Precoated steel sheet for hot press forming, method of manufacturing the same, hot press formed part, method of manufacturing the same, and application

Abstract

The application provides a pre-coated steel sheet for hot stamping forming, a manufacturing method of the pre-coated steel sheet, a hot stamping forming part, a manufacturing method of the hot stamping forming part and application. 2 When the pre-coating weight is 20-60 g / m 2 , the surface roughness Ra and Rpc of the coating after hot forming are ensured to be high by controlling the surface roughness Ra and Rpc of the pre-coating; the uniformity of the pre-coating weight is stably controlled by controlling the shape of the pre-coating steel sheet and the parameters of the air knife after aluminum-silicon plating; the uniformity of the coating liquefaction and flow after heating is ensured by controlling the uniformity of the pre-coating weight and the uniformity of the heating temperature in the heating process during hot forming, so that the surface roughness Ra and Rpc of the coating after hot forming are uniform. The coating of the hot stamping forming part produced by the application has high and uniform surface roughness Ra and Rpc, so that the whole part has good paint adhesion and corrosion resistance.

Description

Technical Field

[0001] This invention pertains to hot stamping forming components, specifically relating to pre-coated steel sheets for hot stamping forming and their manufacturing methods, hot stamping forming parts and their manufacturing methods and applications, for use in vehicles. Background Technology

[0002] In recent years, countries around the world have imposed increasingly stringent requirements on automobile safety, energy conservation, and emissions. High strength and thinning, as well as energy conservation and emission reduction, have been the main development trends in the automotive industry. Among these, hot stamping is one of the most common methods to achieve high strength in automotive parts with tensile strength greater than 1500MPa. It combines austenitizing treatment, high-temperature forming, and rapid cooling to achieve high strength in the product. It is mainly used in the production of high-strength, complex-formed parts and can avoid the disadvantages of cold forming, such as difficulty in forming, large springback, and poor dimensional stability.

[0003] In the hot stamping process, if bare hot-formed steel is used, oxidation and decarburization inevitably occur on the steel plate surface, affecting the strength of the steel plate. Furthermore, hot-formed parts require shot peening or pickling, which affects the dimensional accuracy of the product. To address these issues, coated hot-formed steel has become the mainstream product in the market. Currently, the main hot-formed steel coatings developed domestically and internationally include aluminum-silicon (Al-Si) and zinc-based coatings (such as GI and GA). However, zinc-based coatings are prone to liquid zinc embrittlement during high-temperature stamping, causing stamping cracks. The most maturely applied coating product is Al-Si, which was first proposed by ArcelorMittal and successfully applied in industrial trials. Its typical coating composition (mass percentage) is 87% Al-10Si-3% Fe.

[0004] When Al-Si coated products are used for hot stamping, the coating and substrate undergo interdiffusion during heating to form Fe-Al and Fe-Al-Si alloyed coatings, altering the coating properties. In this case, the coating cannot undergo a good phosphating reaction with the phosphating solution, resulting in poor adhesion for subsequent electrophoretic coatings. To ensure good adhesion for subsequent electrophoretic coatings, the surface roughness of the coating after hot forming needs to be relatively high. High surface roughness improves the mechanical bonding force between the coating and the electrophoretic paint film. For example, general standards specify that the surface roughness of Al-Si coatings after hot forming must meet Ra≥1.8 and Rt≥12; Fiat Chrysler Automobiles standards specify that the surface roughness of Al-Si coatings after hot forming must meet Rz between 10 and 25. Ra, Rt, and Rz mentioned above all represent the peak height or the size of the peak and valley in the roughness profile (Ra refers to the average roughness, which is the absolute value of the average height of the surface profile over a specific length; Rt refers to the sum of the peak height and the valley height; and Rz refers to the micro-irregularity, which is the difference in distance between the highest peak and the lowest valley on the surface of an object).

[0005] Patent CN113481451A, published on October 8, 2021, discloses a pre-coated steel sheet for hot forming, its preparation method, and hot-formed steel components and their applications. This invention discloses that when the pre-coating thickness is thin (5-19 μm), the amount of coating liquefaction after heating is small, and the liquefied coating mainly fills pits, failing to guarantee a large surface roughness Ra after heating. This invention improves the surface roughness Ra of the hot-formed coating by increasing the surface roughness Ra of the pre-coating, ensuring that the surface roughness Ra of the hot-formed coating is ≥1.80 μm, thereby guaranteeing good paint adhesion and corrosion resistance. However, in practical use, when the pre-coating weight is small, even with a larger Ra, poor paint adhesion and corrosion resistance still occur. This is related to the uneven surface roughness Ra of the hot-formed parts and the failure to guarantee a high and uniform Rpc.

[0006] A patent published on June 3, 2021, by Baoshan Iron & Steel Co., Ltd., with publication number WO2021103805A1, discloses a hot-formed part with excellent paint film adhesion and its manufacturing method. In the hot-formed part provided by this patent, the average weight of the aluminum plating layer is 20–120 g / m². 2 For each single side, the surface roughness Ra of the sheet metal is 0.3–2.0 μm, the surface roughness peak count Rpc is 30–150, and the average surface roughness Ra of the hot-formed part is 1.5–2.5 μm, with a surface roughness peak count Rpc of 50–250. However, when the weight of the aluminum plating layer is small, and the surface roughness Ra of the sheet metal is 0.3–2.0 μm, the average surface roughness Ra of the hot-formed part is difficult to meet the above Ra requirements. Moreover, the uniformity of the surface roughness Ra and Rpc of the hot-formed part is affected by the uniformity of the coating thickness of the raw material, the sheet shape, and the hot-forming process.

[0007] Therefore, for aluminum or aluminum alloy coatings, when the pre-coating weight is small, it is particularly important to improve the adhesion and corrosion resistance of the aluminum alloy coating after thermoforming. Summary of the Invention

[0008] The purpose of this invention is to provide a pre-coated steel sheet for hot stamping and its manufacturing method. For situations where the pre-coating weight is low, the surface roughness Ra and Rpc of the pre-coating are controlled to ensure that the hot-formed coating has high surface roughness Ra and Rpc. Furthermore, by controlling the steel sheet shape before aluminizing and the air knife parameters after aluminizing, the uniformity of the pre-coating weight is ensured, guaranteeing uniform liquefaction and flow of the coating after heating, thereby ensuring that the hot-formed coating has a high and relatively uniform surface roughness.

[0009] Another objective of this invention is to provide a hot-stamped forming component and its manufacturing method, which is manufactured by using the above-mentioned pre-coated steel sheet for hot stamping through a hot forming process, while controlling the stability of the heating temperature during hot forming, and the resulting hot-stamped forming component has a high and uniform surface roughness.

[0010] Another object of the present invention is to provide an application of a hot-stamped part for use in land-based motor vehicles.

[0011] The specific technical solution of this invention is as follows:

[0012] A pre-coated steel sheet for hot stamping, wherein a pre-coating is provided on at least one surface of a substrate; the pre-coating is aluminum or an aluminum alloy.

[0013] The total thickness of the pre-coated steel sheet used for hot stamping is 0.7–3.0 mm;

[0014] The weight of the pre-coating is 20-60 g / m² per single surface. 2 .

[0015] The surface roughness of the pre-coated layer is controlled as follows:

[0016] When 20g / m 2 ≤Weight of each pre-coated layer ≤40g / m 2 At that time, the surface roughness Ra of the pre-coated layer is ≥2.0μm and the weight of Rpc+ per single-sided pre-coated layer is ≥100;

[0017] When 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 When the surface roughness Ra of the pre-coated layer is ≥2.5μm and the weight of each pre-coated layer Rpc+ per single side is ≥100.

[0018] When calculating the Rpc+ weight per single-sided pre-coating, the Rpc value in / cm and the weight per single-sided pre-coating in g / m² should be used. 2 Add the values ​​when they are in units.

[0019] The substrate includes low-carbon steel, medium-carbon steel, or high-carbon steel.

[0020] The present invention provides a method for manufacturing a pre-coated steel sheet for hot stamping, comprising the following process flow: manufacturing a substrate → hot-dip galvanizing → finishing;

[0021] The substrate is manufactured by steelmaking, continuous casting, hot rolling, and pickling to obtain a substrate before hot-dip galvanizing. During the hot rolling and pickling processes, the shape of the steel plate is controlled by the rolling parameters to ensure that the flatness of the produced substrate is within 15mm, preferably within 10mm.

[0022] The hot-dip plating process involves immersing the substrate in an aluminum or aluminum alloy plating solution, followed by controlling the pre-coating weight using nitrogen or compressed air sprayed by an air knife. The pre-coating weight of this invention is 20–60 g / m² per single side. 2 The plating bath temperature is 640–680℃, and the immersion time is 2–8 seconds. After hot-dip plating, the air knife parameters mainly consist of air knife pressure, distance, and height. These parameters are adjusted based on changes in the pre-coating weight and strip speed. When production is relatively stable, the strip speed variation is small, and the air knife parameter fluctuation should be controlled within 5%. The pre-coating weight fluctuates with the air knife pressure. The air knife parameter fluctuation is calculated as: |Maximum or minimum air knife parameter value - Average air knife parameter value| / Average air knife parameter value × 100%, taking the larger of the maximum or minimum value.

[0023] This invention ensures good uniformity of the pre-coating weight on the entire steel plate surface after aluminosilicate plating by controlling the steel plate shape and air knife parameters before aluminosilicate plating. Specifically, the weight fluctuation of the pre-coating on the entire steel plate surface is within 30%.

[0024] The finishing process involves adjusting the finishing roll process parameters according to the roughness requirements of the pre-coating. This process also improves mechanical properties and strip straightness. Specifically, the average Ra value of the finishing roll is between 3.5 and 6.0, the roll Rpc is greater than or equal to the pre-coating Rpc, and the finishing elongation is between 1.0 and 2.0%. This invention controls: when 20 g / m... 2 ≤Weight of each pre-coated layer ≤40g / m 2 When the average surface roughness of the pre-coating satisfies: 2.0≤Ra≤4.8 and Rpc+ per single-sided pre-coating weight ≥100; when 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 When the average surface roughness of the pre-coating is satisfied, it meets the following conditions: 2.5≤Ra≤4.8 and Rpc+ weight of each single-sided pre-coating ≥100.

[0025] The present invention provides a hot stamping formed part, which is made by heating, hot stamping or other heat treatment operations on the above-mentioned pre-coated steel sheet for hot stamping.

[0026] The heating method can be resistance heating, radiation heating or induction heating, and is not limited to the above methods.

[0027] The present invention provides a method for manufacturing a hot-stamped part, comprising blanking → heat treatment → hot stamping.

[0028] Blanking: a blank made by punching or cutting a pre-coated steel sheet for hot stamping into the shape required for the hot-formed part;

[0029] The heat treatment involves placing the billet in a heating furnace and heating and holding it at a temperature of 860–1000°C. The furnace atmosphere is either air or nitrogen, and the billet remains in the furnace for 2–10 minutes. A box furnace or a roller-bottom furnace can be used, but regardless of the type, the temperature fluctuation in the heating zone must be controlled within 3% to ensure the uniformity of the surface roughness of the coating after heating.

[0030] The hot stamping process involves rapidly transferring the heat-treated blank into a mold for stamping and cooling, wherein the transfer time does not exceed 15 seconds, the stamping holding time is 5 to 15 seconds, and the cooling and demolding temperature does not exceed 250°C.

[0031] The coating of the hot-stamped part has Ra≥1.5μm and Rpc≥80 / cm.

[0032] The surface roughness Ra of the hot-stamped part has a fluctuation of ≤20% and an Rpc fluctuation of ≤20%. Wherein, surface roughness Ra or Rpc fluctuation = |maximum or minimum surface roughness - average surface roughness| / average surface roughness × 100%, taking the larger of the maximum or minimum surface roughness values.

[0033] The present invention provides an application of a hot-stamped part for use in land-based motor vehicles.

[0034] In the hot stamping forming parts of this invention, Ra and Rpc refer to average values, where the unit of Ra is μm and the unit of Rpc is / cm.

[0035] Compared with existing technologies, for pre-coated aluminum or aluminum alloy coated steel sheets, when the pre-coating weight is between 20 and 60 g / m², 2 During the process, the surface roughness Ra and Rpc of the pre-coating are controlled to ensure that the coating after hot forming has high surface roughness Ra and Rpc. Furthermore, the uniformity of the pre-coating weight is controlled by maintaining the steel plate shape before aluminizing and the air knife parameters after aluminizing; during hot forming, the uniformity of the heating temperature is controlled. By controlling the uniformity of the pre-coating weight and the uniformity of the heating temperature, the uniformity of the coating liquefaction and flow after heating is ensured, ultimately guaranteeing that the coating after hot forming has relatively uniform surface roughness Ra and Rpc. The hot-stamped parts manufactured from the pre-coated steel plate provided by this invention have a high and relatively uniform surface roughness Ra and Rpc, thereby ensuring that the entire part has good paint adhesion and corrosion resistance. Attached Figure Description

[0036] Figure 1 The average weight of the pre-coated layer per single side is 35g / m². 2The surface roughness Ra and Rpc of the coating after heating vary with heating temperature and heating time. Detailed Implementation

[0037] Regarding the technology disclosed in CN113481451A, the inventors discovered that, to ensure good adhesion of Al-Si coatings after thermoforming and subsequent electrophoretic coating, the surface roughness of the thermoformed coating is crucial, not only for the peak height or valley size (Ra), but also for the peak count (Rpc). Even with a large Ra, a small Rpc will not guarantee good paint adhesion and corrosion resistance. More critically, when the pre-coating thickness is thin (5–19 μm), uneven coating thickness or heating temperature leads to significant differences in the flow of the coating after liquefaction in different areas. This makes it difficult to guarantee the uniformity of surface roughness Ra and Rpc across different areas, potentially resulting in abnormally low Ra or Rpc in localized areas, leading to poor paint adhesion and corrosion resistance in those locations. Therefore, strict control of the coating thickness uniformity of the raw materials and the thermoforming process is necessary. Furthermore, significant differences in surface roughness Ra and Rpc across different areas after thermoforming result in noticeable color differences after painting, affecting the final product's appearance.

[0038] Regarding the patent with publication number WO2021103805A1, the inventors discovered that when the average weight of the aluminum plating layer is 20-60 g / m² 2 For each single side, if the surface roughness Ra of the raw material is 0.3 to 2.0 μm, it is difficult to ensure that the surface roughness Ra of the thermoformed part is not less than 1.5 μm. Moreover, the uniformity of the surface roughness Ra and Rpc of the thermoformed part is affected by the uniformity of the coating thickness of the raw material, the plate shape and the thermoforming process.

[0039] Given that existing pre-coated aluminum or aluminum alloy coated steel sheets cannot guarantee a high and uniform surface roughness Ra and Rpc of the hot-formed coating when the pre-coating weight is small, to ensure good adhesion of Al-Si coatings for electrophoresis and other coatings after hot forming, not only the peak height or valley size is important in the roughness profile, but the peak count (Rpc) of the peak height or valley is equally important; moreover, different coating weights have different effects on the roughness of the hot-formed coating. This invention provides a hot-stamped part and its manufacturing method, as well as a pre-coated steel sheet for hot stamping and its manufacturing method. The hot-stamped part has a high and uniform surface roughness Ra and Rpc after hot forming, thereby ensuring the adhesion and corrosion resistance of the aluminum alloy coating after hot forming.

[0040] The inventors discovered that, to ensure the adhesion and corrosion resistance of paint after hot forming of aluminum alloy coatings, the surface roughness Ra and Rpc of hot-stamped parts need to be at relatively high values, satisfying Ra ≥ 1.5 μm and Rpc ≥ 80 / cm. It is important to note that Ra and Rpc are equally important for ensuring paint adhesion and corrosion resistance. That is, even if Ra is large (e.g., Ra ≥ 1.5 μm), if Rpc is small (e.g., Rpc < 80 / cm), or if Rpc ≥ 80 / cm but Ra is small (e.g., Ra < 1.5 μm), both situations will result in poor paint adhesion and corrosion resistance.

[0041] More importantly, the inventors discovered that the surface roughness Ra and Rpc of the aluminum or aluminum alloy coating after heating are mainly related to the weight of the pre-coating and the surface roughness Ra and Rpc of the pre-coating. The specific heating process, when the coating is fully alloyed, has a relatively small impact on the surface roughness of the heated coating. The average effect of commonly used heating temperature and heating time on the weight of a single-sided pre-coating is 35 g / m². 2 The effect of heating on the surface roughness Ra and Rpc of the coating is shown in the figure. Figure 1 The surface roughness Ra of the pre-coated layer is 2.01 μm, and the Rpc is 100 / cm.

[0042] For pre-coated steel sheets used in hot stamping, when the pre-coating weight is large, such as greater than 60 g / m², 2 For each single side, the weight of the pre-coating is the main factor affecting the surface roughness Ra and Rpc of the coating after heating. At this point, regardless of the pre-coating surface roughness Ra and Rpc, the surface roughness Ra and Rpc of the hot-stamped part after hot forming are both relatively high, ensuring that Ra ≥ 1.5 μm and Rpc ≥ 80 / cm. However, when the pre-coating weight is small, such as not exceeding 60 g / m², the surface roughness Ra and Rpc of the coating are relatively high. 2 For each single side, the weight of the pre-coating and the surface roughness Ra and Rpc of the pre-coating both affect the surface roughness Ra and Rpc of the coating after heating. The surface roughness Ra of the coating after heating tends to decrease compared to the pre-coating Ra, while the surface roughness Rpc of the coating after heating tends to increase compared to the pre-coating Rpc. It is important to note that the magnitude of the decrease in Ra and the increase in Rpc before and after heating are positively correlated with the weight of the pre-coating. Therefore, to ensure that the surface roughness Ra and Rpc of the coating after thermoforming are at a high value, the surface roughness Ra and Rpc of the pre-coating need to be controlled.

[0043] The weight of each single-sided pre-coating layer of this invention is not less than 20g / m². 2 Because the pre-coating weight is too low, problems such as incomplete coating are very likely to occur on the surface of the steel plate. In addition, the excessive air knife pressure causes uneven coating weight on the plate surface, resulting in uneven surface roughness of the coating after hot forming.

[0044] As mentioned above, surface roughness can be characterized by the values ​​of Ra and Rpc; the higher the surface roughness, the larger the Ra and Rpc values. Studies have found that to ensure good paint adhesion and corrosion resistance after hot forming of the pre-coated aluminum or aluminum alloy coating, the surface roughness of the hot-formed coating must meet the following requirements: Ra ≥ 1.5 μm and Rpc ≥ 80 / cm. When the pre-coating weight is small, the surface roughness Ra and Rpc of the pre-coating can be controlled to ensure that the surface roughness Ra and Rpc of the hot-formed coating meet the above requirements.

[0045] The present invention controls the concentration of 20 g / m³ to achieve this. 2 ≤Weight of each pre-coated layer ≤40g / m 2 When the coating Ra decreases by less than 0.5 after heating compared to the pre-coating Ra, and the coating Rpc increases by 5 to 100 after thermoforming compared to the pre-coating Rpc, in order to ensure that the surface roughness Ra of the heated coating is ≥1.5μm and Rpc is ≥80 / cm, the surface roughness Ra of the pre-coating is controlled to be ≥2.0μm and Rpc + weight of each single-sided pre-coating is ≥100.

[0046] When 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 When heated, the Ra of the coating decreases by less than 1.0 compared to the Ra of the pre-coating, and the Rpc of the coating increases by 30 to 150 compared to the Rpc of the pre-coating. To ensure that the surface roughness Ra of the heated coating is ≥1.5μm and Rpc is ≥80 / cm, the surface roughness Ra of the pre-coating is ≥2.5μm and Rpc + the weight of the pre-coating per single side is ≥100.

[0047] To achieve high surface roughness Ra and Rpc in the pre-coating process, this invention uses a surface roughness Ra ≥ 3.5 μm for the finishing rolls during the pre-coating steel sheet preparation process. However, when the surface roughness Ra of the rolls exceeds 6.0 μm, it is difficult to guarantee uniform surface roughness. Therefore, this invention uses rolls with a surface roughness between 3.5 and 6.0 μm. Generally, the surface roughness of the pre-coating / roll roughness is ≤ 0.8, at which point the surface roughness Ra of the pre-coating is ≤ 4.8 μm. Furthermore, it should be noted that to ensure the surface roughness Rpc value of the pre-coating, the roll Rpc ≥ the pre-coating Rpc. The surface roughness Ra and Rpc of the rolls also refer to average values.

[0048] More specifically, when 20g / m 2 ≤Weight of each pre-coated layer ≤40g / m 2 To ensure that the surface roughness Ra ≥ 1.5 μm and Rpc ≥ 80 / cm of the coating after thermoforming, the average surface roughness of the pre-coating must satisfy: 2.0 ≤ Ra ≤ 4.8 and Rpc + weight of each single-sided pre-coating ≥ 100.

[0049] When 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 To ensure that the surface roughness Ra of the coating after heating is ≥1.5μm and Rpc ≥80 / cm, the average surface roughness of the pre-coating must satisfy: 2.5≤Ra≤4.8 and Rpc + weight of each single-sided pre-coating ≥100.

[0050] During the finishing process, the average surface roughness Ra of the roll is between 3.5 and 6.0, the roll Rpc is greater than or equal to the pre-coated Rpc, and the finishing elongation is between 1.0 and 2.0%.

[0051] In addition, the inventors also discovered that when the pre-coating weight is small, in order to ensure that the coating has a more uniform surface roughness Ra and Rpc after thermoforming, it is necessary to control the shape of the steel plate before aluminized silicon plating, the stability of the air knife parameters after aluminized silicon plating, and the uniformity of the heating temperature during the heating process in thermoforming.

[0052] When the steel plate shape is poor before aluminizing and siliconizing, mainly due to significant unevenness, it is difficult to ensure uniform coating weight across the entire surface after hot-dip galvanizing using an air knife. Furthermore, when the coating weight fluctuates significantly, the surface roughness Ra in areas with higher coating weight tends to be lower during hot forming. This is primarily because the coating liquefies and flows more rapidly after heating in these areas, and the heavier coating fills a larger amount of pits, resulting in a lower overall roughness Ra in these areas. Additionally, fluctuations in air knife parameters, primarily pressure, distance, and height, lead to poor uniformity of coating weight across the entire surface, resulting in uneven surface roughness after heating. This may cause localized areas with abnormally low surface roughness Ra or Rpc, leading to poor paint adhesion and corrosion resistance in these areas. Moreover, uneven heating in the furnace during the heating process also affects the coating liquefaction and flow rate; areas with higher temperatures experience faster liquefaction and flow, resulting in a lower overall roughness Ra in these areas.

[0053] The stability of the steel plate (substrate) shape before aluminized silicon plating and the air knife parameters after aluminized silicon plating mainly affects the uniformity of the pre-coating weight on the entire steel plate surface after hot-dip plating, thus affecting the uniformity of the coating surface roughness after heating. To ensure good uniformity of the pre-coating weight on the entire steel plate surface after hot-dip plating, stable control of the steel plate shape before aluminized silicon plating and the air knife parameters after aluminized silicon plating is crucial. Specifically, for the steel plate shape before aluminized silicon plating, the unevenness of the entire steel plate should be within 15mm, preferably within 10mm. Unevenness is measured by placing the steel plate freely on a platform (the smaller the unevenness, the better), without applying any pressure other than the weight of the steel plate itself, and measuring the maximum distance between the lower surface of the steel plate and the platform. The air knife parameters after aluminized silicon plating mainly consist of air knife pressure, distance, and height. These parameters are adjusted according to changes in the pre-coating weight and strip speed. When production is relatively stable, the strip speed variation is small, and the fluctuation of the air knife parameters should be controlled within 5%.

[0054] By controlling the steel plate shape and air knife parameters before aluminizing and silicon coating, the uniformity of the pre-coating weight across the entire steel plate surface after hot-dip galvanizing is ensured. Specifically, the pre-coating weight fluctuation across the entire steel plate surface is within 30%. The pre-coating weight fluctuation is calculated as: |Maximum or Minimum Pre-coating Weight - Average Pre-coating Weight| / Average Pre-coating Weight × 100%, taking the larger of the maximum or minimum pre-coating weight. It is important to note that the pre-coating weight fluctuation here includes fluctuations along the rolling direction and perpendicular to the rolling direction. For any pre-coated steel plate, five locations are selected along the rolling direction or perpendicular to the rolling direction (with equidistant distances between adjacent locations). The average pre-coating weight of these five locations is taken as the average pre-coating weight, i.e., Average Pre-coating Weight = (Edge 1 + Quarter + Middle + Three-Quarter + Edge 2) / 5.

[0055] Furthermore, the uniformity of heating temperature in the furnace during the heating process also affects the uniformity of the surface roughness of the coating after heating. This invention discovers that when the heating temperature fluctuation within the furnace is controlled within 3%, the uniformity of the surface roughness of the coating after heating can be guaranteed. Here, heating temperature fluctuation refers to the temperature fluctuation along the steel plate's transport direction or perpendicular to the transport direction within the plane of the pre-coated steel plate in the furnace. Heating temperature fluctuation = |maximum temperature or minimum temperature - average temperature| / average temperature × 100%, using the larger of the maximum or minimum temperature.

[0056] By controlling the shape of the steel plate before aluminized silicon plating, the stability of the air knife parameters after aluminized silicon plating, and the uniformity of the heating temperature during the heating process, the surface coating of the entire steel plate after hot forming has a relatively uniform surface roughness Ra and Rpc. Specifically, the unevenness of the entire steel plate before aluminized silicon plating is ≤15mm, preferably ≤10mm, the air knife parameter fluctuation is ≤5%, the pre-coating weight fluctuation is ≤30%, the heating temperature fluctuation is ≤3%, and finally, the surface roughness Ra fluctuation and Rpc fluctuation of the entire steel plate surface coating after hot forming are ≤20%.

[0057] Surface roughness Ra or Rpc fluctuation = |maximum or minimum surface roughness - average surface roughness| / average surface roughness × 100%, taking the larger number after calculating the maximum or minimum surface roughness.

[0058] For pre-coated aluminum or aluminum alloy coated steel sheets, when the pre-coating weight is small, the coating after hot forming cannot guarantee a high and uniform surface roughness Ra and Rpc. The present invention provides a hot stamping formed part and a pre-coated steel sheet for hot stamping. The part after hot stamping of the pre-coated steel sheet has a coating with a high and uniform surface roughness Ra and Rpc, thereby ensuring that the entire part has good paint adhesion and corrosion resistance.

[0059] The present invention will be further described below with reference to specific embodiments and comparative examples. The following embodiments or experimental data are intended to illustrate the present invention by way of example, and those skilled in the art should understand that the present invention is not limited to these embodiments or experimental data.

[0060] A pre-coated steel sheet for hot stamping is a pre-coated aluminum or aluminum alloy steel sheet with a thickness of 0.7–3.0 mm, wherein the pre-coating weight is 20–60 g / m² per single side. 2 In the following examples, the typical steel plate thickness is 1.4 mm, and the average pre-coating weight per single side is 20 g / m². 2 35g / m 2 60g / m 2 The pre-coating is applied to both the upper and lower surfaces of the steel plate.

[0061] The base steel of the steel plate (substrate) of this invention is the commercially available 22MnB5. The pre-coating can be achieved through hot-dip galvanizing. A typical hot-dip galvanizing solution includes the following mass percentage components: Si 8-11%, Fe 2-4%, with the balance being aluminum or aluminum alloy and unavoidable impurities. This invention is not limited to this coating method and solution composition; other coating methods or other aluminum or aluminum alloy compositions can be used. The main function of Si in the solution is to form a Fe-Al-Si inhibition layer on the steel plate surface, effectively hindering the formation of the brittle phase Fe2Al5 and improving the coating's formability. This effect weakens significantly when the Si content exceeds 10%. Experimental studies show that a Si content of 8-11% in the aluminum solution is generally suitable. The solubility of Fe varies at different solution temperatures. The conventional aluminum-silicon plating solution temperature is 640-680℃, at which point the solubility of Fe in the solution is 2-4%. The specific parameters of the substrate composition and plating solution composition in the embodiments of this invention are shown in Table 1, where Bal. represents the balance excluding other elements.

[0062] Table 1. Substrate and plating solution composition in Example 1

[0063]

[0064] The above-mentioned method for manufacturing pre-coated steel sheets for hot stamping includes the following steps:

[0065] 1) Preparation of substrate before hot-dip galvanizing: The base steel is processed by steelmaking, continuous casting, hot rolling and pickling to obtain the substrate before hot-dip galvanizing. Among them, the hot rolling and pickling processes are controlled by the rolling parameters to control the shape of the steel plate, so that the flatness of the pickled steel plate is within 15mm, preferably within 10mm.

[0066] 2) Hot-dip plating: The substrate is immersed in an aluminum plating solution, and then nitrogen or compressed air is sprayed by an air knife to control the weight of the pre-coating. The weight of the pre-coating in this invention is 20-60 g / m² per side.2 The plating bath temperature is 640–680℃, and the immersion time is 2–8 seconds. The air knife parameters after aluminum-silicon plating are mainly air knife pressure, distance, and height. These parameters are adjusted according to changes in coating weight and strip speed. When production is relatively stable, the strip speed does not change significantly, and the air knife parameter fluctuation should be controlled within 5%. The air knife parameter fluctuation is calculated as: |Maximum or minimum air knife parameter value - Average air knife parameter value| / Average air knife parameter value × 100%, taking the larger of the maximum or minimum value. Generally, the air knife distance and height are not adjusted at this stage, and the coating weight fluctuates with the air knife pressure. This invention primarily controls the air knife pressure fluctuation to within 5%.

[0067] This invention ensures good uniformity of coating weight across the entire steel plate surface after aluminosilicate coating by controlling the steel plate shape and air knife parameters before aluminosilicate coating. Specifically, the weight fluctuation of the coating across the entire steel plate surface is within 30%. The pre-coating weight fluctuation is calculated as: |Maximum or minimum pre-coating weight - Average pre-coating weight| / Average pre-coating weight × 100%, taking the larger of the maximum or minimum pre-coating weight. It is important to note that the pre-coating weight fluctuation refers to both the weight fluctuation along the rolling direction and perpendicular to the rolling direction. For any pre-coated steel plate, five locations are selected along the rolling direction or perpendicular to the rolling direction (with equidistant distances between adjacent locations). The average pre-coating weight of these five locations is taken as the average pre-coating weight, i.e., Average pre-coating weight = (Edge 1 + Quarter + Middle + Three-Quarter + Edge 2) / 5.

[0068] 3) Finishing: Based on the roughness requirements of the pre-coating, the process parameters of the finishing rolls are adjusted for production. This process simultaneously improves mechanical properties and strip straightness. Specifically, the average Ra value of the finishing rolls is between 3.5 and 6.0 μm, the roll Rpc is greater than or equal to the pre-coating Rpc, and the finishing elongation is between 1.0 and 2.0%. In this embodiment of the invention, based on the pre-coating weight, when 20 g / m²... 2 ≤Weight of each pre-coated layer ≤40g / m 2 When the average surface roughness of the pre-coating satisfies: 2.0≤Ra≤4.8 and Rpc+ per single-sided pre-coating weight≥100, when 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 When the average surface roughness of the pre-coating is satisfied, it meets the following conditions: 2.5≤Ra≤4.8 and Rpc+ weight of each single-sided pre-coating ≥100.

[0069] The pre-coated steel sheet for hot stamping manufactured according to the above manufacturing method is hot-formed into a hot-stamped part with a high and uniform surface roughness. The manufacturing method includes:

[0070] 1) Blanking: The pre-coated aluminum or aluminum alloy coated steel sheet with high surface roughness manufactured above is punched or cut into blanks of the required shape for hot-formed parts;

[0071] 2) Heat Treatment: The billet is placed in a heating furnace and heated and held at a temperature of 860–1000℃. The furnace atmosphere is air or nitrogen, and the billet remains in the furnace for 2–10 minutes. A box furnace or a roller hearth furnace can be used. Regardless of the furnace type, the temperature fluctuation in the heating zone must be controlled within 3% to ensure the uniformity of the surface roughness of the coating after heating. Temperature fluctuation refers to the temperature fluctuation along or perpendicular to the transport direction of the pre-coated steel plate within the furnace. Temperature fluctuation = |maximum or minimum temperature - average temperature| / average temperature × 100%, using the larger of the maximum or minimum temperature. In this embodiment, a box furnace is used with a target heating temperature of 930℃ and a target heating time of 5 minutes to ensure complete alloying of the steel plate coating and eliminate the influence of incomplete alloying on surface roughness. Incomplete alloying results in a smaller amount of liquefied filling of pits, leading to a higher overall surface roughness after heating.

[0072] 3) Hot stamping: The heat-treated blank is quickly transferred to the mold for stamping and cooling. The transfer time shall not exceed 15s, the stamping holding time shall be 5 to 15s, and the cooling and demolding temperature shall not exceed 250℃.

[0073] The embodiments and comparative examples of this invention are described with the blanking direction of the pre-coated steel sheet perpendicular to the rolling direction. The fluctuations in pre-coating weight, furnace temperature, and surface roughness of the coating after hot forming are also perpendicular to the rolling direction. The pre-coating weight, furnace temperature, and surface roughness of the coating after hot forming are measured along the rolling direction, and the corresponding fluctuation values ​​are calculated. The surface roughness measurement standard adopts GB / T2523-2008 Measurement Method of Surface Roughness and Peak Count of Cold-Rolled Metal Sheets (Strips), with a cutoff wavelength of 0.8 mm and a test distance of 4.8 mm.

[0074] Next, appropriate processes (phosphating, electrophoresis) are selected for coating the above-mentioned thermoformed parts, and scratch corrosion tests are performed on the coatings at different parts after coating (corresponding to the parts with the weight fluctuation of the pre-coating) to evaluate the paint adhesion and corrosion resistance. When the maximum corrosion expansion width is not greater than 4mm, the requirements are met.

[0075] The phosphating agents and test parameters in Table 2 were used to phosphate the thermoformed samples. The resulting phosphated plates were then subjected to electrophoresis (electrophoretic paint model: Kansai HT-8000C), with a dry film thickness of approximately 18 μm. A cyclic corrosion method was then used. Each cycle consisted of 8 hours of ambient temperature maintenance (25±3℃), during which four 3-minute sprays of salt solution were applied. The salt solution composition was: 0.9 wt% NaCl, 0.1 wt% CaCl2, and 0.075-0.9 wt% NaHCO3. This was followed by 8 hours of wet heat treatment (49±2℃, 100% RH), and finally 8 hours of drying (60±2℃, <30% RH). A total of 26 cycles were performed to evaluate corrosion resistance.

[0076] Table 2 Phosphating process parameters

[0077]

[0078] Table 3 shows the key process parameters for preparing pre-coated aluminum or aluminum alloy coated steel plates for each embodiment and comparative example. Table 4 shows the heating temperature, surface roughness after heating, and scratch corrosion test results.

[0079] Table 3 Key process parameters for pre-coated aluminum or aluminum alloy coated steel plates in each embodiment and comparative example

[0080]

[0081]

[0082]

[0083] Table 4 shows the heating temperature, surface roughness after heating, and scratch corrosion test results for each embodiment and comparative example.

[0084]

[0085]

[0086] The pre-coated steel sheet components provided by this invention, after hot stamping, have coatings with high surface roughness Ra and Rpc, i.e., Ra ≥ 1.5 μm and Rpc ≥ 80 / cm. This is mainly achieved by controlling the high surface roughness of the pre-coating. Specifically, when 20 g / m 2 ≤Weight of each pre-coated layer ≤40g / m 2 When the average surface roughness of the pre-coating satisfies: 2.0≤Ra≤4.8 and Rpc+ per single-sided pre-coating weight≥100, when 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2At that time, the average surface roughness of the pre-coating satisfies: 2.5≤Ra≤4.8 and Rpc+pre-coating weight per single side ≥100. The high surface roughness of the pre-coating is mainly achieved by using a high-roughness finishing roll. Specifically, the average Ra value of the finishing roll is between 3.5 and 6.0 μm, the roll Rpc ≥ the pre-coating Rpc, and the finishing elongation is between 1.0 and 2.0%.

[0087] As shown in Examples 1-5: In Examples 1-4, the target single-sided pre-coating weight was 20 g / m². 2 35g / m 2 The average Ra value of the finishing roll is between 3.5 and 6.0 μm, the roll Rpc is greater than or equal to the pre-coating Rpc, and the finishing elongation is between 1.0 and 2.0%. This ensures that the average surface roughness Ra of the pre-coating satisfies: 2.0 ≤ Ra ≤ 4.8, and the average surface roughness Rpc of the pre-coating is greater than or equal to 80 / cm. It also satisfies: Rpc + weight of pre-coating per single side ≥ 100. The final thermoformed part has an average surface roughness Ra ≥ 1.5 μm and Rpc ≥ 80 / cm. In Example 5, the target weight of the pre-coating per single side is 60 g / m. 2 The average Ra of the finishing roll is 4.50 μm, the average Rpc is 85 / cm, the finishing elongation is 1.2%, the average surface roughness Ra of the pre-coating is 2.55 μm, satisfying 2.5≤Ra≤4.8, the average surface roughness Rpc of the pre-coating is 82 / cm, which also satisfies: Rpc + weight of each single-sided pre-coating ≥100, the average surface roughness Ra of the final thermoformed part coating is 1.76 μm, and Rpc is 181 / cm, satisfying Ra≥1.5 μm and Rpc≥80 / cm.

[0088] More importantly, the pre-coated steel sheet provided by this invention, after being hot-stamped, produces a component with a coating that not only has high surface roughness Ra and Rpc, but also relatively uniform surface roughness Ra and Rpc. This invention controls the steel sheet shape before aluminized silicon plating, the stability of the air knife parameters after aluminized silicon plating, and the uniformity of the heating temperature during the heating process. As shown in Examples 1-5, this ensures that the unevenness of the entire steel sheet (acid-rolled sheet) before aluminized silicon plating is ≤15mm, preferably ≤10mm, the air knife parameter fluctuation is ≤5%, the pre-coating weight fluctuation is ≤30%, the heating temperature fluctuation is ≤3%, and finally, the surface roughness Ra fluctuation and Rpc fluctuation of the entire steel sheet surface coating after hot forming are ≤20%.

[0089] In contrast, in Comparative Example 1, the unevenness of the pickled and rolled steel sheet was relatively large, with 20mm > 15mm, resulting in a pre-coating weight fluctuation of >30%, and some areas having a relatively large coating weight (e.g., 50g / m²). 2After thermoforming, the surface roughness Ra and Rpc at this location are low, with Ra = 1.15 μm < 1.5 μm, resulting in insufficient scratch corrosion. Furthermore, the overall Ra and Rpc fluctuations are both greater than 20%. In Comparative Example 2, the air knife parameters fluctuate significantly, with 6% > 5%, similarly causing a pre-coating weight fluctuation of > 30%, with some areas exhibiting a higher coating weight (e.g., 51 g / m²). 2 In Comparative Example 3, the surface roughness Ra and Rpc of this area are relatively low after hot forming, with Ra = 1.12 μm < 1.5 μm, resulting in unsatisfactory scratch corrosion. Furthermore, the overall Ra and Rpc fluctuations are both greater than 20%. In Comparative Example 4, the unevenness of the pickled and rolled plate and the fluctuations in air knife parameters are relatively small, ensuring minimal fluctuation in the pre-coating weight. However, the heating temperature fluctuation is large, 3.2% > 3%, and the temperature in the middle of the steel plate is 960℃, significantly higher than other parts. During heating, the coating liquefies more rapidly in this area, resulting in low surface roughness Ra and Rpc (Ra = 1.10 μm < 1.5 μm), causing unsatisfactory scratch corrosion. Furthermore, the overall Ra and Rpc fluctuations are both greater than 20%. In Comparative Example 4, the target weight of the pre-coating on a single side is 20 g / m². 2 The average surface roughness of the pre-coated layer is Ra = 1.4 μm and Rpc = 62 / cm, which does not meet the requirements of 2.0 ≤ Ra ≤ 4.8 and Rpc + weight of each single-sided pre-coated layer ≥ 100. After thermoforming, the average surface roughness is Ra = 1.37 μm < 1.5 μm and Rpc = 65 / cm ​​< 80 / cm. Therefore, the scratch corrosion propagation width is > 4 mm, which does not meet the requirements.

[0090] When the pre-coating weight is small (20g / m²) 2 ≤Weight of each pre-coated layer ≤60g / m 2 Ensuring high and uniform surface roughness Ra and Rpc of the coating after thermoforming is crucial for guaranteeing the adhesion and corrosion resistance of aluminum alloy coatings after thermoforming.

[0091] Through the above embodiments, when the pre-coated aluminum or aluminum alloy coating is relatively small, the pre-coated thermoformed coating obtained by the present invention has a high and relatively uniform surface roughness Ra and Rpc, which can ensure good paint adhesion and corrosion resistance.

[0092] The above embodiments have described in detail the purpose and effects of the present invention. It should be understood that the above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. All modifications, equivalent substitutions, improvements, etc., made by those skilled in the art or by adopting the technical concept and technical solution of the present invention within the spirit and principles of the present invention and without departing from the design concept of the present invention are within the protection scope of the present invention.

Claims

1. A pre-coated aluminum or aluminum alloy steel sheet for hot stamping, characterized in that, The pre-coating weight of the pre-coated steel sheet used for hot stamping is controlled to be 20~60g / m² per single side. 2 ; The surface roughness control of the pre-coating: when 20g / m 2 ≤Weight of each pre-coated layer ≤40g / m 2 When the surface roughness Ra of the pre-coated layer is ≥2.0μm and the weight of the pre-coated layer per single side is ≥100, and when 40g / m 2 <Weight of each pre-coated layer ≤60g / m² 2 At that time, the surface roughness Ra of the pre-coated layer is ≥2.5μm and the weight of Rpc+ per single-sided pre-coated layer is ≥100; The weight fluctuation of the pre-coating is within 30%; The manufacturing method of the pre-coated steel sheet for hot stamping includes the following process flow: substrate manufacturing → hot-dip galvanizing → finishing. The substrate unevenness should be within 15mm. The hot-dip galvanizing requires that the air knife parameter fluctuation be controlled within 5%.

2. A method for manufacturing a pre-coated steel sheet for hot stamping as described in claim 1, comprising the following process flow: manufacturing a substrate → hot-dip galvanizing → finishing, characterized in that, The substrate being manufactured must have a flatness within 15mm; the hot-dip plating process requires that the fluctuation of the air knife parameters be controlled within 5%.

3. The manufacturing method according to claim 2, characterized in that, The hot-dip plating process involves a plating solution temperature of 640~680℃ and an immersion time of 2~8s.

4. The manufacturing method according to claim 2, characterized in that, The finishing process includes: the average Ra value of the finishing roll is between 3.5 and 6.0 μm, the roll Rpc is greater than or equal to the pre-coated Rpc, and the finishing elongation is between 1.0 and 2.0%.

5. A hot-stamped part, characterized in that, The hot stamping part is manufactured by heat treatment using the pre-coated steel sheet for hot stamping as described in claim 1. The surface roughness of the coating of the hot stamping part is Ra≥1.5μm and Rpc≥80 / cm, and the surface roughness Ra of the coating of the hot stamping part fluctuates ≤20% and Rpc fluctuates ≤20%.

6. The hot-stamped part according to claim 5, characterized in that, The heat treatment involves controlling the temperature fluctuation of the heating zone to within 3%.

7. A method for manufacturing a hot-stamped part as described in claim 5 or 6, characterized in that, The manufacturing method includes heat treatment, wherein the heating furnace temperature is 860~1000℃ and the billet stays in the heating furnace for 2~10 minutes.

8. An application of the hot-stamped part according to claim 5 or 6, characterized in that, Used for land-based motor vehicles.

Images