Preparation method of hot air expansion part of hot forming steel with coating

Abstract

The invention provides a preparation method of a hot gas expansion part of hot forming steel with a coating, and belongs to the field of hot gas expansion forming manufacturing. The method comprises the following steps: sequentially carrying out staged heating consisting of a preheating section, a transition section and an austenitizing section on a hot-formed steel pipe blank with a coating; the pipe blank heated in stages is transferred to a hot air expansion mold, and ventilation expansion composed of low-pressure air expansion and high-pressure air expansion is sequentially carried out, so that the peripheral surface of the pipe blank is attached to a hot air expansion mold cavity, and a formed part is obtained; and cooling water is introduced into the hot air expansion mold for quenching, so that the cooling rate of the formed part is higher than the phase change critical rate, and the hot air expansion part is obtained. By optimizing the technological process, volatilization, oxidation or excessive alloying of the plating layer caused by high temperature is avoided, and breakage and falling of the plating layer caused by forming stress concentration are prevented, so that the integrity, uniformity and binding force of the plating layer after hot gas bulging forming and subsequent treatment are improved, and it is ensured that the plating layer meets the quality requirement.

Description

Technical Field

[0001] This application relates to the field of hot gas expansion forming manufacturing technology, and in particular to a method for preparing hot gas expansion parts with coated hot-formed steel. Background Technology

[0002] The application of hot-formed steel tubular parts in the automotive manufacturing industry provides crucial support for improving vehicle performance and lightweighting. These parts, formed through a hot-expansion process, not only possess excellent high-strength properties, significantly enhancing vehicle collision resistance, but also achieve weight reduction by simplifying the structure, thus achieving an efficient balance between safety and economy. Traditionally, hot-formed steel is mostly used in bare sheet form, but its production process has significant drawbacks: oxide scale easily forms on the surface during high-temperature heating. This oxide scale not only accelerates mold wear, shortens mold life, and affects production accuracy, but also requires additional shot blasting for removal. This step not only prolongs the production cycle but also significantly increases production costs, hindering the improvement of production efficiency and economic benefits.

[0003] To address the oxide scale problem, aluminum-silicon coated hot-formed steel has emerged and gained widespread application. This material, through a hot-dip galvanizing process, coats the surface of a bare steel plate with an aluminum-silicon coating (approximately 90% aluminum and 10% silicon). Upon high-temperature heating, it forms a dense oxide film, effectively inhibiting the oxidation reaction of the base steel and significantly reducing oxide scale formation. However, aluminum-silicon coatings still face coating quality issues during thermal expansion: the degree of alloying between the coating and the substrate is difficult to control at high temperatures, easily leading to localized over-alloying causing coating embrittlement, or insufficient alloying resulting in reduced coating adhesion, affecting the coating's protective effect on the substrate. Furthermore, its corrosion resistance still falls short of the requirements for highly corrosive environments. To further improve the corrosion resistance of hot-formed steel, zinc-based coated hot-formed steel has been developed and put into application. Through a hot-dip galvanizing process, a zinc-based coating is coated on both sides of the steel plate. With its dual protective mechanism (physical barrier and cathodic protection), it exhibits superior corrosion resistance while maintaining the high formability and high strength of hot-formed steel, making it particularly suitable for corrosive environments such as humidity and salt spray. However, the quality stability of zinc-based coatings faces even more severe challenges in the hot gas expansion process: due to the low melting and boiling points of zinc, it is prone to severe volatilization during high-temperature heating, leading to uneven coating thickness, localized losses, or even exposure of the substrate, resulting in loss of protective function; simultaneously, liquid zinc or zinc-iron alloys at high temperatures can easily penetrate the substrate, causing brittle fracture (LME effect), accompanied by damage to the coating-substrate interface, leading to coating peeling; furthermore, stress concentration during hot gas expansion can also cause coating cracking and peeling, further damaging the integrity of the coating. These problems all make it difficult for zinc-based coatings to meet quality requirements after hot gas expansion, becoming a key bottleneck restricting their application. Therefore, both aluminum-silicon coatings and zinc-based coatings on hot-formed steel suffer from quality problems such as coating volatilization, uncontrolled alloying, decreased adhesion, and cracking and peeling during the hot gas expansion process. How to ensure that the coating maintains its integrity, uniformity, and effective protective performance after high-temperature forming, and meets the quality requirements of the coating, has become a core technical problem that urgently needs to be solved in the hot gas expansion process of coated hot-formed steel. Summary of the Invention

[0004] This application provides a method for preparing hot-expanded parts made of coated hot-formed steel to solve the following technical problem: how to improve the coating quality pass rate of coated hot-formed steel during the hot-expanding process and ensure that the coating meets the requirements.

[0005] This application provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0006] The coated hot-formed steel tube blank is subjected to staged heating consisting of a preheating section, a transition section, and an austenitizing section.

[0007] The tube blank, after being heated in stages, is transferred to a hot air expansion mold and then subjected to air expansion consisting of low-pressure air expansion and high-pressure air expansion in sequence, so that the outer peripheral surface of the tube blank fits into the cavity of the hot air expansion mold to obtain a shaped part.

[0008] Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than the critical phase transformation rate, thereby obtaining the hot gas expansion part.

[0009] Optionally, when the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤2min;

[0010] The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤2min;

[0011] The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤3min;

[0012] The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤2min;

[0013] The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤3min.

[0014] Optionally, when the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the total heat preservation time of the preheating section, the transition section and the austenitizing section is ≤10min.

[0015] Optionally, when the outer diameter of the coated hot-formed steel pipe blank is >30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤1.5min;

[0016] The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤1.5min;

[0017] The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤4min;

[0018] The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤4min;

[0019] The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤4min.

[0020] Optionally, when the outer diameter of the coated hot-formed steel pipe blank is >30mm, the total heat preservation time of the preheating section, the transition section and the austenitizing section is ≤11min.

[0021] Optionally, the pressure of the low-pressure air expansion is ≤30MPa, and the pressure of the high-pressure air expansion is >30MPa.

[0022] Optionally, the initial temperature of the ventilation expansion is ≤782℃.

[0023] Optionally, the initial temperature of the ventilated expansion is ≤740℃.

[0024] Optionally, the fillet radius of the hot-expanded part is ≥1mm, the maximum thinning rate of the hot-expanded part is ≤15%, and the microstructure of the hot-expanded part is martensite.

[0025] Optionally, the coated hot-formed steel is zinc-based coated hot-formed steel or aluminum-silicon coated hot-formed steel.

[0026] The technical solutions provided in this application have the following advantages compared with the prior art:

[0027] This application provides a method for preparing a hot-expanded part of coated hot-formed steel. The method includes: sequentially heating a coated hot-formed steel tube blank in stages, consisting of a preheating section, a transition section, and an austenitizing section; transferring the heated tube blank to a hot-expanding mold and sequentially performing air-expanding forming consisting of low-pressure air expansion and high-pressure air expansion, so that the outer peripheral surface of the tube blank fits into the cavity of the hot-expanding mold to obtain a formed part; and quenching the hot-expanding mold by circulating cooling water, so that the cooling rate of the formed part is higher than the critical phase transformation rate, thereby obtaining the hot-expanded part.

[0028] First, the stability of the coating is controlled by staged heating. The staged heating method of preheating section, transition section and austenitizing section is adopted. By gradually increasing the temperature and accurately controlling the temperature, the excessive volatilization or oxidation of the coating caused by direct impact of high temperature is avoided. At the same time, it promotes uniform alloying between the coating and the substrate, reduces defects at the interface between the coating and the substrate, and ensures the integrity of the coating structure.

[0029] Secondly, step-by-step air expansion reduces coating damage. First, low-pressure air expansion allows the tube blank to initially fit the mold, dispersing the forming stress and avoiding direct high pressure that could cause the coating to crack or peel off due to excessive local stress. Then, high-pressure air expansion achieves precise forming, ensuring that the coating deforms synchronously with the tube blank, reducing relative slippage damage between the coating and the substrate, and maintaining coating continuity.

[0030] The coating state is fixed again by rapid quenching. Cooling water is quenched into the hot gas expansion mold to make the formed part cool rapidly and the cooling rate is higher than the critical phase transformation rate. This not only ensures the transformation of the matrix structure, but also fixes the alloying state of the coating and the matrix in time, avoiding coating embrittlement or interface separation that may occur in the medium temperature zone, and ensuring the stability of coating performance.

[0031] Therefore, by optimizing the process flow during the hot expansion of coated hot-formed steel, the volatilization, oxidation, or over-alloying of the coating caused by high temperature can be avoided, as can the coating cracking and peeling caused by stress concentration during forming. Furthermore, the damage to the coating-substrate interface caused by liquid zinc (for zinc-based coatings) penetrating into the substrate can be suppressed. This improves the integrity, uniformity, and adhesion of the coating after hot expansion forming and subsequent processing, ensuring that the coating meets quality requirements. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0033] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic flowchart illustrating a method for preparing a hot-expanded part made of coated hot-formed steel, as provided in an embodiment of this application. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "comprise" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0037] Figure 1 This is a schematic flowchart illustrating a method for preparing a hot-expanded part made of coated hot-formed steel, as provided in an embodiment of this application.

[0038] like Figure 1 As shown in the figure, this application provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0039] S1. The coated hot-formed steel pipe billet is sequentially heated in stages, consisting of a preheating section, a transition section, and an austenitizing section.

[0040] In some embodiments, the coated hot-formed steel is zinc-based coated hot-formed steel or aluminum-silicon coated hot-formed steel.

[0041] The method for manufacturing hot-expanded parts provided in this application is applicable to zinc-based coated hot-formed steel, aluminum-silicon coated hot-formed steel, and other coated hot-formed steels. Since zinc-based coated hot-formed steel faces numerous problems during hot expansion, this method is most effective for it.

[0042] Zinc-based coated hot-formed steel mainly consists of a substrate and a coating. The substrate is made of hot-formed steel, and its coating system is flexible and adjustable. The coating can include both a nickel plating layer and a zinc plating layer, or it can consist of only a zinc plating layer. Zinc is the most abundant element in the coating, which usually also contains a certain amount of elements such as aluminum, as well as unavoidable impurities such as iron and silicon. In the preparation of zinc-based coated hot-formed steel, the basic coating structure is a single coating, that is, only a zinc plating layer, in which the zinc content is ≥90%, usually containing a small amount of aluminum (1-5%) and the aforementioned unavoidable impurities, which can achieve basic corrosion resistance protection. Composite coatings consist of a combination of nickel and zinc plating layers, and can be designed into different structures according to functional requirements: In the nickel-zinc structure, the nickel plating layer is located between the zinc layer and the substrate, which can enhance the bonding force between the coating and the substrate. This is because nickel has a low atomic diffusion rate, which can inhibit the excessive growth of zinc-iron alloys; In the zinc-nickel structure, the nickel plating layer covers the surface of the zinc layer, which can improve corrosion resistance, as the nickel plating layer can prevent corrosive media from directly contacting the zinc layer; In the double nickel-zinc structure, nickel is plated between the substrate and the zinc layer, as well as on the surface of the zinc layer. It is suitable for highly corrosive environments, but the cost is relatively high.

[0043] When adding a nickel plating layer, the plating layer can be placed between the zinc layer and the substrate, or outside the zinc layer. In other words, nickel plating can be performed between the substrate and the plating layer, on the plating surface, or both between the substrate and the plating layer and on the plating surface. However, the choice of nickel plating process requires a comprehensive consideration of advantages and costs: the nickel layer can act as a diffusion barrier, reducing zinc penetration into the substrate during high-temperature forming, lowering the risk of liquid zinc embrittlement (LME), and improving the surface hardness and scratch resistance of the plating layer. However, nickel plating increases material costs. Therefore, to control costs, nickel plating can be omitted. The specific application can be selective based on the service environment of the part; for example, composite plating may be preferred in coastal areas or high-humidity conditions.

[0044] When preparing hot-formed steel with aluminum-silicon coating or other coatings, nickel plating is usually not required; it can be prepared directly through coating methods such as electroplating or hot-dip plating.

[0045] In some embodiments, when the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤2min;

[0046] The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤2min;

[0047] The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤3min;

[0048] The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤2min;

[0049] The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤3min.

[0050] In some embodiments, when the outer diameter of the coated hot-formed steel tube blank is ≤30mm, the total holding time of the preheating section, the transition section and the austenitizing section is ≤10min.

[0051] In some embodiments, when the outer diameter of the coated hot-formed steel pipe blank is >30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤1.5min;

[0052] The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤1.5min;

[0053] The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤4min;

[0054] The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤4min;

[0055] The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤4min.

[0056] In some embodiments, when the outer diameter of the coated hot-formed steel tube blank is >30mm, the total holding time of the preheating section, the transition section and the austenitizing section is ≤11min.

[0057] This application significantly improves the alloying degree of zinc-based coated hot-formed steel parts by employing staged heating and precise control of heating parameters and process paths at each stage. In this process, a low-temperature holding stage is first used to promote the pre-alloying of the zinc-iron alloy, while simultaneously reducing severe zinc volatilization. The subsequent high-temperature stage ensures sufficient austenitization of the matrix. Because the required degree of alloying varies depending on the outer diameter of the billet, the specific parameters for each stage will differ slightly. The specific functions of each stage in the staged heating process are as follows:

[0058] Preheating section: When the outer diameter of the pipe is ≤30mm, the heating temperature is 390℃-420℃, and the holding time is ≤2min; when the outer diameter of the pipe is >30mm, the heating temperature is 390℃-420℃, and the holding time is ≤1.5min. Its function is to preheat the pipe blank at a lower temperature, reduce thermal stress, and promote pre-alloying between the coating and the substrate to form a thin alloy layer, which enhances the bonding force between the coating and the substrate. It also prevents excessive volatilization of the coating (such as zinc-based coatings) due to excessively high temperatures.

[0059] Transition stage: When the outer diameter of the tube is ≤30mm, the heating temperature is 580℃-680℃, and the holding time is ≤2min; when the outer diameter of the tube is >30mm, the heating temperature is 580℃-680℃, and the holding time is ≤1.5min. During this stage, the temperature continues to rise, further promoting the alloying of the coating and the substrate, gradually thickening the alloy layer, providing better protection for subsequent high-temperature heating, and simultaneously making the tube blank temperature more uniform, reducing the temperature gradient, and preparing for austenitization.

[0060] Austenitizing stage: This stage is divided into three heating phases. In the first phase, when the outer diameter of the tube is ≤30mm, the heating temperature is 680℃-780℃, and the holding time is ≤3min; when the outer diameter is >30mm, the holding time is ≤4min. The purpose is to initiate austenitization of the substrate while the coating continues its alloying reaction. In the second phase, when the outer diameter is ≤30mm, the heating temperature is 800℃-860℃, and the holding time is ≤2min; when the outer diameter is >30mm, the holding time is ≤4min. The aim is to accelerate the austenitization process of the substrate, enabling the material to achieve good plasticity, creating conditions for subsequent gas expansion forming, and simultaneously controlling the temperature to prevent excessive volatilization of the coating. In the third stage, when the outer diameter of the tube is ≤30mm, the heating temperature is 880℃-910℃ and the holding time is ≤3min; when the outer diameter of the tube is >30mm, the holding time is ≤4min. This ensures that the matrix is ​​fully austenitized, and that the required martensitic structure can be obtained after subsequent quenching, thereby improving the strength of the parts. In addition, the shorter holding time can reduce the loss of the coating at high temperatures.

[0061] For example, when the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the heating temperature of the preheating section can be 390℃, 400℃, 410℃, 415℃, 420℃, etc., and the holding time can be 1min, 1.2min, 1.5min, 1.8min, 2min, etc.; the heating temperature of the transition section can be 580℃, 600℃, 630℃, 650℃, 680℃, etc., and the holding time can be 1min, 1.3min, 1.6min, 1.9min, 2min, etc.; the heating temperature of the first stage of the austenitizing stage can be 680℃, 700℃, 730℃, 760℃, 780℃, etc., and the holding time of the first stage can be 1min, 1.5min, 1.6min, 1.8min, 2min, etc. The heating temperatures for the second stage of the austenitizing section can be 800℃, 820℃, 840℃, 850℃, 860℃, etc., and the holding times for the second stage can be 0.5min, 1min, 1.5min, 1.8min, 2min, etc. The heating temperatures for the third stage of the austenitizing section can be 880℃, 890℃, 900℃, 905℃, 910℃, etc., and the holding times for the third stage can be 1min, 1.5min, 2min, 2.5min, 3min, etc. The total holding time for the preheating stage, transition stage, and austenitizing stage can be 7min, 8min, 8.5min, 9min, 10min, etc.

[0062] Total heat preservation time: When the outer diameter of the pipe is ≤30mm, the total heat preservation time is ≤10min; when the outer diameter of the pipe is >30mm, the total heat preservation time is ≤11min. Controlling the total heat preservation time is to ensure that the billet is fully heated, the coating is alloyed and the matrix is ​​austenitized, while avoiding excessive volatilization of the coating or uneven alloying due to prolonged high temperature, as well as reducing energy consumption and improving production efficiency.

[0063] For example, when the outer diameter of the coated hot-formed steel pipe blank is >30mm, the heating temperature of the preheating section can be 390℃, 395℃, 405℃, 415℃, 420℃, etc., and the holding time can be 0.5min, 1min, 1.2min, 1.4min, 1.5min, etc.; the heating temperature of the transition section can be 580℃, 590℃, 620℃, 660℃, 680℃, etc., and the holding time can be 0.5min, 1min, 1.2min, 1.4min, 1.5min, etc.; the heating temperature of the first stage of the austenitizing section can be 680℃, 710℃, 740℃, 770℃, 780℃, etc., and the holding time of the first stage can be 1min. The heating temperatures for the second stage of the austenitizing section can be 800℃, 810℃, 830℃, 850℃, 860℃, etc., and the holding times for the second stage can be 1min, 2min, 3min, 3.5min, 4min, etc.; the heating temperatures for the third stage of the austenitizing section can be 880℃, 895℃, 900℃, 905℃, 910℃, etc., and the holding times for the third stage can be 1min, 2min, 3min, 3.5min, 4min, etc.; the total holding times for the preheating stage, transition stage, and austenitizing stage can be 8min, 9min, 10min, 10.5min, 11min, etc.

[0064] S2. The tube blank, after being heated in stages, is transferred to a hot air expansion mold, and then subjected to air expansion consisting of low-pressure air expansion and high-pressure air expansion in sequence, so that the outer peripheral surface of the tube blank fits into the cavity of the hot air expansion mold to obtain a shaped part.

[0065] It should be noted that during the tube manufacturing process, it is essential to ensure that the weld seam is free of cracks after tube fabrication, and the selected coolant must not damage the coating. Welding can be performed using high-frequency welding or laser welding. The sprayed coolant must be non-corrosive, highly lubricating, and resistant to high temperatures.

[0066] In some embodiments, the pressure of the low-pressure gas expansion is ≤30MPa, and the pressure of the high-pressure gas expansion is >30MPa.

[0067] The pressure of low-pressure air expansion is limited to ≤30MPa. Its function is to initially deform the tube blank, allowing it to initially conform to the mold cavity and make adaptive adjustments to the tube blank. If high pressure is applied directly, the material may crack or deform unevenly due to instantaneous stress concentration. Low-pressure air expansion can effectively avoid these problems and lay the foundation for subsequent high-pressure air expansion.

[0068] The pressure of the high-pressure gas expansion is limited to >30MPa. Based on the low-pressure gas expansion, the tube blank is further deformed by high-pressure gas, making it completely fit the mold cavity, achieving precise forming of the part and ensuring the shape accuracy and dimensional tolerance of the part. The higher pressure can accelerate the expansion speed, improve production efficiency, and at the same time allow the material to better fill the details of the mold cavity.

[0069] For example, the pressure of low-pressure air expansion can be 10MPa, 15MPa, 20MPa, 25MPa, 30MPa, etc.; the pressure of high-pressure air expansion can be 35MPa, 40MPa, 45MPa, 50MPa, 55MPa, etc.

[0070] In some embodiments, the initial temperature of the ventilated expansion is ≤782°C.

[0071] In some embodiments, the initial temperature of the ventilated expansion is ≤740°C.

[0072] The initial temperature for ventilated expansion is limited to ≤782℃, with ≤740℃ being the optimal choice. This is because 782℃ is the melting point of zinc-iron alloy. Temperatures below this level can prevent the zinc-iron alloy in the coating from melting and forming liquid zinc, thus preventing liquid zinc from penetrating into the substrate grain boundaries and causing the substrate to become brittle. This avoids the occurrence of liquid metal embrittlement (LME) and ensures the forming quality and performance of the parts.

[0073] For example, the initial ventilation inflation temperature can be 700℃, 720℃, 740℃, 760℃, 782℃, etc.; more preferably, the initial ventilation inflation temperature can be 680℃, 700℃, 720℃, 730℃, 740℃, etc.

[0074] S3. Cooling water is passed through the hot gas expansion mold for quenching, so that the cooling rate of the formed part is higher than the critical phase transformation rate, and a hot gas expansion part is obtained.

[0075] After bulging, the tube is formed into a specific part shape. Simultaneously, a hot air expansion mold is attached to the outer wall of the tube. Cooling water flows through the hot air expansion mold, allowing heat exchange between the mold and the tube part. The cooling water carries away heat, ensuring a high cooling rate for the tube part. This results in a cooling rate for the hot-expanded part exceeding the critical phase transformation rate of the material, causing the part's microstructure to transform into martensite.

[0076] Therefore, in this embodiment, cooling water is circulated into the hot gas expansion mold to make the cooling rate of the formed part higher than the critical phase transformation rate, so that the part's microstructure is transformed into martensite. Martensite has high strength and high hardness, which can significantly improve the mechanical properties of the part and meet the high strength requirements of the part in actual use. At the same time, rapid cooling also helps to fix the alloying state of the coating and the substrate, reducing possible subsequent microstructure changes and coating problems.

[0077] In some embodiments, the fillet radius of the hot-expanded part is ≥1mm, the maximum thinning rate of the hot-expanded part is ≤15%, and the microstructure of the hot-expanded part is martensite.

[0078] It should be noted that during the part design process, overly sharp rounded corners should be avoided to prevent localized strain concentration that could lead to forming failure. This application's embodiments utilize simulation technology to optimize the part's shape features. By simulating material flow and stress distribution under different parameters such as rounded corner radii and cross-sectional change rates, the optimal geometric configuration is accurately determined, ensuring uniform material deformation during hot air expansion forming and improving the part's forming success rate and dimensional accuracy. The design of hot air expansion parts requires ensuring a rounded corner radius ≥ 1mm and a maximum thinning rate ≤ 15% during hot air expansion. For example, the rounded corner radius of hot air expansion parts can be 1mm, 1.5mm, 2mm, 2.5mm, 3mm, etc.; the maximum thinning rate of hot air expansion parts can be 5%, 8%, 10%, 12%, 15%, etc.

[0079] After hot gas expansion forming and quenching, the shape accuracy and microstructure of the parts need to be tested. If the test is passed, the part can be mass-produced and applied. If the part fails the test, the process parameters need to be readjusted and optimized, and then production and trial production continue until it passes the test.

[0080] Therefore, this application achieves dynamic optimization of the heating process through refined temperature control curve design; innovatively improves the pressurization sequence to ensure uniform stress distribution during forming; and optimizes the mold structure design to enhance the constraint and guidance of material deformation. The synergistic effect of these multi-dimensional technological innovations ensures high-quality forming of zinc-based coated hot-formed steel parts while effectively suppressing zinc volatilization at high temperatures. This fundamentally overcomes the technical barriers of traditional processes where liquid zinc causes substrate brittleness and coating loss, while also increasing the alloying degree and bonding between the coating and the substrate.

[0081] In summary, the embodiments of this application, during the hot-formed steel with coating, optimize process parameters and procedures to avoid coating volatilization, oxidation, or over-alloying caused by high temperatures, prevent coating cracking and peeling due to stress concentration during forming, and suppress damage to the coating-substrate interface caused by liquid zinc (for zinc-based coatings) penetrating into the substrate. This improves the integrity, uniformity, and adhesion of the coating after hot-formed steel with coating and subsequent processing, ensuring that the coating meets quality requirements. Specific measures are as follows:

[0082] (1) Precise control of staged heating to suppress zinc volatilization and over-alloying: Stepped heating and time-limited holding divide the heating process into a preheating stage (390℃~420℃), a transition stage (580℃~680℃), and an austenitizing stage (heating to 880℃~910℃ in three stages), and strictly control the holding time of each stage (total time ≤10~11min). The low-temperature stage (preheating and transition stages) can promote the pre-alloying of zinc-iron alloy, reduce the free zinc content, and avoid the violent volatilization of zinc caused by direct heating at high temperatures; the high-temperature stage (austenitizing stage) reduces the loss of coating at high temperatures by shortening the holding time while ensuring sufficient austenitization of the substrate. At the same time, differentiated parameter adaptation is used to design different heating parameters for different tube blank outer diameters (≤30mm and >30mm) to ensure that tube blanks of different thicknesses can achieve uniform alloying of the coating and sufficient phase transformation of the substrate, avoiding local zinc volatilization or insufficient alloying caused by uneven heating.

[0083] (2) Strictly control the initial expansion temperature to avoid embrittlement of liquid zinc: Limit the expansion temperature to ≤782℃ (optimal ≤740℃). 782℃ is the melting point of zinc-iron alloy. Below this temperature, the zinc-iron alloy in the coating can be prevented from melting and forming liquid zinc, fundamentally avoiding embrittlement (LME effect) caused by liquid zinc penetrating into the substrate grain boundaries. By controlling the transfer time of the tube blank from heating to expansion, ensure that the initial expansion temperature is within a safe range, providing temperature protection for the stable bonding of the coating and the substrate.

[0084] (3) Step-by-step air expansion forming to reduce stress damage between the coating and the substrate: Through a combination of low-pressure air expansion (≤30MPa) and high-pressure air expansion (>30MPa), the low-pressure stage allows the billet to initially adhere to the mold, dispersing stress and avoiding excessive local deformation or coating cracking caused by direct high pressure; the high-pressure stage achieves precise forming, ensuring that the part fits the mold while reducing tearing or wear on the coating during material flow. Step-by-step pressurization allows the coating and the substrate to deform synchronously, reducing interface stress and coating loss.

[0085] (4) Optimize the coating system design to enhance coating stability: A flexible and adjustable zinc-based coating structure can be used to achieve basic corrosion resistance with a single zinc layer (zinc content ≥90%), or to improve performance with composite coatings (nickel-zinc, zinc-nickel, double nickel-zinc structures). The nickel layer acts as a diffusion barrier, reducing zinc penetration into the substrate at high temperatures, lowering the risk of LME (Liquid Metal Electrode Surface) and enhancing coating hardness and scratch resistance, while reducing coating loss during the forming process. Furthermore, the nickel plating process is selectively chosen based on the service environment (e.g., high-corrosion conditions), controlling costs while further reducing zinc volatilization and substrate brittleness risks through the protective effect of the nickel layer.

[0086] (5) Rapid quenching to fix the structure and ensure the stability of the coating and the substrate: After gas expansion forming, the heat is quickly removed by the cooling water in the mold, so that the cooling rate of the part is higher than the critical rate of phase transformation. This ensures that the substrate is transformed into a high-strength martensite structure while rapidly fixing the alloying state of the coating and the substrate, avoiding the formation of brittle phases at the interface between the coating and the substrate that may occur in the medium temperature zone, and further reducing coating loss and interface damage.

[0087] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0088] Example 1

[0089] This embodiment provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0090] S11. Select a zinc-based coated hot-formed steel pipe billet with an outer diameter of 25mm and heat it in stages, sequentially passing through a preheating section, a transition section, an initial austenitization section, a complete austenitization section, and a plasticity optimization section. The preheating section has a heating temperature of 400℃ and a holding time of 1.5min; the transition section has a heating temperature of 630℃ and a holding time of 1.5min; the austenitization section has a heating temperature of 730℃ and a holding time of 2min; the austenitization section has a heating temperature of 810℃ and a holding time of 1.5min; the austenitization section has a heating temperature of 890℃ and a holding time of 2min; the total holding time for all sections is 8.5min.

[0091] S21. Transfer the tube blank after staged heating to the hot air expansion mold for air expansion; first perform low-pressure air expansion with a pressure of 25MPa, then perform high-pressure air expansion with a pressure of 35MPa; the initial temperature of air expansion is 730℃, so that the outer circumferential surface of the tube blank fits into the cavity of the hot air expansion mold to obtain the formed part.

[0092] S31. Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than its phase transformation critical rate, thus obtaining the hot gas expansion part; the maximum thinning rate of the hot gas expansion part is 9%, and the microstructure is martensite. The coating of the part is qualified (the zinc content in the coating meets the requirements, and the maximum length of LME crack is <10um).

[0093] Example 2

[0094] This embodiment provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0095] S11. Select a zinc-based coated hot-formed steel pipe billet with an outer diameter of 20mm and heat it in stages, sequentially passing through a preheating section, a transition section, an initial austenitization section, a complete austenitization section, and a plasticity optimization section. The preheating section has a heating temperature of 395℃ and a holding time of 2 minutes; the transition section has a heating temperature of 600℃ and a holding time of 2 minutes; the austenitization section has a heating temperature of 690℃ and a holding time of 2 minutes; the austenitization section has a heating temperature of 800℃ and a holding time of 1.5 minutes; the austenitization section has a heating temperature of 890℃ and a holding time of 2 minutes; the total holding time for all sections is 9.5 minutes.

[0096] S21. Transfer the tube blank after staged heating to the hot air expansion mold for air expansion; first perform low-pressure air expansion with a pressure of 25MPa, then perform high-pressure air expansion with a pressure of 36MPa; the initial temperature of air expansion is 750℃, so that the outer circumferential surface of the tube blank fits into the cavity of the hot air expansion mold to obtain the formed part.

[0097] S31. Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than its phase transformation critical rate, thus obtaining the hot gas expansion part; the maximum thinning rate of the hot gas expansion part is 10%, and the microstructure is martensite. The coating of the part is qualified (the zinc content in the coating meets the requirements, and the maximum length of LME crack is <10um).

[0098] Example 3

[0099] This embodiment provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0100] S11. Select a zinc-based coated hot-formed steel pipe billet with an outer diameter of 31 mm and heat it in stages, sequentially passing through a preheating section, a transition section, an initial austenitization section, a complete austenitization section, and a plasticity optimization section. The preheating section has a heating temperature of 410℃ and a holding time of 1.5 min; the transition section has a heating temperature of 650℃ and a holding time of 1.5 min; the austenitization section has a heating temperature of 780℃ and a holding time of 2 min; the austenitization section has a heating temperature of 800℃ and a holding time of 2 min; the austenitization section has a heating temperature of 900℃ and a holding time of 1 min; the total holding time for all sections is 8 min.

[0101] S21. Transfer the tube blank after staged heating to the hot air expansion mold for air expansion; first perform low-pressure air expansion with a pressure of 25MPa, then perform high-pressure air expansion with a pressure of 35MPa; the initial temperature of air expansion is 720℃, so that the outer circumferential surface of the tube blank fits into the cavity of the hot air expansion mold to obtain the formed part.

[0102] S31. Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than its phase transformation critical rate, thus obtaining the hot gas expansion part; the maximum thinning rate of the hot gas expansion part is 10%, and the microstructure is martensite. The coating of the part is qualified (the zinc content in the coating meets the requirements, and the maximum length of LME crack is <10um).

[0103] Comparative Example 1

[0104] This embodiment provides a method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising:

[0105] S11. Select a zinc-based coated hot-formed steel pipe blank with an outer diameter of 30mm, and heat it in stages at a temperature of 900℃ for 8 minutes.

[0106] S21. Transfer the tube blank after staged heating to the hot air expansion mold for air expansion; first perform low-pressure air expansion with a pressure of 25MPa, then perform high-pressure air expansion with a pressure of 35MPa; the initial temperature of air expansion is 720℃, so that the outer circumferential surface of the tube blank fits into the cavity of the hot air expansion mold to obtain the formed part.

[0107] S31. Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than its phase transformation critical rate, thus obtaining a hot gas expansion part; the maximum thinning rate of the hot gas expansion part is 12%, and the microstructure is martensite. The coating of the part is unqualified (the zinc content in the coating is lower than the threshold, and the maximum length of the LME crack is >10um).

[0108] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0109] (1) In this embodiment of the application, by optimizing the temperature control curve, improving the pressurization sequence and mold structure design, the high-temperature volatilization of zinc is effectively suppressed while ensuring the successful forming of zinc-based coated hot-formed steel parts, thus fundamentally solving the technical bottleneck of LME failure and coating loss in traditional processes.

[0110] (2) In the embodiments of this application, the provided method can be used for zinc-based coated hot-formed steel, aluminum-silicon coated hot-formed steel, and other coated hot-formed steel. Because zinc-based coated hot-formed steel experiences more problems during thermal expansion, this method is the most effective.

[0111] (3) In this embodiment, by strictly controlling the initial temperature of gas expansion (≤782℃, optimal ≤740℃), which is lower than the melting point of zinc-iron alloy, the coating is prevented from melting and forming liquid zinc, thus eliminating the brittle fracture (LME effect) caused by liquid zinc penetrating into the substrate grain boundaries and ensuring the safety of part forming. At the same time, by using a staged heating strategy (preheating stage → transition stage → multi-stage austenitization), zinc-iron pre-alloying is promoted at low temperature, and zinc volatilization is reduced at high temperature with time-limited holding. Combined with the control of the total holding time, the loss of coating during the high-temperature process is significantly reduced, and the bonding stability between the coating and the substrate is improved.

[0112] (4) In this embodiment, a step-by-step pressurization process of "low-pressure air expansion (≤30MPa) + high-pressure air expansion (>30MPa)" is adopted. First, the stress is dispersed to achieve preliminary mold application, and then precise forming is performed. This avoids cracks or uneven deformation caused by direct high pressure, ensuring that the outer periphery of the part is tightly fitted to the mold cavity, the fillet radius is ≥1mm, the maximum thinning rate is ≤15%, and the shape accuracy is high. At the same time, supercritical cooling (cooling rate is higher than the phase transformation critical rate) is achieved by water quenching through the mold, so that the part structure is completely transformed into martensite, which has high strength and high hardness, meeting the core mechanical property requirements of hot-formed steel; at the same time, rapid cooling fixes the alloying state of the coating and the substrate, reducing subsequent changes in the structure.

[0113] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a hot-expanded part made of coated hot-formed steel, the method comprising: The coated hot-formed steel tube blank is subjected to staged heating consisting of a preheating section, a transition section, and an austenitizing section. The tube blank, after being heated in stages, is transferred to a hot air expansion mold and then subjected to air expansion consisting of low-pressure air expansion and high-pressure air expansion in sequence, so that the outer peripheral surface of the tube blank fits into the cavity of the hot air expansion mold to obtain a shaped part. Cooling water is quenched into the hot gas expansion mold to make the cooling rate of the formed part higher than the critical phase transformation rate, thereby obtaining the hot gas expansion part.

2. The method according to claim 1, characterized in that, When the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤2min; The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤2min; The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤3min; The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤2min; The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤3min.

3. The method according to claim 2, characterized in that, When the outer diameter of the coated hot-formed steel pipe blank is ≤30mm, the total heat preservation time of the preheating section, the transition section and the austenitizing section is ≤10min.

4. The method according to claim 1, characterized in that, When the outer diameter of the coated hot-formed steel pipe blank is >30mm, the heating temperature of the preheating section is 390℃~420℃, and the holding time is ≤1.5min; The heating temperature of the transition section is 580℃~680℃, and the holding time is ≤1.5min; The heating temperature of the first stage of the austenitizing section is 680℃~780℃, and the holding time of the first stage is ≤4min; The second stage heating temperature of the austenitizing section is 800℃~860℃, and the second stage holding time is ≤4min; The heating temperature of the third stage of the austenitizing section is 880℃~910℃, and the holding time of the third stage is ≤4min.

5. The method according to claim 4, characterized in that, When the outer diameter of the coated hot-formed steel pipe blank is >30mm, the total heat preservation time of the preheating section, the transition section and the austenitizing section is ≤11min.

6. The method according to claim 1, characterized in that, The pressure of the low-pressure gas expansion is ≤30MPa, and the pressure of the high-pressure gas expansion is >30MPa.

7. The method according to claim 1, characterized in that, The initial temperature for ventilation expansion is ≤782℃.

8. The method according to claim 7, characterized in that, The initial temperature for ventilation expansion is ≤740℃.

9. The method according to claim 1, characterized in that, The radius of the fillet of the hot-expanded part is ≥1mm, the maximum thinning rate of the hot-expanded part is ≤15%, and the microstructure of the hot-expanded part is martensite.

10. The method according to claim 1, characterized in that, The coated hot-formed steel is zinc-based coated hot-formed steel or aluminum-silicon coated hot-formed steel.

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