AlSi layer, and to reduce the adhesion of Al-Fe metal compounds to the mold surface, Publication No. CN102066615A provides a solution of forming a surface coating layer containing ZnO having a wurtzite crystal structure on the Al plating layer so that the Al plating layer is lubricated, the plating thickness can be prevented from becoming uneven during heating, and the formability of the hot stamped steel sheet is improved. However, in this technique, it is necessary to adhere the ZnO particles to the surface of the Al plating layer using an adhesive such as an organic resin and a silane coupling agent. These adhesives will be oxidized and decomposed at heating temperatures of 700-1000°C, causing a large amount of ZnO to peel off from the surface of the Al plating layer. During this process, the peeled ZnO particles may diffuse and contaminate the heating furnace channels during the heating process, or adhere to the mold surface during the hot stamping process, thereby causing mold contamination. Furthermore, residual ZnO particles on the surface of the Al plating layer can degrade the weldability of the formed part.

[0009] Publication No. CN104093880A discloses a solution containing at least one Zn compound selected from the group consisting of Zn hydroxide, Zn phosphate, and Zn organic acid as a surface coating layer on an Al plating layer. This technique also requires at least one of an organic resin, a silane coupling agent, and a silica sol as an adhesive. However, Zn hydroxide, Zn phosphate, and Zn organic acid are all oxidized to zinc oxide (ZnO) at heating temperatures of 700-1000°C, and the aforementioned adhesives can also be oxidized and decomposed. Therefore, similar to the technique of CN102066615A described above, this technique also does not address the problem of poor adhesion of Zn-containing oxide particles on the surface of the Al plating layer after hot stamping, as well as the problem of detached Zn oxide particles contaminating the furnace channels and molds.

[0010] Publication No. CN104220641A describes the addition of a compound containing one or more transition metal elements to a surface coating layer containing ZnO as an additive to improve lubricity. This is demonstrated. However, like the two aforementioned patents, this technology also requires an organic adhesive to facilitate the film formation and adhesion of ZnO and transition metal compounds. After the organic adhesive decomposes under heating conditions of 700-1000°C, it is also impossible to maintain the adhesion of ZnO and transition metal compounds on the surface of the Al-plated layer of the steel sheet.

[0011] Publication No. CN105121691A discloses that the heating performance of a steel sheet in hot pressing can be improved by including an Al plating layer containing 0.02 to 2 mass% of one or more elements selected from Mg, Ca, Sr, Li, Na, and K, and a surface coating layer laminated on the Al plating layer and containing ZnO. However, changing the composition of the Al plating layer also does not solve the problem that after heating the ZnO surface coating layer containing an organic adhesive at 700 to 1000°C, the ZnO cannot form an effective bond on the surface of the Al plating layer due to the decomposition of the adhesive.

[0012] Publication No. CN107636200A provides a surface treatment solution for hot-pressed plated steel sheets comprising an aqueous dispersion of ZnO and a water-dispersible organic resin, wherein the water-dispersible organic resin acts as a film-forming adhesive after the surface treatment solution is applied to the surface of the plated steel sheet and dried to form a film. From the processing procedure of the hot-pressed steel sheet: Under heating conditions of 700-1000°C, it is found that after the organic resin decomposes due to the high temperature, the ZnO particles cannot form an effective bond on the surface of the plated layer, similar to the technique described above.

[0013] In summary, the technique of applying wurtzite-type ZnO particles and aqueous resins to the surface of an Al or AlSi plating layer to improve thermal lubricity and chemical conversion processability has some effect. However, with regard to aqueous resins, thermal decomposition occurs under high-temperature heat treatment conditions, and as a result, a large number of ZnO particles cannot form effective adhesion on the surface of the plating layer. Consequently, the coating often exhibits obvious thermal lubricity, requiring 3 g / m². 2 Higher film weights are required, such as with ultra-wurtzite-type ZnO compounds. The advantages in thermal lubricity, chemical conversion processability after hot forming, and corresponding improvements in coating corrosion resistance, as demonstrated in practical applications, are also unstable. Furthermore, the use of other adhesives, such as silane coupling agents, cannot solve the problem of reduced adhesion of the coating layer due to adhesive decomposition after high-temperature heat treatment. It also introduces thermal decomposition products such as silica, which impairs weldability, adhesion, and chemical conversion processability between coatings, as well as affecting the adhesion and corrosion resistance of the coating after application. [Overview of the project]

[0014] Taking into consideration the aforementioned drawbacks of the prior art, the object of the present invention is to provide a coated steel sheet for hot stamping and an aqueous surface treatment liquid applicable thereto.

[0015] In this invention, by designing the composition of an aqueous surface treatment liquid and applying it to the surface of a coated steel sheet, the coated steel sheet for hot stamping has good high-temperature lubricity during the hot stamping process, the workability of the coated steel sheet for hot stamping is improved, surface wear of the hot stamping mold is reduced, the frequency of mold repair is decreased, and production efficiency is improved. Furthermore, after hot stamping the coated steel sheet for hot stamping, good weldability and adhesion can be achieved, chemical conversion processability is good, and corrosion resistance after coating is improved.

[0016] Specifically, the present invention employs the following technical solutions:

[0017] According to a first aspect of the present invention, there is provided a coated steel sheet for hot stamping, including a composite coating layer formed by applying an aqueous surface treatment liquid onto the coated steel sheet; wherein the coated steel sheet includes a substrate and an Al or AlSi-containing coating disposed on one or both sides of the substrate; and the composite coating layer contains an aqueous anionic polymer resin A, a tungsten-containing compound B, a zinc-containing compound C, a boron-containing compound D, a water-soluble thickener E, and an aqueous surface improvement aid F. Preferably, the composite coating layer has a dry film weight of 1 to 5 g / m².

[0018] According to a second aspect of the present invention, there is provided an aqueous surface treatment liquid for forming the above composite coating layer on a coated steel sheet, wherein the aqueous surface treatment liquid is formed by dispersing a solid substance in water; 2 wherein the solid substance contains the following components in parts by mass:

[0019] aqueous anionic polymer resin A: 15 to 65 parts; tungsten-containing compound B: 10 to 40 parts as parts by mass of tungsten element; zinc-containing compound C: 3 to 15 parts as parts by mass of zinc element; boron-containing compound D: 0.004 to 0.03 parts as parts by mass of boron element; water-soluble thickener E: 0.01 to 0.5 parts; aqueous surface improvement aid F: 0.5 to 2 parts. Preferably, the aqueous anionic polymer resin A is one or more selected from an aqueous acrylic resin, an aqueous epoxy resin, an aqueous polyester resin, and an aqueous alkyd resin; and / or the tungsten-containing compound B is one or more selected from tungsten oxide, tungstate, or tungsten metal salt; and / or

[0020] the zinc-containing compound C is selected from zinc oxide, zinc salt, or organic zinc compound; and / or the boron-containing compound D is selected from boron oxide, borate, or organic boron compound; and / or the aqueous surface improvement aid F is selected from a surfactant, a leveling agent, and a defoaming agent. Boron-containing compound D is selected from boron oxide, borate, or organoboron compounds; and / or Water-soluble thickener E is one or more selected from water-soluble hydroxymethylcellulose, water-dispersible alkali-swellable polyacrylic acid emulsion, and water-dispersible nonionic polyurethane-associative thickening resin; and / or The aqueous surface improvement aid F is one or two selected from water-dispersible modified polydimethylsiloxane and water-dispersible polyether-modified silicone; and / or Here, aqueous anionic polymer resin A is 20 to 50 parts by mass; and / or Tungsten-containing compound B contains 15 to 35 parts by mass of tungsten.

[0021] Preferably, the aqueous anionic polymer resin A has a number average molecular weight greater than 8,000, and the mass fraction of harmful substances resulting from the combustion of the aqueous anionic polymer resin A is less than 1% by weight of the total solids of the aqueous anionic polymer resin A; and / or When tungsten-containing compound B is a water-insoluble tungsten compound, the tungsten-containing compound B has a particle size of 5 to 1,000 nm; and / or When zinc-containing compound C is a water-insoluble zinc compound, the zinc-containing compound C has a particle size of 5 to 1,000 nm; and / or The mass ratio of the zinc element in the zinc-containing compound C to the boron element in the boron-containing compound D is 150:1 to 750:1, preferably 250:1 to 670:1; and / or Water-soluble thickener E has a number-average molecular weight of 50,000 to 100,000.

[0022] Preferably, tungsten-containing compound B has a particle size of 20 to 500 nm; and / or Zinc-containing compound C has particle sizes ranging from 20 to 500 nm.

[0023] A third aspect of the present invention provides a method for manufacturing a coated steel sheet for hot stamping, comprising the following steps: applying an aqueous surface treatment solution to a surface-cleaned Al or AlSi coated steel sheet; then curing the aqueous surface treatment solution at 80 to 180°C to form a composite coating layer on the Al or AlSi coated steel sheet, thereby obtaining a coated steel sheet for hot stamping.

[0024] According to a fourth aspect of the present invention, the present invention provides a hot stamping method for the above-described hot stamping coated steel sheet, comprising the steps of cutting the hot stamping coated steel sheet into a mold, heating it, and forming a stamp.

[0025] Preferably, during the heating process, the hot stamping coated steel sheet cut into the mold is heated from 50°C to the target plate temperature and held for 30 to 60 seconds; then the heated hot stamping coated steel sheet is transferred to a stamp forming mold that has a cooling effect.

[0026] Preferably, during the heating process, the average heating rate is 3-12°C / second, and the final plate temperature is 850-980°C; and / or The stamping process is carried out in air at a cooling rate of ≥27°C / second before the temperature of the coated steel sheet for hot stamping drops to 400°C. The method of cooling the steel sheet is as follows: The stamping mold has a water cooling function. During hot stamping, the cooling rate is controlled by adjusting the flow rate and pressure holding time of the cooling water in the mold. Alternatively, forced cooling can be carried out by water mist spraying, provided that the cooling rate meets the requirements. There are no special restrictions on the upper limit of the cooling rate, which can be selected according to the actual needs. Considering the process cost, it is preferable to set the upper limit of the cooling rate to 50°C / second.

[0027] According to a fifth aspect of the present invention, a hot-stamped and formed coated steel sheet obtained by the hot-stamping method described above is provided, wherein the hot-stamped and formed coated steel sheet has a yield strength of ≥950 MPa, a tensile strength of ≥1300 MPa, and an elongation after break of ≥5% A 50mm It has a hardness of ≥400 HV10 or ≥40 HRC, and a cold bending angle VDA of ≥45°. The coated steel sheet for hot stamping and the aqueous surface treatment liquid applicable thereto provided by the present invention have the following beneficial effects:

[0028] 1. In this invention, by designing the composition of an aqueous surface treatment liquid and applying it to the surface of a coated steel sheet, the coated steel sheet for hot stamping has good high-temperature lubricity during the hot stamping process, the workability of the coated steel sheet for hot stamping is improved, surface wear of the hot stamping mold is reduced, the frequency of mold repair is decreased, and production efficiency is improved. Furthermore, after hot stamping the coated steel sheet for hot stamping, good weldability and adhesion can be achieved, chemical conversion processability is good, and corrosion resistance after coating is improved.

[0029] 2. The present invention provides Al or AlSi coated steel sheets for hot forming and an aqueous surface treatment liquid. The aqueous surface treatment liquid and the products treated therewith do not contain harmful elements such as chromium and have little impact on the environment.

[0030] 3. The hot-stamping coated steel sheet of the present invention is coated on its surface with a composite coating layer containing tungsten (W), zinc (Zn), and an organic resin. The composite coating layer has a corrosion-preventive effect equivalent to that of rust-preventive oil and can withstand long-term transportation or storage without the protection of rust-preventive oil. Furthermore, the coated steel sheet for hot stamping has stable thermal lubricity during the hot stamping process, as well as good chemical conversion processability, corrosion resistance after coating, and spot welding performance after hot stamping.

[0031] 4. The aqueous surface treatment solution of the present invention is easy to apply and has stable performance.

[0032] 5. The manufacturing method and hot stamping method for coated steel sheets for hot stamping according to the present invention have a short process flow and are easy to implement. [Brief explanation of the drawing]

[0033] Other features, purposes, and advantages of the present invention will become more apparent by referring to the accompanying drawings and reading the following detailed description of non-limiting embodiments: [Figure 1] Figure 1 is a schematic diagram of the structure of the coated steel sheet for hot stamping according to the present invention. [Figure 2] Figure 2 is a schematic diagram of a testing machine for testing the high-temperature friction coefficient of a plate strip according to one embodiment of the present invention. [Figure 3] Figure 3 is a schematic longitudinal cross-sectional view of a sample plate of the adhesive component used to test its adhesion.

[0034] Reference Code List 10 circuit boards 11 Al or AlSi-containing coating 12 Composite coating layer 20 Test sample boards 21 Motor 22 Coupling 23 Transmission shaft 24 Recorders 25 Force Sensor 26 Upper mold 27 Cooling water channels 28 Lower mold 29 Heating Furnace [Modes for carrying out the invention]

[0035] Detailed description of the embodiment To better understand the above technical solutions of the present invention, the present invention will be further described in combination with the following embodiments.

[0036] As shown in Figure 1, the present invention provides a coated steel sheet for hot stamping, comprising a composite coating layer 12 formed by applying a coated steel sheet and an aqueous surface treatment liquid onto the coated steel sheet; wherein the coated steel sheet comprises a substrate 10 and an Al or AlSi-containing coating 11 disposed on one or both sides of the substrate 10. The composite coating layer 12 contains an aqueous anionic polymer resin A, a tungsten-containing compound B, a zinc-containing compound C, a boron-containing compound D, a water-soluble thickener E, and an aqueous surface improvement aid F.

[0037] The composite coating layer 12 contains 1-5 g / m² 2 It has a dry film weight of 1 g / m². That is, the thickness of the composite coating layer 12 on each surface is 1 g / m². 2 and 5g / m 2 It is controlled between the following. The reason is as follows: The film weight of the composite coating layer is 1 g / m 2 When the amount of coating is less than 5 g / m², a low coating amount leads to a decrease in the processing lubrication function of the steel sheet during hot stamping formation. 2 When it exceeds this, on the one hand, the surface treatment cost per unit area of ​​steel plate increases. On the other hand, an excessively thick coating layer may prevent the organic components in the composite coating layer 12 from being completely decomposed during the heat treatment process, and some inorganic substances in the coating layer may not form good adhesion with the surface of the Al or AlSi coated steel sheet, and residues from the combustion of inorganic substances may contaminate the furnace and mold.

[0038] The parts by mass of each solid component in the composite coating layer 12 are as follows: aqueous anionic polymer resin A: 15 to 65 parts; tungsten-containing compound B: 10 to 40 parts by mass of tungsten element; zinc-containing compound C: 3 to 15 parts by mass of zinc element; boron-containing compound D: 0.004 to 0.03 parts by mass of boron element; water-soluble thickener E: 0.01 to 0.5 parts; aqueous surface improvement aid F: 0.5 to 2 parts.

[0039] Aqueous anionic polymer resin A is one or more composite resins selected from aqueous acrylic resins, aqueous epoxy resins, aqueous polyester resins, and aqueous alkyd resins. Aqueous anionic polymer resin A should have a number average molecular weight greater than 8,000. The mass fractions of harmful nitrogen, sulfur, and chlorine elements that may result from the combustion of aqueous anionic polymer resin A should be less than 1% by weight of the total solids of the aqueous anionic polymer resin A. Aqueous anionic polymer resin A can be dispersed in water in the form of an aqueous solution, an aqueous dispersion, or an emulsion.

[0040] The composite coating layer 12 contains 15 to 65 parts by mass of aqueous anionic polymer resin A. When the amount of aqueous anionic polymer resin A is less than 15 parts by mass, the adhesion of the composite coating layer formed by the aqueous anionic polymer resin A to the surface of the Al or AlSi coated steel sheet for hot stamping may decrease. When the amount of aqueous anionic polymer resin A exceeds 65 parts by mass, the relative proportion of tungsten-containing compound B, which mainly plays a role in high-temperature lubrication in the composite coating layer on the surface of the Al plated or AlSi plated steel sheet for hot stamping, decreases, and sufficient lubrication may not be provided during the hot stamping process. More preferably, the composite coating layer 12 contains 20 to 50 parts by mass of aqueous anionic polymer resin A.

[0041] Tungsten-containing compound B has high-temperature lubrication properties, and its tungsten content is 10 to 40 parts by mass. The reasons are as follows: When the tungsten content of tungsten-containing compound B having high-temperature lubrication properties is less than 10 parts by mass, it cannot perform its high-temperature lubrication function. When the tungsten content of tungsten-containing compound B having high-temperature lubrication properties is greater than 40 parts by mass, the performance of the composite coating layer deteriorates, which can affect properties such as adhesion, water resistance, and corrosion resistance of the coating and the Al or AlSi coated steel sheet, and can also increase production costs. Preferably, the tungsten content of tungsten-containing compound B having high-temperature lubrication properties is 15 to 35 parts by mass.

[0042] Preferably, the tungsten-containing compound B having a high-temperature lubricating function is formed during the hot stamping process and contains compounds having a lubricating function such as tungsten oxide and tungstate. When the tungsten-containing compound B is coated on an Al or AlSi-based coated steel sheet, it has a lubricating effect at the interface between the steel sheet and the mold. The source of the tungsten-containing compound B is tungsten trioxide (WO3), tungsten dioxide (WO2) or blue tungsten oxide (WO 2.72 , WO 2.90 , W 20 O 58 and (NH4) x ·WO3), etc., tungsten oxide; or sodium tungstate (Na2WO4), potassium tungstate (K2WO4), lithium tungstate (Li2WO4), magnesium tungstate (MgWO4), calcium tungstate (CaWO4), cobalt tungstate (CoWO4), cadmium tungstate (CdWO4), ferrous tungstate (FeWO4), ammonium tungstate um [(NH4)6W7O 24 , and zinc tungstate (5ZnO·12WO3), etc., tungstate compounds; or tungsten metal salts such as tungsten chloride [tungsten hexachloride (WCl6), tungsten pentachloride (WCl5), tungsten tetrachloride (WCl4), tungsten dichloride (WCl2)]; or other types of tungsten-containing compounds such as nanoscale tungsten carbide (WC), tungsten disulfide (WS2) and tungsten diselenide (WSe2). The tungsten-containing compound B having a high-temperature lubricating function may contain one or more tungsten oxides, tungstates, tungsten salts or other tungsten-containing compounds. These may be the same type of tungsten oxide, tungstate, tungsten salt or other tungsten-containing compound, or they may be different types of tungsten oxide, tungstate, tungsten salt, other tungsten-containing compounds, or mixtures thereof.

[0043] Tungsten-containing compound B, which has high-temperature lubrication properties, undergoes thermal decomposition and thermochemical reactions during the hot stamping and heating process (limiting temperature reaches 850°C to 980°C). Therefore, from the standpoint of environmental protection and operational safety, tungsten-containing compounds that decompose at high temperatures and produce harmful substances should be avoided. Preferably, tungsten-containing compound B is tungsten trioxide (WO3), sodium tungstate (Na2WO4), potassium tungstate (K2WO4), magnesium tungstate (MgWO4), ammonium tungstate [(NH4)6W7O 24 It is at least one selected from ] and nano-tungsten carbide (WC).

[0044] The tungsten-containing compound B, which has high-temperature lubrication properties, can be uniformly dispersed in the composite coating layer 12 in the form of a powder or solute. Preferably, in order to improve the dispersibility of the tungsten-containing compound B, which has high-temperature lubrication properties, an aqueous anionic polymer resin A having good compatibility with it can be selected to match it. For the tungsten-containing compound B in powder form, a dispersant can be added in an amount of 0.01 to 0.5 parts by mass to improve the dispersibility of the tungsten-containing compound B in the aqueous anionic polymer resin A matrix, so that the tungsten-containing compound B, which has high-temperature lubrication properties, maintains its basic stability in physical properties below 200°C and does not aggregate or separate in the composite coating layer 12.

[0045] Tungsten-containing compound B, which has high-temperature lubrication properties, such as tungsten oxide, tungstate salts, tungsten salts, or other tungsten-containing compounds insoluble in water, has a particle size of 5 to 1000 nm, preferably 20 to 500 nm, in powder form.

[0046] Tungsten-containing compound B, which has high-temperature lubrication properties, is a suitable high-temperature lubricant in hot stamping processes. This is because tungsten-containing compound B, such as tungsten oxide, tungstate salts, tungsten salts, or other tungsten-containing compounds, can partially or completely decompose (or convert) to tungsten oxide (WOx), a solid lubricant with a high ionic potential at high temperatures. For example, tungstate salts undergo substitution reactions with Al2O3 on the surface of Al or AlSi coatings at high temperatures:

[0047] [ka]

[0048] As a result, WO3 with high-temperature lubrication properties can be formed. The formed tungsten oxide exhibits good adhesion to Al or AlSi coatings at high temperatures. Furthermore, zinc tungstate (5ZnO·12WO3), nanotungsten carbide (WC), tungsten disulfide (WS2), tungsten diselenium (WSe2), etc., also possess good high-temperature oxidation resistance and high-temperature lubrication properties themselves.

[0049] The part by mass of zinc in zinc-containing compound C is 3 to 15 parts. The reasons are as follows: When the part by mass of zinc in zinc-containing compound C is less than 3 parts, it cannot effectively improve the chemical conversion processability of the part after hot forming and the corrosion resistance after coating. When the part by mass of zinc in zinc-containing compound C is more than 15 parts, it can lead to reduced adhesion between the surface composite coating and the Al or AlSi coated steel sheet, and the zinc-containing conversion layer formed after hot forming may not adhere well to the surface of the coated steel sheet. When zinc-containing compound C is a zinc-containing compound that is insoluble in water, in order to ensure its uniform dispersion in the surface composite coating layer, its particle size in powder form should be 5 to 1000 nm, preferably 20 to 500 nm. Sources of zinc-containing compound C include zinc oxide (ZnO), zinc salts such as zinc nitrate (ZnNO3) and zinc chloride (ZnCl2), or zinc acetate (Zn(Ac)2) and zinc stearate (C 36 H 70 It may be an organozinc compound such as O4Zn.

[0050] The part by mass of boron in boron-containing compound D is 0.004 to 0.03 parts. The reason for this is as follows: When the part by mass of boron in boron compound D is less than 0.004, it cannot effectively improve the adhesion of the coating to the surface of the Al or AlSi coated steel sheet after hot forming. When the part by mass of boron in boron compound D is greater than 0.03, the reactivity of the composite coating layer with chemical treatment agents and the coating performance after hot forming will deteriorate. The source of boron-containing compound D may be boron oxide such as boron oxide (B2O3); borates such as magnesium borate (MgHBO3), sodium tetraborate (Na2B4O7), and sodium borohydride (NaBH4); or organoboron compounds such as borane, carborane, methyl borate, and organoborates.

[0051] The mass ratio (W) of zinc-containing compound C as zinc element to boron-containing compound D as boron element Zn / W BThe ratio is 150:1 to 750:1. Boron-containing compound D is a suitable adhesion promoter in the hot forming process because boron oxide, borate, or organoboron compounds can decompose to B2O3 under heat treatment. B2O3 functions as a flux and network-forming agent and, when combined with zinc-containing compound C, forms a zinc borate (e.g., B2O6Zn3) solid solution, which improves the adhesion of the surface composite coating layer to the surface of the Al or AlSi-based coated steel sheet after hot forming.

[0052] The amount of water-soluble thickener E to be added is 0.01 to 0.5 parts. The reasons for this are as follows: When the amount of water-soluble thickener is less than 0.01 parts by mass, it has no noticeable thickening effect. When the amount of water-soluble thickener exceeds 0.5 parts by mass, it is very likely to result in poor uniformity of the surface composite coating layer on the surface of the Al or AlSi-based coated steel sheet, and will worsen the leveling of the aqueous surface treatment solution after coating.

[0053] The water-soluble thickener E is one or more selected from water-soluble hydroxymethylcellulose, water-dispersible alkali-swellable polyacrylic acid emulsion, and water-dispersible nonionic polyurethane-associated thickening resin. The number-average molecular weight of the water-soluble thickener E (such as water-soluble hydroxymethylcellulose, water-dispersible alkali-swellable polyacrylic acid emulsion, and water-dispersible nonionic polyurethane-associated thickening resin) is 50,000 to 100,000.

[0054] The amount of aqueous surface improvement aid F to be added is 0.5 to 2 parts. The reasons for this are as follows: When the mass of the aqueous surface improvement aid is less than 0.5 parts, the aqueous surface treatment solution cannot perform a good spreading and leveling role on the surface of the Al or AlSi-based coated steel sheet, and a sufficiently spread and uniform composite coating layer cannot be formed on the surface of the Al or AlSi-based coated steel sheet. When the mass of the aqueous surface improvement aid exceeds 2 parts, it is very likely to cause defects such as streaks and pinholes that occur during the drying process of the aqueous surface treatment solution after coating.

[0055] Aqueous surface improvement aid F contains water-dispersible modified polydimethylsiloxane and water-dispersible poly It is one or two selected from tel-modified silicones.

[0056] The aqueous surface treatment solution described above is applied to an Al or AlSi coated steel sheet whose surface has been cleaned, and the aqueous surface treatment solution is solidified at 80 to 180°C to form a composite coating layer on the Al or AlSi coated steel sheet, thereby obtaining a coated steel sheet for hot stamping. The Al or AlSi coated steel sheet having the composite coating layer obtained by the above method on its surface has excellent hot stamping workability, excellent weldability after formation, and good coating properties. Methods for applying the aqueous surface treatment solution to the Al or AlSi coated steel sheet include roller coating, dipping, or spraying. The aqueous surface treatment solution can be applied to one or both sides of the Al or AlSi coated steel sheet.

[0057] The reason the surface treatment solution solidifies at 80-200°C is as follows: At 80-180°C, the water and water-soluble low-boiling point additives, which are solvents in the aqueous surface treatment solution, can evaporate, promoting crosslinking and film formation of the aqueous anionic polymer resin A. When the curing temperature is lower than 80°C, the dried film of the composite coating layer formed by the aqueous surface treatment solution and the Al or AlSi coated steel sheet will not be completely crosslinked, which can lead to a decrease in the adhesive strength of the dried film. When the curing temperature is higher than 180°C, on the one hand, the energy cost of surface treatment of the steel sheet will increase, and on the other hand, some of the performance of the composite coating layer will change, thereby adversely affecting the dried film of the composite coating layer. In this invention, there are no particularly strict limitations on the heating and drying method of the aqueous surface treatment solution coated on the surface of the Al or AlSi coated steel sheet. The heating and drying method can be various heating methods such as hot air heating, induction heating, and infrared heating.

[0058] The present invention also provides a hot stamping method for a hot stamping coated steel sheet, comprising the steps of cutting out a hot stamping coated steel sheet, either an Al or AlSi coated steel sheet having the composite coating layer described above on its surface, into a mold, heating it, and forming a stamp. During the heating process, the hot stamping coated steel sheet cut into the mold is heated from 50°C to the target plate temperature and held for 30 to 60 seconds; then the heated hot stamping coated steel sheet is transferred to a stamp-forming mold having a cooling effect. During the heating process, the average heating rate is 3 to 12°C / second, and the target plate temperature is 850 to 980°C; the stamp-forming process is carried out at a cooling rate of ≥27°C / second in air before the temperature of the hot stamping coated steel sheet drops to 400°C.

[0059] The hot stamping coated steel sheet and aqueous surface treatment liquid of the present invention will be further described below in combination with specific examples / embodiments. [Examples]

[0060] Examples 1-32 and Comparative Examples 1-8 In Examples 1-32 and Comparative Examples 1-8, the substrates were all molten aluminum silicon (AlSi) alloy coated steel sheets. The aqueous surface treatment solution was applied to the surface of the cleaned AlSi alloy coated steel sheet and dried at 80-180°C, resulting in a coating of 1-5 g / m². 2 A composite coating layer having the dry film weight of [amount] was formed on the surface of the AlSi alloy coated steel sheet.

[0061] The selected steel sheet is preferably subjected to a hot stamping process to obtain steel sheets having various strength levels, including various properties related to mechanical deformation and fracture characteristics such as tensile strength, yield strength, elongation after fracture, hardness, and cold bending angle. In one embodiment of the present invention, after hot stamping, the yield strength is ≥ 950 MPa, the tensile strength is ≥ 1300 MPa, and the elongation after fracture is ≥ 5% (A 50mmSelect a steel sheet capable of achieving a hardness of HV10 ≥ 400 (or HRC ≥ 40) and a cold bending angle VDA (1.4 mm) ≥ 45°. There are no special restrictions on the upper limit of the performance of the coated steel sheet after hot stamping. Of course, steel Considering the overall production costs of the sheets, for coated steel sheets formed by hot stamping, the upper limit of the yield strength can be set to 1200 MPa, the upper limit of the tensile strength can be set to 1500 MPa, the upper limit of the elongation after fracture can be set to 10%, the upper limit of the hardness HV10 can be set to 500, the upper limit of the HRC can be set to 50, and the upper limit of the cold bending angle VDA can be set to 80°.

[0062] The above mechanical properties (including yield strength, tensile strength, and elongation after fracture) are based on GB / T 228.1-2010 “Metallic materials - Tensile testing - Part 1: Method of test at room” The hardness of coated steel sheets (HV10) is measured according to GB / T 4340-2012 “Metallic materials - Vickers hardness test”. Rockwell hardness (HRC) is measured according to GB / T 230-2018 “Metallic materials - Rockwell Measure according to the "hardness test". The cold bending angle VDA is measured according to the VDA238-100 standard.

[0063] The chemical composition of steel sheets is, in mass percent, carbon (C): 0.20-0.25 wt%, silicon (Si): 0.01-0.4 wt%, manganese (Mn): 1.0-1.4 wt%, titanium (Ti): 0.02-0.05 wt%, boron (B): 0.001-0.005 wt%, and the remainder consisting of Fe and other unavoidable impurities.

[0064] In steel sheets, the addition of carbon (C) ensures the mechanical strength of the steel sheet. When the C content is less than 0.2% by weight, the yield strength and tensile strength described above cannot be obtained. On the other hand, when the C content exceeds 0.25 wt%, the steel sheet can become even harder, but melt cracking (melt cracking) can occur. Cracks can easily form during hot stamping, leading to a decrease in elongation at fracture and cold bending angle performance. Therefore, the carbon content is preferably 0.20 to 0.25% by weight.

[0065] Si and C are also elements that ensure the mechanical strength of the steel sheet. When the Si content is less than 0.01% by weight, the effect of increasing mechanical strength is not shown, and the yield strength and tensile strength mentioned above cannot be obtained. On the other hand, Si is an element that is easily oxidized. If the Si content exceeds 0.4% by weight, the surface energy of the steel sheet will decrease, and the wettability during hot-dip plating will be affected, resulting in incomplete plating or no plating at all. Therefore, the Si content is preferably 0.01 to 0.4% by weight.

[0066] Mn is an element that improves the hardenability and strength of steel sheets after hot stamping. Furthermore, Mn reacts with the impurity element sulfur (S) to form manganese sulfide (MnS), which prevents the steel sheet from developing hot brittleness due to sulfur (S). When the Mn content is less than 1.0 wt%, the desired yield strength and tensile strength cannot be obtained. On the other hand, when the Mn content exceeds 1.4 wt%, the excess residual γ phase after hot stamping can lead to a decrease in yield strength and tensile strength. Therefore, the Mn content is preferably 1.0 to 1.4 wt%.

[0067] Ti can improve the strength of steel sheets and the heat resistance of Al-based coatings. When the Ti content is less than 0.02% by weight, the effect of improving mechanical strength and oxidation resistance cannot be obtained. On the other hand, when the Ti content is higher than 0.05% by weight, the excess Ti is likely to produce carbides and nitrides that soften the steel with elements such as C and N, and may not be able to obtain the desired yield strength and tensile strength. Therefore, the Ti content is preferably 0.02 to 0.05% by weight.

[0068] B is an element that plays a role in improving the mechanical strength of steel sheets during the quenching process. When the B content is less than 0.001% by weight, there is no effect in improving the strength after quenching, and the desired yield strength and tensile strength cannot be obtained. On the other hand, when the B content exceeds 0.005% by weight, the rolling load during hot rolling increases significantly, and inclusions easily form in the steel sheet, causing the steel sheet to become brittle and reducing the fatigue strength of the steel sheet. Therefore, the B content is preferably 0.001 to 0.005% by weight.

[0069] Furthermore, the component composition of the pre-coated steel sheet (substrate) described above is merely illustrative. The substrate of the present invention may also contain other components. For example, the substrate may also contain deoxidizing elements such as 0.01 to 0.06 wt% aluminum (Al) and 0.01 to 0.35 wt% chromium (Cr). Al and Cr have similar effects to Mn and also have the effect of improving the hardenability of the steel sheet. Furthermore, the steel sheet may also contain unavoidable impurities introduced during the manufacturing process.

[0070] The Al or AlSi coating is formed on one or both sides of the substrate as described above. In this embodiment, the Al or AlSi coating is formed on one or both sides of the substrate by a fusion coating method, but it is not limited to such a coating method. The function of the Al or AlSi coating is to prevent the formation of oxide scale (mainly Fe oxide) on the surface of the steel sheet during the heating process of hot stamping formation. Therefore, it is preferable to apply the coating to both sides of the steel sheet. The Al content in the Al coating is 70% by weight or more, while the content of other elements is not particularly limited.

[0071] In addition to Al, the AlSi coating used in this embodiment also contains silicon (Si). Si forms tough phases such as Fe2SiAl7, Fe2SiAl2, and Fe(Al,Si) at the interface between the substrate and the coating during molten coating, thereby suppressing the formation of the Fe-Al brittle phase at the interface. When the amount of Si added to the coating is less than 5% by weight, the ratio of the Fe-Al brittle phase at the interface between the coating and the substrate is high during molten coating, and cracks penetrating the coating easily form during processing, which impairs the corrosion resistance of the processed part. Furthermore, when the amount of Si added is greater than 11% by weight, the corrosion resistance and weldability of the coating decrease; therefore, the amount of Si added to the AlSi coating is preferably 5 to 11% by weight.

[0072] In addition to Al and Si, the AlSi coating also contains 2 to 4 wt% Fe which dissolves from the apparatus and the substrate during the plating process, and may also contain unavoidable impurity elements such as Zn, Mg, Ca, and Mn.

[0073] The AlSi coating undergoes alloying with Fe in the substrate to varying degrees during melting and hot stamping heating. Therefore, the coating is not necessarily composed of uniformly structured layers and may contain phase-separated layers (alloy layers) with partially different degrees of alloying. AlSi coated steel sheets are obtained by immersing a pre-treated steel strip in a molten liquid consisting of Al and 5 to 11 mass (weight)% Si on a continuous melting AlSi production line. The amount of AlSi coating on each surface of the substrate is preferably 25 to 90 g / m². 2 The coating amount is 25g / m 2 When the amount is less than 90 g / m², the antioxidant effect of the Al or AlSi-based coating described above cannot be fully obtained. Furthermore, the coating amount is 90 g / m². 2 When the amount is greater, the surface roughness of the coated steel sheet increases, and the hot stamping heating process is accompanied by the formation of an Al-Fe alloy layer with high surface hardness, further increasing the surface roughness and resulting in an increase in the coefficient of friction between the steel sheet and the mold during the hot stamping process.

[0074] The present invention focuses on a specific composite coating layer formed on a coated steel sheet, and therefore the composition and performance of the coated steel sheet used are not particularly limited. The description of the coating of the coated steel sheet is merely illustrative. Users can choose from various types of coated steel sheets according to their actual needs.

[0075] Table 1 lists the parts by mass of each component in the solid content of the aqueous surface treatment solutions in Examples 1-32 and Comparative Examples 1-8. All of the following aqueous surface treatment solutions are used to form a composite coating layer on the surface of a coated steel sheet having an AlSi coating.

[0076] [Table 1-1]

[0077] [Table 1-2]

[0078] [Table 1-3]

[0079] Table 2 shows the results for coated steel sheets with AlSi coating in Examples 1-32 and Comparative Examples 1-8. The specifications, the curing temperature used to form the composite coating layer, and the dry film weight of the composite coating layer are listed.

[0080] [Table 2-1]

[0081] [Table 2-2]

[0082] After the aqueous surface treatment solution solidified on the surface of the coated steel sheet, a coated steel sheet for hot stamping was obtained. Performance tests of the coated steel sheets for hot stamping in Examples 1-32 and Comparative Examples 1-8 were performed before and after hot stamping, respectively. The hot stamping method includes the steps of cutting the coated steel sheet for hot stamping into a mold, heating it, and forming a stamp. The specific process conditions are listed in Table 3. In Examples 1-32 and Comparative Examples 1-8, the mold used to form the stamp of the coated steel sheet for hot stamping was a flat mold with a water-cooling function.

[0083] [Table 3-1]

[0084] [Table 3-2]

[0085] Performance tests of the coated steel sheets for hot stamping were conducted according to the test methods described below. The test results are listed in Table 4 (Performance tests 1-3 below were performed on the coated steel sheets before hot stamping, and performance tests 4-9 were performed on the coated steel sheets after hot stamping).

[0086] 1) Adhesion test of the surface coating (before hot stamping) Adhesion Test: 3M 610 tape was applied to the surface of a hot-stamping coated steel sheet, ensuring complete adhesion. The tape was then quickly peeled off by applying force at a 60° angle to the sample surface. The content of element W or element Zn (comparative example) on the steel sheet surface before and after the test was determined using XRF (X-ray fluorescence spectrometer) to obtain the residual amount (mass fraction) of the corresponding element. The meaning of the symbols in the corresponding test results in Table 4 is as follows: ◎(Excellent): Remaining amount 95-100% ○ (Good): 80-94% remaining. △ (Medium): Remaining amount 50-79% × (Defective): Remaining amount <49%

[0087] 2) Solvent resistance test The surface of the hot-stamping coated steel sheet was wiped 30 times back and forth using a fine cloth soaked in 80% by volume ethanol. The content of element W or element Zn (comparative example) on the surface of the steel sheet before and after wiping was determined using XRF, and the residual amount (mass fraction) of the corresponding element after wiping was obtained. The meaning of the symbols in the corresponding test results is as follows: ◎(Excellent): Remaining amount 95-100% ○ (Good): 80-94% remaining. △ (Medium): Remaining amount 50-79% × (Defective): Remaining amount <49%

[0088] 3) Heat resistance and humidity resistance test Hot stamping coated steel sheets were placed in a constant temperature and humidity chamber at 49°C and 98% humidity for 120 hours. The heat and humidity resistance of the coated steel sheets were evaluated by the surface rust area. The specific evaluation was performed according to ISO-10289:Methods for corrosion testing of metallic and other inorganic coatings on metallic substrates -- Rating of test specimens and manufactured articles subjected to corrosion tests. The meaning of the symbols in the corresponding test results is as follows: ◎(Excellent): Surface rust area ≤ 5% ○ (Good): Surface rust area 5%~10% △ (Medium): Surface rust area 11%~50% × (Defective): Surface rust area > 51%

[0089] 4) Adhesion test of the surface coating (after hot stamping) Adhesion Test: 3M 610 tape was applied to the surface of a test sample plate, ensuring complete adhesion of the tape to the steel plate surface. The tape was then quickly peeled off by applying force at a 60° angle to the sample surface. The content of element W or element Zn (comparative example) on the surface of the steel plate before and after the test was determined using XRF, and the residual amount (mass fraction) of the corresponding element was obtained. The meaning of the symbols in the corresponding test results is as follows: ◎(Excellent): Remaining amount 95-100% ○ (Good): 80-94% remaining. △ (Medium): Remaining amount 50-79% × (Defective): Remaining amount <49%

[0090] 5) Thermal lubricity test The thermal lubricity test of the coated steel sheet for hot stamping was performed using a testing machine for testing the high-temperature friction coefficient of the sheet strip shown in Figure 2. A 500 mm × 100 mm test sample plate 20 was welded with a thermocouple and then inserted into a heating furnace 29 and held at 930°C for 4 minutes. The test sample plate 20 was pulled out by operating a stepping motor 21 to drive the coupling 22. When the surface temperature of the test sample plate 20 had dropped to 700°C, monitored by a recorder 24, a load (load pressure) of 3 MPa was applied to the surface of the test sample plate by a mold (the mold has a surface area of ​​10 mm × 10 mm and includes an upper mold 26 and a lower mold 28). The mold surface was cooled through a cooling water channel 27 inside the mold. The coupling 22 was driven by the stepping motor 21 to pull the test sample plate 20 at a speed of 20 mm / second. The pulling load was recorded by a force sensor 25. The thermal friction coefficient was obtained according to the formula: Thermal friction coefficient = Drawing load / (2 × Load pressure × Mold surface area). The meaning of the signs in the corresponding test results is as follows: ◎(Excellent): Thermal friction coefficient ≤ 0.35 ○ (Good): Thermal friction coefficient 0.35~0.4 △(Medium): Thermal friction coefficient 0.4~0.5 × (Defective): Thermal friction coefficient > 0.5

[0091] 6) Spot weldability test The test sample plate was cut to dimensions of 30mm x 80mm. Its use in spot welding. The possible current range (the difference between the upper and lower current limits) was measured. The measurement conditions were as follows: The lower current limit was set for a melt diameter of 4.25d. 1 / 2 The current is defined as the current that reaches (where d is the plate thickness). For example, for a plate thickness of 1.4 mm, the lower limit current is 5.0 kA. The upper limit current is defined as the current at which welding spatter first occurs. Electrodes: Chromium copper material, DR (Dezincification resistance) of brass) type (tip diameter: 6mm, 40R radius) Pressure: 4300N Power-on time: 20 cycles (50Hz) The meaning of the symbols in the corresponding test results is as follows: ○ (Good): Usable current range ≥ 1.5kA × (Defective): Available current range <1.5kA

[0092] 7) Adhesion test The test sample plates were cut to dimensions of 25 mm x 100 mm. After wiping the surface of the sample plates with acetone to remove surface dirt, the sample plates were immersed in rust-preventive oil for 3-5 seconds, and then removed from the oil and left vertically for 24 hours. Next, a structural adhesive having a length of approximately 13 mm and a thickness of approximately 0.1 mm (the thickness of the adhesive was determined by adding no more than 10 glass beads with a diameter of 0.1 mm to the bonding area) was applied to the short side edge of one oil-coated sample plate, and the edge of the other oil-coated sample plate was covered at the bonding position (the two sample plates were placed side by side in the width direction). The bonded configuration of the two sample plates is shown in Figure 3. The two sample plates were then clamped together so that the bonded portion remained in the configuration shown in Figure 3. The portion was then heated in an oven at 170°C for 20 minutes, and then left under standard conditions (101.3 kPa; 25°C) for 24 hours before testing. Two samples were pulled in opposite directions. For example, sample 1 was pulled in the F1 direction and sample 2 in the F2 direction to break the adhesive bond. F1 and F2 were on the longitudinal axis of the sample plate on which they acted. Tensile forces F1 and F2 were applied by stretching a clamp at a tensile speed of 50 mm / min. The failure mode when the adhesive bond broke was recorded (the failure phenomenon of the adhesive bond was evaluated according to GB / T16997-1997 “Adhesives - Designation of main failure patterns”, where CF: failure phenomenon occurring on the structural adhesive body; AF: failure phenomenon where the structural adhesive separates from the bonded substrate surface; PF: failure phenomenon in the adhesive bonded substrate or AlSi coating). The meaning of the symbols in the corresponding test results is as follows: ○(Good): The failure mode in which the structural adhesive separates from the surface of the bonded substrate is CF. × (Defective): The failure mode is AF or PF, where the structural adhesive separates from the surface of the bonded substrate.

[0093] 8) Chemical transformation processability test After removing surface contaminants from the test sample plate with an alkaline degreasing agent (Parkerizing FC-E2032), the test sample was rinsed with pure water and immersed in a surface conditioning agent (Parkerizing PL-Z) for 30 seconds. Then, the test sample plate was transferred and immersed in a phosphate treatment solution (Parkerizing PB-L3020) at 35°C for 2 minutes. The meaning of the symbols in the corresponding test results is as follows: ○ (Good): Precipitated zinc phosphate crystal coating × (Poor): No zinc phosphate crystal coating precipitated.

[0094] 9) Corrosion resistance test after coating: After the above chemical transformation process was completed, the test sample plate was coated with 20 μm electrophoretic paint (Kansai HG-350), and then baked and solidified in an oven at 170°C for 20 minutes. The coated test sample plate was then subjected to the method specified in the Japanese JASO M610 standard. The coating was evaluated accordingly. The coating was pre-marked with a knife and tested for 180 cycles (60 days). The maximum blister width on one side, starting from the marking line, was measured. The meaning of the symbols in the corresponding test results is as follows: ◎(Excellent): Blister width ≤ 3mm ○ (Good): Blister width 3-6mm × (Defective): Blister width > 6mm

[0095] [Table 4-1]

[0096] [Table 4-2]

[0097] [Table 4-3]

[0098] After subjecting the coated steel sheets for hot stamping in Examples 1 to 32 to the above test, the evaluation results were obtained. All results were marked "◎" and "○," indicating from Table 4 that the coated steel sheets for hot stamping, including the composite coating layer, possess excellent or good overall properties in terms of solvent resistance, corrosion resistance, high-temperature thermal lubricity, weldability, adhesion, and coating properties after hot stamping.

[0099] By combining Table 1 and Table 4, it can be seen that, compared to Examples 1-32, the low content of aqueous anionic polymer resin A in the surface treatment agents of Comparative Examples 1-3 and 8 results in inferior film formation characteristics of the applied aqueous surface treatment solution after drying, as well as inferior adhesion, solvent resistance, and corrosion resistance of the composite coating layer. Furthermore, in Comparative Examples 4 and 7, the absence of tungsten-containing compound B means that there are no tungsten oxide compounds with high-temperature lubrication properties in the hot stamping process, resulting in inferior high-temperature thermal lubrication. In Comparative Example 5, the amount of boron-containing compound D added is low, and the mass ratio (W) of elemental zinc (Zn) and boron (B) is low. Zn / W B ) reached 10,000, and the zinc-containing compound C did not form a good bond with the coated steel sheet during the heating stage of the hot stamping process, resulting in a loose zinc oxide layer being formed on the surface of the coated steel sheet, leading to poor weldability and adhesion of the resulting coated steel sheet for hot stamping. In Comparative Example 6, the amount of boron-containing compound D added was excessively large, and the mass ratio (W) of elemental zinc (Zn) and boron (B) was Zn / W B The ratio is 50, and boron-containing compound D affects the formation of zinc phosphate crystal coating during the chemical conversion process after hot pressing, resulting in poor corrosion resistance of the coated steel sheet for hot stamping after coating.

[0100] In summary, the present invention involves coating the surface of a coated steel sheet with an aqueous surface treatment solution to achieve good high-temperature thermal lubricity in the hot stamping process and to improve the processability of the coated steel sheet for hot stamping. Therefore, compared to materials not treated with the aqueous surface treatment solution, complex deformation formation can be achieved, wear on the surface of the hot stamping die can be reduced, the frequency of die repairs can be reduced, and production efficiency can be improved. The parts after the hot stamping process can achieve good weldability and adhesion, have good chemical conversion processability, and also exhibit improved corrosion resistance after coating. According to the present invention, the scope of application of the hot stamping process for coated steel sheets for hot stamping is expanded, and the applicability of related parts in the automotive and machining industries is also improved.

[0101] Those skilled in the art will understand that the above embodiments are merely illustrative of the present invention and are not intended to limit it. Any changes and modifications to the above embodiments within the spirit of the present invention would fall within the scope of the claims of the present invention.

Claims

1. A coated steel sheet for hot stamping, the coated steel sheet for hot stamping includes a composite coating layer formed by applying a coated steel sheet and an aqueous surface treatment liquid onto the coated steel sheet; The coated steel sheet comprises a substrate and an Al or AlSi-containing coating formed on one or both sides of the substrate; The composite coating layer comprises the following components in parts by mass, for a coated steel sheet: Aqueous anionic polymer resin A: 15 to 65 parts; Tungsten-containing compound B: 10 to 40 parts by mass of tungsten element; Zinc-containing compound C: 3 to 15 parts by mass of element zinc; Boron-containing compound D: 0.004 to 0.03 parts by mass of element boron; Water-soluble thickener E: 0.01 to 0.5 parts; Aqueous surface improvement aid F: 0.5 to 2 parts.

2. The composite coating layer has a density of 1 to 5 g / m². ２ A coated steel sheet for hot stamping according to claim 1, having a dry film weight of [value missing].

3. The hot stamping coated steel sheet according to claim 1, wherein the substrate contains the following components in mass percent: C: 0.20 to 0.25 wt%, Si: 0.01 to 0.4 wt%, Mn: 1.0 to 1.4 wt%, Ti: 0.02 to 0.05 wt%, B: 0.001 to 0.005 wt%, and the remainder being Fe and other unavoidable impurities.

4. The coated steel sheet for hot stamping according to Claim 1, wherein the Al-containing coating contains 70% by weight or more of Al.

5. The coated steel sheet for hot stamping according to Claim 1, wherein the AlSi coating contains 5 to 11% by weight of Si.

6. An aqueous surface treatment liquid for forming a composite coating layer according to any one of claims 1 to 5, The solid phase substance is an aqueous surface treatment solution containing the following components in parts by mass: Aqueous anionic polymer resin A: 15 to 65 parts; Tungsten-containing compound B: 10 to 40 parts by mass of tungsten element; Zinc-containing compound C: 3 to 15 parts by mass of element zinc; Boron-containing compound D: 0.004 to 0.03 parts by mass of element boron; Water-soluble thickener E: 0.01 to 0.5 parts; Aqueous surface improvement aid F: 0.5 to 2 parts.

7. The aqueous surface treatment liquid according to claim 6, wherein the aqueous anionic polymer resin A is one or more selected from aqueous acrylic resin, aqueous epoxy resin, aqueous polyester resin, and aqueous alkyd resin.

8. The aqueous surface treatment solution according to claim 6, wherein the tungsten-containing compound B is one or more selected from tungsten oxide, tungstate salts, or tungsten metal salts.

9. The aqueous surface treatment solution according to claim 6, wherein the zinc-containing compound C is selected from zinc oxide, zinc salts, or organozinc compounds.

10. The aqueous surface treatment solution according to claim 6, wherein the boron-containing compound D is selected from boron oxide, a borate, or an organoboron compound.

11. The aqueous surface treatment liquid according to claim 6, wherein the water-soluble thickener E is one or more selected from water-soluble hydroxymethylcellulose, water-dispersible alkali-swellable polyacrylic acid emulsion, and water-dispersible nonionic polyurethane-associative thickening resin.

12. The aqueous surface treatment liquid according to claim 6, wherein the aqueous surface improvement aid F is one or two selected from water-dispersible modified polydimethylsiloxane and water-dispersible polyether-modified silicone.

13. The aqueous surface treatment liquid according to claim 6, wherein the aqueous anionic polymer resin A is 20 to 50 parts by mass.

14. The aqueous surface treatment liquid according to claim 6, wherein the tungsten-containing compound B is 15 to 35 parts by mass as tungsten element.

15. The aqueous surface treatment liquid according to claim 7, wherein the aqueous anionic polymer resin A has a number average molecular weight greater than 8,000, and the mass fraction of harmful substances resulting from the combustion of the aqueous anionic polymer resin A is less than 1% by weight of the total solids of the aqueous anionic polymer resin A.

16. The aqueous surface treatment liquid according to claim 7, wherein when tungsten-containing compound B is a water-insoluble tungsten compound, the tungsten-containing compound B has a particle size of 5 to 1,000 nm.

17. The aqueous surface treatment liquid according to claim 7, wherein when the zinc-containing compound C is a water-insoluble zinc compound, the zinc-containing compound C has a particle size of 5 to 1,000 nm.

18. The aqueous surface treatment liquid according to claim 7, wherein the mass ratio of the zinc element in the zinc-containing compound C to the boron element in the boron-containing compound D is 150:1 to 750:

1.

19. The aqueous surface treatment liquid according to claim 7, wherein the water-soluble thickener E has a number average molecular weight of 50,000 to 100,000.

20. The aqueous surface treatment solution according to claim 15, wherein the tungsten-containing compound B has a particle size of 20 to 500 nm.

21. The aqueous surface treatment solution according to claim 15, wherein the zinc-containing compound C has a particle size of 20 to 500 nm.

22. A method for manufacturing a hot stamping coated steel sheet according to any one of claims 1 to 5, comprising the following steps: applying an aqueous surface treatment solution onto a surface-cleaned coated steel sheet; then curing the applied aqueous surface treatment solution at 80 to 180°C to form a composite coating layer on the coated steel sheet, thereby obtaining a hot stamping coated steel sheet.

23. A hot stamping method for a hot stamping coated steel sheet according to any one of claims 1 to 5, comprising the steps of cutting the hot stamping coated steel sheet into a mold, heating it, and forming a stamp.

24. The hot stamping method according to claim 23, wherein during the heating process, a hot stamping coated steel sheet cut into a mold is heated from 50°C to the target plate temperature and held for 30 to 60 seconds; and then the heated hot stamping coated steel sheet is transferred to a stamp forming mold having a cooling effect.

25. The hot stamping method according to claim 24, wherein during the heating process, the average heating rate is 3 to 12°C / second, and the final plate temperature is 850 to 980°C.

26. The hot stamping method according to claim 24, wherein the stamp forming process is carried out in air at a cooling rate of 27°C / second or more before the temperature of the coated steel sheet for hot stamping drops to 400°C.

27. A hot-stamped and formed coated steel sheet has a yield strength of ≥950 MPa, a tensile strength of ≥1300 MPa, and an elongation A after break of ≥5%. ５０ｍｍ The hot stamping method according to claim 23, wherein the material has a hardness HV10 of ≥400 or a hardness HRC of ≥40, and a cold bending angle VDA of ≥45°.

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