Direct hot forming method and parts for improving synchronous forming precision of large-thickness-difference plated hot-formed steel

Abstract

The present application belongs to the technical field of high-strength steel part production, and particularly relates to a direct forming process for improving the dimensional accuracy of plated hot-formed steel and a part. The present application controls the time interval of each stage of the forming process, quantitatively calculates the real-time temperature of the sheet in air and hot bath medium, controls the sheet forming starting temperature to be less than or equal to 700 DEG C, the temperature when the die is in place to be greater than or equal to 300 DEG C, and the hot bath time to be greater than or equal to 0.5 s, and obtains a quantitative process window that is common to sheet of different thickness specifications. The part prepared by the present process has a thickness of 0.8-2.2 mm, a gage RPS point position tolerance of less than or equal to 0.5 mm, a flatness deviation of less than or equal to 0.5 mm / m, a room temperature tensile strength of 500-2200 MPa, and a transition layer thickness between the coating and the substrate of less than or equal to 4 microns, effectively inhibiting LME micro-cracks, and solving the problems of poor dimensional accuracy and uneven performance of tailor-welded blanks or co-molded parts.

Description

Technical Field

[0001] This invention belongs to the field of high-strength steel parts manufacturing technology, specifically relating to a direct hot forming method and parts for improving the synchronous forming accuracy of hot-formed steel with large thickness difference coating. Background Technology

[0002] With the automotive industry's increasing demands for lightweight body construction and collision safety, hot stamping has become the mainstream process for manufacturing ultra-high-strength steel structural components for car bodies. It is widely used in the production of key body structural components such as A-pillars, B-pillars, and door rings, meeting the weight reduction requirements of automotive lightweighting while significantly improving the passive safety performance of the vehicle body. However, hot stamping dies are limited by multiple factors, including die material properties, heat treatment processes, and the difficulty of cooling channel processing, resulting in extremely high die manufacturing costs. Currently, the industry often adopts a multi-cavity die design, merging multiple independent dies into a single integrated die. This approach significantly reduces the amount of die steel used, machining time, and heat treatment costs, while also significantly shortening the hot stamping cycle production time and reducing the heating energy consumption per unit part, thus effectively distributing the expensive die manufacturing costs and the operating costs of the hot stamping production line. Furthermore, to meet the differentiated performance requirements of different areas of the body structure—for example, the body's anti-collision areas require extremely high strength to withstand collision impacts, while energy-absorbing areas require high toughness to absorb collision energy—and to achieve integrated forming of parts, laser welding plate technology has been widely used in the field of hot forming. This technology uses laser welding to form a whole blank from steel plates of different thicknesses, strength grades, or coating types, and then performs uniform hot stamping processing. This enables the "on-demand design" performance distribution of different areas of a single part, taking into account the functional requirements of different parts of the vehicle body.

[0003] However, when hot stamping continuous production of sheet metal of different thicknesses using the same set of molds and the same process cycle, the difference in contact pressure between the thick and thin sheets and the mold, as well as the different heat conduction paths, leads to inconsistent cooling rates in different thickness regions, with the cooling rate of the thicker sheet region being relatively low. This not only may result in incomplete microstructure transformation in the thicker sheet region, failing to obtain the expected full martensitic structure, but also, due to the significant differences in the timing and degree of martensitic phase transformation in different thickness regions—the martensitic phase transformation in the thicker sheet is significantly delayed compared to the thinner sheet—and the volume expansion accompanying the martensitic phase transformation, these differences in the timing and degree of phase transformation generate huge residual stresses inside the part. Ultimately, this leads to severe springback, twisting, or localized deformation after demolding, making it difficult for the dimensional accuracy of the part to meet the process requirements of welding and assembly of the body-in-white. Therefore, how to ensure that hot-formed steel sheets of different thicknesses can obtain sufficient and matched martensitic structures within the same process window, while effectively controlling the springback deformation generated during the cooling phase transformation, and improving the overall dimensional accuracy of the parts, is a technical challenge that urgently needs to be overcome in the field of high-strength steel hot forming technology. Summary of the Invention

[0004] The purpose of this invention is to provide a direct hot forming method and components for simultaneously improving the forming accuracy of hot-formed steel with large thickness differences. This method enables hot-formed steel sheets of different specifications to simultaneously obtain qualified performance and dimensional accuracy under the same mold and the same process cycle, providing a solution for the consistency of dimensional accuracy of components of different thicknesses or welded parts.

[0005] The objective of this invention is achieved through the following technical solution: This invention provides a direct hot forming method for simultaneously improving the forming accuracy of hot-formed steel with large thickness differences in coatings, comprising the following steps: S1. Heat the coated hot-formed steel sheet in a heating furnace to the complete austenitizing temperature; add a heat bath medium to the mold box and heat it to the target temperature; S2. The heated coated hot-formed steel sheet is transferred from the heating furnace to a support frame inside the hot-forming mold; the lower mold surface of the hot-forming mold is partially or completely immersed in the hot bath medium, and a support frame higher than the mold surface is provided in the lower mold cavity; the height of the support frame is above or below the highest liquid level of the hot bath medium; t is defined. Z The time interval during which the heated sheet metal is transferred from the heating furnace to the material support rack inside the thermoforming mold; S3. The mold descends, the hot-formed steel sheet is formed in the mold, pressure is maintained and quenched to obtain the parts; Define t T The time interval t is defined as the time between the heated sheet metal contacting the support frame and the upper mold surface. YThe time interval from when the heated sheet enters the hot bath medium to when the mold is in place; the total time interval t = t_t from when the heated sheet is transferred out of the heating furnace to when the mold is in place. Z +t T +t Y ; Define t A t is the time interval during which the heated sheet metal remains in the air. W The time interval between the heated sheet entering the hot bath medium and the mold forming the final shape; When the height of the material support frame is above the highest liquid level of the hot bath medium in the lower mold, t A =t Z +t T , t W =t Y ; When the height of the material support frame is below the highest liquid level of the hot bath medium in the lower mold, t A =t Z , t W =t T +t Y ; In steps S2-S3, the temperature T of the hot-formed steel sheet is... X Satisfying the formula: T X =T0 k A ×t A k W ×t W Where T0 is the furnace exit temperature of the sheet metal, kJ / m³ A k is the cooling rate of the heated sheet metal in the air medium. W The cooling rate of the heated sheet material in the hot bath medium; The hot bath time t W ≥0.5s; the initial forming temperature T of the hot-formed steel sheet after heating. XS ≤700℃, sheet metal temperature T when the mold is in place XZ ≥300℃; S4. Obtain parts with forming accuracy that meet technical requirements.

[0006] Furthermore, in step S1, the heating temperature of the hot-formed steel sheet is 880~950℃, and the holding time is 3~10min to ensure that the sheet is completely austenitized.

[0007] Furthermore, the cooling rate k of the heated sheet metal in the air medium A Based on the sheet thickness d, the formula is: k A =9.85+19.01×e 1.08(d 1.2) Where d takes values ​​ranging from 0.75mm to 2.25mm; when the air temperature is greater than 30℃, k AH =0.8×k A When the air medium temperature is < -10℃, k AL =1.2×k A .

[0008] Furthermore, the cooling rate k of the heated sheet material in the hot bath medium... W Based on the sheet metal temperature TX, it is divided into two ranges: k W1 =1.8×d×T W 188.3×d 4.1×T W +438, T X ∈[200,420]℃; k W2 =5.7×d×T W 609×d 13.3×T W +1447, T X ∈[420,800]℃; Where d∈[0.75,2.25]mm, T W ∈[80,100]℃, T W The temperature of the heat bath medium; The heat bath medium includes water and its aqueous solution, and may be mixed with cooling agents, rust inhibitors, or lubricants; depending on the heat bath medium, α is used. H For k W Make corrections, i.e., k WS =α H ×k W The value of αH is in the range of 0.1 ≤ α H ≤2.

[0009] Furthermore, the coating of the hot-formed steel is a zinc-based coating, including pure zinc coating, zinc-iron alloy coating, and zinc-aluminum-magnesium coating; the coating thickness on one side is 30~100g / m².

[0010] The present invention also provides a coated hot-formed steel part with excellent dimensional accuracy, the part being prepared by the direct forming process according to any one of claims 1 to 6; the part having no obvious warping, springback, or localized shrinkage deformation; and the metallographic structure of the part being mainly composed of uniform martensite.

[0011] Furthermore, the component comprises a surface coating, an ultra-thin transition layer, and a steel substrate; the ultra-thin transition layer is located at the corrosion boundary between the coating and the substrate, extending from the corrosion boundary towards the depth of the substrate, and is formed by the diffusion of zinc element from the coating into the substrate; the thickness of the ultra-thin transition layer is ≤4μm, and its zinc element content decreases along the depth direction from the corrosion boundary and is not higher than 15wt%; the corrosion boundary is obtained by etching the quenched component with a nitric acid alcohol solution.

[0012] Furthermore, the chemical element composition of the coated hot-formed steel sheet is as follows: C 0.05wt%~0.35wt%, Si≤1.2wt%, Mn 0.5wt%~2.2wt%, Cr≤1.5wt%, Mo≤0.5wt%, Ni≤0.5wt%, Ti≤0.04wt%, Nb≤0.2wt%, V≤0.2wt%, B 0.002wt%~0.006wt%, P≤0.020wt%, S≤0.003wt%, Al≤0.8wt%, N≤0.006wt%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, the maximum depth of LME microcracks in the component is ≤10μm, and the room temperature tensile strength is ≥1350MPa.

[0014] The beneficial effects of this invention are as follows: The direct hot forming process for improving the dimensional accuracy of coated hot-formed steel provided by this invention, through quantitative control of the air cooling and hot bath time windows, can ensure that regions of different thicknesses can obtain sufficient and matched martensitic structure and mechanical properties. It effectively suppresses differences in thermal expansion and martensitic phase transformation timing deviations between regions of different thicknesses, reducing springback or twisting after demolding, resulting in significantly better dimensional accuracy of hot-stamped parts than traditional processes. Simultaneously, this process can reduce the risk of LME microcracks in parts, controlling the thickness of the extremely thin transition layer between the coating and the substrate to within 4μm, ensuring that the room temperature tensile strength of the parts is ≥1350MPa, meeting the mechanical property requirements of 1500MPa-level hot-formed steel. It systematically solves the problems of uneven performance, thermal expansion deformation, and cooling springback in welded or co-molded parts, enabling hot-formed steel sheets of different specifications to simultaneously obtain qualified performance and dimensional accuracy under the same mold and process cycle, providing a solution for the consistency of dimensional accuracy in co-molded or welded parts of different thicknesses.

[0015] The direct hot forming process for improving the dimensional accuracy of coated hot-formed steel provided by this invention does not rely on empirical parameters or manual adjustments. Instead, it provides a quantitative process window applicable to sheets of different thicknesses based on measured temperature drop curves and simulation calculations. This process window is quantifiable, reproducible, and highly stable. The process transforms the temperature change trend of the sheet into a function related to sheet thickness, hot bath temperature, air residence time, and hot bath time. Precise temperature monitoring can be achieved by adjusting the automated transfer cycle of the mechanical end effector and the die running position parameters of the hot pressing production line. It eliminates the need for infrared temperature measurement methods with significant errors, making it highly operable for hot pressing production lines and easy for industrial application.

[0016] The direct hot forming process for improving the dimensional accuracy of coated hot-formed steel provided by this invention is applicable to all zinc-based coated hot-formed steel products, including hot-formed steel with zinc coatings such as pure zinc coating, zinc-iron alloy coating, and zinc-aluminum-magnesium coating. It eliminates the need for individual adjustments for thin or thick sheet metal, simplifying the raw material supply chain and reducing manufacturing costs and management complexity. Simultaneously, this process is adaptable to multi-cavity mold designs and laser-welded plate technology, enabling the stable implementation of solutions that combine multiple molds into one and weld steel plates of different thicknesses and properties into a single blank for hot stamping. This significantly reduces mold steel, processing time, and heat treatment costs, substantially lowering the hot stamping cycle production time and heating energy consumption per unit part, thus amortizing expensive mold costs and hot stamping production line operating costs. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the forming process window of the present invention; Figure 2 This is a cooling rate curve of the 1.2mm sheet metal of the present invention in air medium (25°C); Figure 3 This is a cooling rate curve of the 2.0mm sheet material of the present invention in hot water at 100℃; Figure 4 This is a process window intersection diagram under the conditions of a hot bath temperature of 95℃ and a plate thickness of 1.2~1.8mm according to the present invention; Figure 5 This is a schematic diagram of the transition layer characterization in Embodiment 5 of the present invention; Figure 6 This is a metallographic structure and maximum microcrack depth diagram of Embodiment 5 of the present invention; Figure 7 This is a metallographic structure and maximum microcrack depth diagram of Comparative Example 4 of the present invention; Figure 8 This is a metallographic structure and maximum microcrack depth diagram of Comparative Example 5 of the present invention; Figure 9 This is a metallographic structure and maximum microcrack depth diagram of Comparative Example 6 of the present invention; Detailed Implementation Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0019] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0020] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0021] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0022] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0023] This invention uses multiple sets of air-cooling experimental data of zinc-based coated hot-formed steel sheets to fit the relationship between the steel sheet thickness d and the cooling rate k in the air. A Relationship formula: k A =9.85+19.01×e 1.08(d 1.2) In the formula, k A d is the air cooling rate (°C / s); d is the steel plate thickness (mm); 9.85 is the limiting cooling rate (°C / s), reflecting the theoretical minimum cooling rate when the steel plate thickness increases infinitely; 19.01 is the initial excess cooling rate (°C / s), reflecting the sensitivity of the steel plate thickness to the cooling rate; 1.08 is the attenuation constant (mm). -1 This reflects the rate at which the cooling rate decreases as the steel plate thickness increases, and is related to the material's thermal diffusivity.

[0024] The experimental conditions for the above formula are as follows: the material is galvanized hot-formed steel with 22MnB5 as the base material, the plate thickness is 0.8~2.5mm, the initial temperature of the steel plate is 900±10℃, the ambient temperature is 0~30℃, the steel plate is placed horizontally in still air (without forced convection), and both sides are exposed (not in contact with the mold) for cooling.

[0025] The test method is as follows: a K-type thermocouple is welded to the center of the steel plate surface, and a multi-channel temperature acquisition instrument is used to monitor and measure the actual temperature of the plate in real time with a sampling frequency of 10Hz.

[0026] The fitting method is as follows: Based on Newton's law of cooling, the cooling rate of an object in air is proportional to the temperature difference between it and the environment. For steel plates of different thicknesses *d*, the cooling rate decreases with increasing plate thickness, and the rate of decrease gradually slows down, eventually approaching a non-zero lower limit (ambient temperature), which conforms to the exponential decay law. Therefore, the exponential decay model *v = c + a × e* is selected. b(d d0) The least squares nonlinear regression method was used, and the correlation coefficient R was obtained based on 24 sets of valid experimental data. 2 =0.968, with an average relative error of 4.93%.

[0027] The core physical principle of the above formula is Newton's law of cooling. For a hot steel plate placed in the air, the rate of change of its temperature T with time t satisfies the following differential equation: dT / dt= k (T T evn ) The solution to this equation is T(t) = T evn +(T0 T evn )×e kt Where T is the steel plate temperature; T0 is the initial temperature of the steel plate; k is the cooling rate constant, which is related to the steel plate thickness, heat dissipation area, air velocity, etc.; T evnLet be the ambient temperature and be the final temperature at which the steel plate cools. The physical meaning of the formula is: the hotter the steel plate, the faster it dissipates heat into the air; as the temperature decreases, the heat dissipation rate slows down. Theoretically, the steel plate temperature will only infinitely approach the ambient temperature. Initially, the temperature difference between the steel plate and the environment is large, resulting in a fast cooling rate. Later, as the temperature difference decreases, the cooling rate slows down. The exponential decay function can reflect the time-varying nature of the cooling rate and accurately describe the core physical law that "the greater the temperature difference, the faster the cooling rate."

[0028] This invention obtains the corresponding cooling rate by testing the sheet temperature of materials of different thicknesses in different temperature-controlled heat baths. The data is then fitted and extrapolated using a bilinear interpolation formula to obtain the cooling rate k of the heated sheet in the heat bath. W Calculation formula: k W1 =1.8×d×Tw 188.3×d 4.1×T w +438 T x ∈[200,420) k W2 =5.7×d×Tw 609×d 13.3×T w +1447 T x ∈[420,800] In the formula, d∈[0.75,2.25]mm, T w ∈[80,100]℃; k W d is the cooling rate in the hot bath medium (°C / s), d is the thickness of the hot-formed sheet (mm), and T is the cooling rate in the hot bath medium (°C / s). w The temperature of the heat bath medium is (°C).

[0029] The value range of 'd' in the above formula is based on measured data, and in the automotive parts field, this thickness specification range basically covers all the specifications involved in body design. The heat bath temperature is not lower than 80℃. Based on the measured data of the heat bath temperature drop curve, the temperature drop curve of sheet metal with a heat bath temperature above 80℃ has good repeatability, and the fitted curve has good reproducibility with the measured curve. When the heat bath temperature is 70℃, the vaporization film is not stable enough, the data repeatability is poor, and it is difficult to fit the data. Furthermore, actual industrial production data verifies that parts (especially those with a thickness less than 1.6mm) at 70℃ will undergo martensitic transformation instantaneously due to excessively rapid cooling, resulting in excessively high strength and stamping cracks. xThe value range is [200, 800]℃. This is because when the sheet material is cooled to around 200℃, the temperature difference with the heat bath medium decreases, the cooling rate slows down but remains essentially unchanged, and 200℃ is already below the Mf point (the end temperature of martensite transformation). The material has already completed its structural transformation, its strength increases, and problems such as springback and cracking are unavoidable. Values ​​below 200℃ are not meaningful for reference. 800℃ is higher than the Γ phase (Γ... Fe3Zn 10 The decomposition temperature is approximately 782℃. If the part is formed at a temperature higher than this, the microcracks in the part are likely to exceed 10μm and are not of reference value.

[0030] Depending on the different heat bath medium, use α H For k W Make corrections, i.e., k WS =α H ×k W , where α H The range of values ​​for is 0.1 ≤ α H ≤2. The heat bath medium includes, but is not limited to, water and its solutions; other solvents may be mixed in, such as cooling agents, rust inhibitors, lubricants, etc., and the heat bath temperature shall not be lower than 80°C.

[0031] The coating of the hot-formed steel described in this invention is a zinc-based coating, including pure zinc coating, zinc-iron alloy coating, zinc-aluminum-magnesium coating, and other zinc-containing coatings; the single-sided coating thickness is 30~100g / m². The chemical element composition of the hot-formed steel sheet is: C 0.05wt%-0.35wt%, Si≤1.2wt%, Mn 0.5wt%-2.2wt%, Cr≤1.5wt%, Mo≤0.5wt%, Ni≤0.5wt%, Ti≤0.04wt%, Nb≤0.2wt%, V≤0.2wt%, B 0.002wt%-0.006wt%, P≤0.020wt%, S≤0.003wt%, Al≤0.8wt%, N≤0.006wt%, with the balance being Fe and unavoidable impurities.

[0032] A schematic diagram of the forming process window of the present invention is shown below. Figure 1 As shown, where d∈[0.75,2.25]mm, T w =90℃.

[0033] Example 1 This embodiment is used to extract the cooling rate of coated hot-formed steel sheets in air.

[0034] Experimental materials: galvanized hot-formed steel (1500MPa grade hot-formed steel) with 22MnB5 as the base material, with a plate thickness of 1.2mm.

[0035] Experimental equipment: multi-channel temperature acquisition instrument, K-type thermocouple, heating furnace.

[0036] Experimental steps: Weld a K-type thermocouple to the center of the steel plate surface, place the plate in a heating furnace and heat it to 890℃. After holding it at that temperature for 6 minutes, remove it and place it horizontally in still air (without forced convection), with both sides exposed (not in contact with the mold) for cooling. Use a multi-channel temperature acquisition instrument to monitor and measure the actual temperature of the plate in real time, with a sampling frequency of 10Hz and an ambient temperature of 25℃.

[0037] Experimental results: The temperature drop curve of a 1.2mm thick plate in air was obtained, as shown below. Figure 2 As shown.

[0038] Based on the same experimental method, air cooling experiments were conducted on galvanized hot-formed steel plates with thicknesses of 0.8 mm, 1.0 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2.0 mm, and 2.2 mm. The air cooling rate of each thickness was measured and compared with the fitted value calculated by the above fitting formula. The results are shown in Table 1.

[0039] Table 1 Comparison of measured and fitted data of sheet cooling rate in air medium

[0040] As shown in Table 1, the relative error between the air cooling rate formula obtained by the present invention and the measured data is within 5%, indicating high fitting accuracy and the ability to accurately reflect the cooling characteristics of hot-formed steel sheets with different coating thicknesses in the air medium.

[0041] Example 2: Rapid Cold Extraction in a Hot Bath Medium This embodiment is used to extract the cooling rate of coated hot-formed steel sheets in a hot bath medium.

[0042] Experimental materials: 1500MPa grade zinc-based coated hot-formed steel, 2.0mm thick.

[0043] Experimental equipment: multi-channel temperature acquisition instrument, K-type thermocouple, heating furnace, hot bath.

[0044] Experimental steps: Weld a K-type thermocouple to the center of the steel plate surface, place the plate in a heating furnace and heat it to 890℃. After holding it at that temperature for 6 minutes, remove it and quickly place it in a hot bath at 100℃. Use a multi-channel temperature acquisition instrument to monitor and measure the actual temperature of the plate in real time, with a sampling frequency of 10Hz and an ambient temperature of 25℃.

[0045] Experimental results: The temperature drop curve of a 2.0 mm thick plate in a 100℃ hot bath medium was obtained, as shown below. Figure 3 As shown. By Figure 3 It can be seen that the cooling process of the sheet metal is divided into an air transfer section and a hot bath cooling section, with the cooling rate of the hot bath cooling section being significantly higher than that of the air cooling section.

[0046] Example 3: Hot forming of welded plates with different thicknesses This embodiment is used to verify the application effect of the process of the present invention in the hot forming of welded plates of different thicknesses.

[0047] Experimental materials: Zinc-based coated hot-formed steel sheets with thicknesses of 1.2 mm, 1.4 mm, 1.6 mm and 1.8 mm, which were welded into integral welded plates using laser welding technology.

[0048] Experimental equipment: heating furnace, thermoforming press, hot bath mold, gauge, gap gauge.

[0049] Experimental conditions: The heat bath medium is water, and the heat bath temperature is T. w =95℃; the material support rack is located above the highest liquid level of the hot bath medium; the sheet material exit temperature T0=900℃; the heat preservation time is 6min; the pressure preservation time is 20s.

[0050] Process parameter calculation: According to the temperature calculation formula T of the present invention X =T0 k A ×t A k W ×t W , with T X ∈[300,700]℃ as the boundary condition, combined with k for each thickness of plate. A and k W The value is calculated to obtain the hot bath time t. W When =0.8s, the air residence time t A The value range is [7.60, 16.63]s. The process windows and common intersection areas for different plate thicknesses are as follows: Figure 4 As shown, the common upper boundary is determined by the thickest plate (d=1.8mm, slowest cooling), and the common lower boundary is determined by the thinnest plate (d=1.2mm, fastest cooling) and t. W The effective process parameter window is determined by ≥0.5s. Figure 4 The green shaded area in the text.

[0051] Experimental procedure: Place the welded plate in a heating furnace and heat to 900℃, hold for 6 minutes, then remove and proceed according to t A The air residence time is 14 seconds, and the material is transferred to the support rack inside the thermoforming mold. The mold descends to complete the forming process, and the part is removed after holding the pressure for 20 seconds.

[0052] Dimensional accuracy inspection method: Place the formed part on the fixture, fix the part on the fixture using locating pins, and install the clamping device. Use a gap gauge in conjunction with the fixture for measurement. Use the fixture surface as the reference surface and the lower surface of the part as the measuring surface, ensuring that the reference surface and the measuring surface are parallel. The straight edge of the gap gauge should be completely in contact with the reference surface and perpendicular to it. When taking the reading, take the contact point between the part and the gap gauge as the reading point. The difference between the gap value between the measuring surface and the reference surface and the required gap value (5.0mm) is the positional tolerance of the inspection point. If the difference is greater than 0, it is marked as +; if it is less than 0, it is marked as -.

[0053] Experimental results: After forming, the positional tolerance of the RPS positioning points of the gauge for all thickness specifications of the parts was ≤ ±0.5mm, the flatness deviation was ≤0.5mm / m, there was no obvious warping, springback and local diameter reduction deformation, the forming accuracy met the technical requirements, and the gap value measurement data of each detection point is shown in Table 2.

[0054] Example 4: Co-mold thermoforming of materials with different plate thicknesses This embodiment is used to verify the application effect of the process of the present invention in co-mold thermoforming of materials with different thicknesses.

[0055] Experimental materials: zinc-based coated hot-formed steel sheets with thicknesses of 1.6 mm and 2.2 mm respectively.

[0056] Experimental equipment: heating furnace, thermoforming press, hot bath mold, gauge, gap gauge.

[0057] Experimental conditions: The heat bath medium was an aqueous solution with added rust inhibitor; the heat bath temperature was T. w =85℃; the material support rack is located below the highest liquid level of the hot bath medium; the sheet material exit temperature T0=900℃; the heat preservation time is 7min; the pressure preservation time is 15s.

[0058] Process parameter calculation: According to the temperature calculation formula T of the present invention X =T0 k A ×t A k W ×t W Using TX∈[300,700]℃ as the boundary condition, combined with k for each thickness of plate... A and k W The value is calculated to obtain the hot bath time t. W At 4s, the air residence time t A The value range is [2.00, 19.70]s.

[0059] Experimental procedure: Place two sheet metals of different thicknesses into a heating furnace and heat to 900℃. After holding at that temperature for 7 minutes, remove the sheet metal and transfer it to the support rack in the thermoforming mold according to an air residence time of tA=10s. The mold descends to complete the forming process. After holding the pressure for 15s, remove the part.

[0060] Experimental results: The positional tolerance of the RPS positioning points of the gauge for 1.6mm and 2.2mm thick parts after forming is ≤±0.5mm, the flatness deviation is ≤0.5mm / m, there is no obvious warping, springback and local diameter reduction deformation, and the forming accuracy meets the technical requirements.

[0061] Example 5: Verification of U-shaped mold hot stamping forming This embodiment is used to verify the effects of the process of the present invention on LME microcracks, transition layer thickness and mechanical properties of components.

[0062] Experimental materials: Zinc-based coated hot-formed steel, 1.8 mm thick.

[0063] Experimental equipment: U-shaped thermoforming mold, heating furnace, metallographic microscope, scanning electron microscope, universal testing machine.

[0064] Experimental conditions: The heat bath medium is water, and the heat bath temperature is T. w =90℃; sheet metal exit temperature T0=890℃; air residence time t A =8.5s; hot bath time t W =0.8s; holding time 10s.

[0065] Experimental steps: Place the sheet metal into a heating furnace and heat it to 890℃. After holding the temperature for 6 minutes, take it out and perform hot stamping forming according to the above process parameters. After holding the pressure, take out the U-shaped parts.

[0066] Performance testing methods: Mechanical property testing: The tensile strength of the parts was tested in accordance with GB / T228.1-2010 "Metallic materials - Tensile testing - Part 1: Test at room temperature".

[0067] LME microcrack characterization: Metallographic microscopes were used to observe LME microcracks in the coating at various locations on the parts, and the maximum depth and number of microcracks were counted.

[0068] Transition layer thickness characterization: The coating phase was observed using a scanning electron microscope in backscatter mode, and the Zn / Fe element content at the coating boundary was characterized using EDS line scanning to determine the transition layer thickness. The transition layer is located at the corrosion boundary between the coating and the substrate, extending from the corrosion boundary into the substrate depth. It is formed by the diffusion of zinc from the coating into the substrate, and its zinc content decreases along the depth direction from the corrosion boundary, not exceeding 15% (mass percentage). The corrosion boundary was obtained after the quenched component was etched with a nitric acid-alcohol solution.

[0069] Experimental results: Mechanical properties: The tensile strength of the parts is 1489MPa, which meets the requirement that the tensile strength of hot-formed steel of grade 1500MPa is greater than 1350MPa. The metallographic structure is mainly uniform martensite.

[0070] LME microcracks: The maximum depth of LME microcracks in the components is 3μm, and the number of microcracks is 3. The microcracks are controlled within the acceptable range.

[0071] Transition layer thickness: The thickness of the extremely thin transition layer of the component is 1.4 μm, not exceeding 4 μm, which meets the requirements of this invention. A microscopic schematic diagram of the transition layer characterization in Example 5 is shown below. Figure 5 As shown, the variation of Zn and Fe element content with distance clearly indicates the extent of the transition layer. The metallographic structure and maximum microcrack depth images of Example 5 are shown below. Figure 6 As shown, the microstructure is uniform, and the microcracks are small and few in number.

[0072] Comparative Example 1: Hot forming of welded plates with excessively long air residence time This comparative example is used to verify the impact of air residence time exceeding the process window of this invention on the dimensional accuracy of the welded board.

[0073] The experimental materials, equipment, conditions, and testing methods were the same as in Example 3, except that the air residence time t was changed. A The time was adjusted to 25 seconds, which exceeds the t calculated by this invention. A The value range is [7.60, 16.63]s.

[0074] Experimental results: The positional tolerance of the RPS positioning points of the gauge at thicknesses of 1.2mm and 1.4mm after forming is ±2.0mm, which does not meet the requirements for forming accuracy; the positional tolerance of the RPS positioning points of the gauge at thicknesses of 1.6mm and 1.8mm is ±0.8mm, which meets the requirements for forming accuracy. The gap measurement data of each detection point are shown in Table 2.

[0075] Table 2 Measurement data of component gap values ​​in Example 3 and Comparative Example 1

[0076] Comparative Example 2: Hot forming of welded plates with excessively low heat bath temperature This comparative example is used to verify the effect of the heat bath temperature being lower than the lower limit required by the present invention on the dimensional accuracy of the welded board.

[0077] The experimental materials, equipment, procedures and testing methods are the same as in Example 3, except that the heat bath temperature Tw is adjusted to 70°C, which is lower than the lower limit of 80°C required by this invention.

[0078] Experimental results: Due to the excessively rapid cooling rate of the hot bath at 70℃, the position tolerance of the RPS positioning point of the gauge at the thickness positions of 1.2mm and 1.4mm after forming is ±5.0mm, and the measurement forming accuracy does not meet the requirements; the position tolerance of the RPS positioning point of the gauge at the thickness positions of 1.6mm and 1.8mm is ±0.7mm, and the measurement forming accuracy does not meet the requirements.

[0079] Comparative Example 3: Co-mold thermoforming with excessively long air residence time This comparative example is used to verify the impact of air residence time exceeding the process window of this invention on the dimensional accuracy of common mold thermoforming.

[0080] The experimental materials, equipment, conditions, and testing methods are the same as in Example 4, except that the air residence time t is changed. A The time was adjusted to 25 seconds, which exceeds the t calculated by this invention. A The value range is [2.00, 19.70]s.

[0081] Experimental results: The position tolerance of the RPS positioning point of the fixture for the 2.2mm thick part after forming is ±0.5mm, and the forming accuracy meets the requirements; the position tolerance of the RPS positioning point of the fixture for the 1.6mm thick part is ±1.5mm, and the forming accuracy does not meet the requirements.

[0082] Comparative Example 4: Hot forming of U-shaped parts with excessively high forming temperature This comparative example is used to verify the effect of forming temperature exceeding the upper limit required by the present invention on LME microcracks and transition layer thickness of the parts.

[0083] The experimental materials, equipment, conditions, and methods were the same as in Example 5, except that the air residence time t was changed. A The time was adjusted to 3 seconds, so that the forming temperature was about 770℃, which is close to the decomposition temperature of the Γ phase.

[0084] Experimental results: Mechanical properties: The tensile strength of the component is 1513 MPa, which meets the requirements.

[0085] LME microcracks: The maximum depth of LME microcracks in the component was 38μm, and the number was 87, which is a serious exceedance of the standard.

[0086] Transition layer thickness: The transition layer thickness of the component is 5.6 μm, which is higher than the upper limit of 4 μm required by this invention. Comparative Example 4's metallographic structure and maximum microcrack depth images are shown below. Figure 7 As shown, numerous deep and dense microcracks are visible.

[0087] Comparative Example 5: Conventional mold cooling thermoforming without a heat bath This comparative example is used to compare the performance differences between the traditional heatless mold cooling process and the process of the present invention.

[0088] The experimental materials, equipment, procedures, and testing methods are the same as in Example 5, except that the hot bath process is omitted, and a traditional internal water channel cooling scheme is used. The sheet metal is cooled in the air to approximately 600°C before forming, i.e., t W =0s, air residence time t A =15s.

[0089] Experimental results: Mechanical properties: The tensile strength of the component is 1128MPa, which is lower than the requirement of 1350MPa, and the mechanical properties are unqualified.

[0090] LME microcracks: The maximum depth of LME microcracks in the component was 14μm, and the number was 52, which exceeded the standard.

[0091] Transition layer thickness: The transition layer thickness of the component is 4.9 μm, which is higher than the upper limit required by this invention. Comparative Example 5's metallographic structure and maximum microcrack depth images are shown below. Figure 8 As shown, non-martensitic phases are present in the microstructure, and there are a large number of microcracks.

[0092] Comparative Example 6: Cold Water Bath Thermoforming This comparative example is used to verify the impact of excessively low heat bath temperature, resulting in a forming temperature below the lower limit required by this invention, on the forming quality of the parts.

[0093] The experimental materials, equipment, procedures, and testing methods are the same as in Example 5, except that the heat bath temperature T is changed. w Adjust to 25℃ (cold water bath), hot bath time t W =1s, so that the forming temperature is below 300℃.

[0094] Experimental results: Mechanical properties: The tensile strength of the component is 1537MPa, which meets the requirements.

[0095] LME microcracks: The maximum depth of LME microcracks in the component was 141μm, and the number was 14, which is a serious exceedance of the standard.

[0096] Forming quality: The parts have completed martensitic transformation during forming, resulting in severe springback, and the U-shaped parts have cracked, failing to meet the usage requirements.

[0097] Transition layer thickness: The transition layer thickness of the component is 1.8 μm, which meets the requirements. Comparative Example 6's metallographic structure and maximum microcrack depth images are shown below. Figure 9 As shown, extremely deep microcracks and cracking defects are visible.

[0098] In summary, the LME microcrack conditions and transition layer thicknesses of the various embodiments and comparative examples of the present invention are shown in Table 3.

[0099] Table 3. Microcrack conditions and transition layer thickness of LME in each embodiment and comparative example.

[0100] As can be seen from the comparison of the above embodiments and comparative examples, by using the direct forming process described in this invention, and by quantitatively controlling the air cooling time t... A and hot bath time t W By controlling the initial forming temperature of the sheet metal to no higher than 700℃ and the sheet metal temperature to no lower than 300℃ when the mold is in place, with a hot bath time ≥0.5s, the mechanical properties and dimensional accuracy of hot-formed steel parts with coatings of different thicknesses can be guaranteed simultaneously. This effectively suppresses the generation of LME microcracks and keeps the transition layer thickness within 4μm. This invention is applicable to welded plates and common mold hot forming scenarios. The process window is quantitative, reproducible, and has good stability, facilitating industrial-scale application.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A direct hot forming method for improving the synchronous forming accuracy of hot-formed steel with large thickness difference coatings, characterized in that, Includes the following steps: S1. Heat the coated hot-formed steel sheet in a heating furnace to the complete austenitizing temperature; add a heat bath medium to the mold box and heat it to the target temperature; S2. The heated coated hot-formed steel sheet is transferred from the heating furnace to a support frame inside the hot-forming mold; the lower mold surface of the hot-forming mold is partially or completely immersed in the hot bath medium, and a support frame higher than the mold surface is provided in the lower mold cavity; the height of the support frame is above or below the highest liquid level of the hot bath medium; t is defined. Z The time interval during which the heated sheet metal is transferred from the heating furnace to the material support rack inside the thermoforming mold; S3. The mold descends, the hot-formed steel sheet is formed in the mold, pressure is maintained and quenched to obtain the parts; Define t T The time interval t is defined as the time between the heated sheet metal contacting the support frame and the upper mold surface. Y The time interval from when the heated sheet enters the hot bath medium to when the mold is in place; the total time interval t = t_t from when the heated sheet is transferred out of the heating furnace to when the mold is in place. Z +t T +t Y ; Define t A t is the time interval during which the heated sheet metal remains in the air. W The time interval between the heated sheet entering the hot bath medium and the mold forming the final shape; When the height of the material support frame is above the highest liquid level of the hot bath medium in the lower mold, t A =t Z +t T , t W =t Y ; When the height of the material support frame is below the highest liquid level of the hot bath medium in the lower mold, t A =t Z , t W =t T +t Y ; In steps S2-S3, the temperature T of the hot-formed steel sheet is... X Satisfying the formula: T X =T0 k A ×t A k W ×t W Where T0 is the furnace exit temperature of the sheet metal, kJ / m³ A k is the cooling rate of the heated sheet metal in air. W The cooling rate of the heated sheet material in the hot bath medium; The hot bath time t W ≥0.5s; the initial forming temperature T of the hot-formed steel sheet after heating. XS ≤700℃, sheet metal temperature T when the mold is in place XZ ≥300℃; S4. Obtain parts with forming accuracy that meet technical requirements.

2. The direct thermoforming method according to claim 1, characterized in that, In step S1, the heating temperature of the hot-formed steel sheet is 880~950℃, and the holding time is 3~10min to ensure that the sheet is completely austenitized.

3. The direct thermoforming method according to claim 1, characterized in that, The cooling rate k of the heated sheet material in the air medium A Based on the sheet thickness d, the formula is: k A =9.85+19.01×e 1.08(d 1.2) Where d takes values ​​ranging from 0.75mm to 2.25mm; when the air temperature is greater than 30℃, k AH =0.8×k A When the air medium temperature is < -10℃, k AL =1.2×k A .

4. The direct thermoforming method according to claim 1, characterized in that, The cooling rate k of the heated sheet material in the hot bath medium W Based on the sheet temperature T X Divided into two intervals: k W1 =1.8×d×T W 188.3×d 4.1×T W +438,T X ∈[200,420]℃; k W2 =5.7×d×T W 609×d 13.3×T W +1447,T X ∈[420,800]℃; Where d∈[0.75,2.25]mm, T W ∈[80,100]℃, T W The temperature of the heat bath medium; The heat bath medium includes water and its aqueous solution, and may be mixed with cooling agents, rust inhibitors, or lubricants; depending on the heat bath medium, α is used. H For k W Make corrections, i.e., k WS =α H ×k W The value of αH is in the range of 0.1 ≤ α H ≤2.

5. The direct thermoforming method according to claim 1, characterized in that, The coating of the hot-formed steel is a zinc-based coating, including pure zinc coating, zinc-iron alloy coating, and zinc-aluminum-magnesium coating; the coating thickness on one side is 30~100g / m².

6. A coated hot-formed steel part with excellent dimensional accuracy, characterized in that, The component is prepared by the direct forming process according to any one of claims 1 to 6; the component has no obvious warping, springback, or localized diameter reduction deformation.

7. The coated hot-formed steel part according to claim 6, characterized in that, The component comprises a surface coating, an ultra-thin transition layer, and a steel substrate; the ultra-thin transition layer is located at the corrosion boundary between the coating and the substrate, extending from the corrosion boundary towards the depth of the substrate, and is formed by the diffusion of zinc element from the coating into the substrate; the thickness of the ultra-thin transition layer is ≤4μm, and its zinc element content decreases along the depth direction from the corrosion boundary and is not higher than 15wt%; the corrosion boundary is obtained by etching the quenched component with nitric acid alcohol solution.

8. The coated hot-formed steel parts according to claim 6, characterized in that, The chemical element composition of the coated hot-formed steel sheet is as follows: C 0.05wt%~0.35wt%, Si≤1.2wt%, Mn 0.5wt%~2.2wt%, Cr≤1.5wt%, Mo≤0.5wt%, Ni≤0.5wt%, Ti≤0.04wt%, Nb≤0.2wt%, V≤0.2wt%, B 0.002wt%~0.006wt%, P≤0.020wt%, S≤0.003wt%, Al≤0.8wt%, N≤0.006wt%, with the balance being Fe and unavoidable impurities.

9. The coated hot-formed steel parts according to claim 6, characterized in that, The maximum depth of LME microcracks in the components is ≤10μm, and the room temperature tensile strength is 500~2200MPa.

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