Hot stamping method for pre-coated steel sheets
Patent Information
- Application Number
- HK42023072817
- Authority / Receiving Office
- HK · HK
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-05-11
- Publication Date
- 2026-09-25
- Estimated Expiration
- 2041-10-20
AI Technical Summary
Existing hot stamping forming processes cannot simultaneously improve the alloying degree of the pre-coated layer and the fracture performance of the steel substrate. In particular, the fracture performance of the pre-coated steel sheet is lower than that of the uncoated layer, and the hot forming process window is narrow, making it difficult to meet automotive industry standards.
Before hot stamping, the pre-coated steel sheet undergoes a preheating treatment, including heating, holding, and cooling steps. Specifically, the sheet is heated to 850-960℃ and held for 7-15 minutes, with a cooling rate of not less than 5℃/s to below 300℃, which can be repeated multiple times to optimize the microstructure of the steel sheet substrate.
By preheating, the alloying degree of the pre-coated layer and the fracture performance of the steel substrate are significantly improved, the temperature and time range of the hot forming process is expanded, and the process stability and production efficiency are enhanced.
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Abstract
Description
Technical Field
[0001] This invention relates to a hot stamping forming method for pre-coated steel sheets. Background Technology
[0002] With the goals of "carbon peaking" and "carbon neutrality" being proposed, energy conservation and emission reduction in the automotive industry have become a top priority, and lightweighting of vehicles is an effective way to implement energy conservation and emission reduction. Among many lightweight materials, ultra-high strength steel is an ideal material for achieving vehicle lightweighting while ensuring vehicle safety due to its advantages such as high strength, low cost, and mature technology.
[0003] Hot stamping is a forming process that involves stamping and deforming fully austenitic steel sheets at high temperatures while simultaneously cooling them rapidly to obtain ultra-high strength steel components. Hot stamping offers advantages such as high forming precision and good weight reduction, and is particularly suitable for processing complex, integrated, thin-walled parts, such as A-pillars, B-pillars, and roof beams of automotive bodies.
[0004] In actual hot stamping processes, to avoid oxidation and decarburization on the steel surface caused by high temperatures, pre-coating is usually applied to the steel sheet surface. The most widely used method is the high-temperature resistant Al-Si coating patent invented by ArcelorMittal in 1999, and hot-formed steel sheets with this pre-coated Al-Si coating were commercialized in 2007. However, in the more than 20 years since the invention of the Al-Si coating, the contradiction between the alloying degree of the pre-coating layer and the fracture performance of the steel substrate has remained unresolved.
[0005] First, one of the problems is to improve the alloying degree of the pre-coated layer.
[0006] If the degree of alloying is insufficient, the pre-coated steel sheet will cause scaling and wear problems in places such as furnace rollers and forming dies, which is very detrimental to actual production. At the same time, insufficient alloying will also lead to a lower proportion of tough phase and a higher proportion of brittle phase in the intermetallic compound of the pre-coated layer, which is prone to coating cracks and affects the coating, corrosion resistance and other properties of the pre-coated steel sheet.
[0007] In the hot stamping process of pre-coated steel sheets, higher heating temperatures and longer holding times are usually required to ensure the degree of alloying. For example, CN101583486B (hereinafter referred to as Patent Document 1) discloses a hot stamping process for pre-coated steel, which is currently the main production process for pre-coated steel. It specifies the austenitizing heating and holding process when the Al-Si coating thickness is 20-33μm: when the steel sheet thickness is 0.7-1.5mm, the temperature and time are limited to a quadrilateral composed of (3 minutes, 930℃), (6 minutes, 930℃), (13 minutes, 880℃), and (4.5 minutes, 880℃); when the steel sheet thickness is 1.5-3mm, the temperature and time are limited to a quadrilateral composed of (4 minutes, 940℃), (8 minutes, 940℃), (13 minutes, 900℃), and (6.5 minutes, 900℃). The aforementioned invention patent also specifies the ratio of Fe2Al5 phase to FeAl phase in the microstructure of the pre-coated layer after hot stamping, so as to ensure the coating, welding, corrosion resistance and other properties of the pre-coated steel components after hot stamping.
[0008] The second problem is how to improve the fracture performance of pre-coated steel plate substrates.
[0009] To improve fracture performance, lower heating temperatures and shorter holding times are typically required to prevent excessive austenite grain growth at high temperatures. However, this requirement contradicts the need to increase the alloying degree of the pre-coating layer. Existing technologies can only strike a balance between the two, and cannot simultaneously improve both the alloying degree of the pre-coating layer and the fracture performance of the steel substrate.
[0010] It is worth noting that current pre-coated steel hot-stamped components still cannot meet the fracture performance standards of all automakers. These fracture performance standards include cryogenic fracture, bending fracture, and hydrogen-induced delayed fracture. Suboptimal cryogenic fracture, bending fracture, or hydrogen-induced delayed fracture performance can all lead to localized cracking failure of body components during vehicle service or collisions, endangering passenger safety. In particular, the Al-Si coating can alloy at high temperatures, forming brittle intermetallic compounds, brittle high-alumina, high-silicon ferrite, and brittle high-carbon martensite. These brittle structures further reduce the fracture performance of hot-stamped steel components. In actual industrial production, after hot forming, the fracture performance of pre-coated steel sheets is significantly lower than that of steel sheets without a pre-coating layer.
[0011] In addition, the existing thermoforming process has a narrow temperature and time window, which places relatively high demands on actual industrial production.
[0012] For example, CN106466697B (hereinafter referred to as Patent Document 2) discloses another hot stamping forming process for Al-Si coated pre-coated steel sheets, specifying a heating and holding process of 900-950℃ for 2.5-10 minutes, with a preferred furnace temperature of 935-950℃ or 945-950℃ and a preferred dwell time of 2.5-5 minutes. Although the heating and holding process in Patent Document 2 is only slightly higher than that in Patent Document 1, in actual production, the heating and holding process in Patent Document 2 will result in excessively large austenite grains in the steel sheet substrate, leading to unsatisfactory fracture performance and making it difficult to apply to vehicle body parts.
[0013] In summary, existing hot forming processes cannot simultaneously improve the alloying degree of the pre-coated layer and the fracture performance of the steel substrate. In particular, the fracture performance of the pre-coated steel sheet still urgently needs improvement to meet the fracture performance standards of the automotive industry. Furthermore, expanding the hot forming process window based on existing hot forming processes will also benefit actual industrial production. Summary of the Invention
[0014] Therefore, the purpose of this invention is to solve the aforementioned technical bottleneck problems and provide a hot stamping forming method for improving the alloying degree of the pre-coated layer of hot-stamped steel sheets, a hot stamping forming method for improving the fracture performance of hot-stamped steel sheets, or a hot stamping forming method for simultaneously improving both the alloying degree of the pre-coated layer and the fracture performance of hot-stamped steel sheets. Another objective of this invention is to expand the temperature and time range of the hot stamping forming process and improve the stability of the hot stamping forming process.
[0015] The inventors discovered that performing a pre-coated steel sheet heat treatment before hot stamping can effectively achieve the above-mentioned objectives. This pre-heat treatment mainly includes a heating and holding step and a cooling step.
[0016] Based on this, the present invention provides a hot stamping forming method for pre-coated steel sheets. The method includes hot stamping the pre-coated steel sheet using a die at a hot stamping temperature to obtain a hot stamped component. The method further includes performing a pre-heat treatment on the pre-coated steel sheet before hot stamping, the pre-heat treatment comprising:
[0017] (1) Heating and heat preservation: The pre-coated steel plate is heated to 850-920℃ and kept for 7 to 15 minutes, or heated to 920-960℃ and kept for 5 to 10 minutes, so as to austenitize the pre-coated steel plate substrate and alloy the pre-coated layer.
[0018] (2) Cooling: After the heating and heat preservation step is completed, the pre-coated steel plate is cooled to below 300°C at a cooling rate of not less than 5°C / s; and
[0019] (3) Optionally repeat the above heating, heat preservation and cooling steps once or multiple times.
[0020] In one embodiment of the present invention, in addition to iron, the pre-coated steel substrate also contains the following components expressed in weight percent: carbon 0.2-0.4%; manganese 0.5-1.5%; boron 0-0.005%; not more than 1% of one or more alloying elements selected from aluminum, silicon, chromium, molybdenum, niobium, and vanadium; and other unavoidable impurity elements.
[0021] In another embodiment of the invention, in addition to iron, the pre-coated steel substrate also contains the following components expressed in weight percent: carbon 0.3-0.5%; manganese 0.5-2.5%; boron 0-0.005%; not more than 3% of one or more alloying elements selected from aluminum, silicon, chromium, molybdenum, niobium, and vanadium; and other unavoidable impurity elements.
[0022] Increasing the carbon, manganese, and silicon content can improve the hardenability of the steel substrate and facilitate the formation of martensite after the pre-heat treatment, achieving a grain refinement effect. However, excessive carbon, manganese, and silicon content can adversely affect the fracture properties of the steel substrate. Adding a very small amount of boron can improve the hardenability of the steel substrate without affecting fracture properties. Aluminum can deoxidize during smelting while protecting the effectiveness of boron. Other elements such as chromium and molybdenum can also improve hardenability, but disadvantageously, they significantly increase the cost of the steel plate. Vanadium and niobium can refine austenite grains and produce precipitation strengthening, improving the strength of hot-stamped components. As a preferred example, the pre-coated steel substrate can be commercially available 22MnB5 or 34MnB5 steel.
[0023] In embodiments of the present invention, the thickness of the pre-coated steel substrate is 0.8-2.5 mm, and the thickness of the pre-coated layer is 6-36 μm. In some embodiments, the thickness of the pre-coated layer is 15-27 μm. In some specific embodiments, the thickness of the pre-coated steel substrate is 1.0-1.3 mm, which is a typical thickness for steel plates used in vehicle body safety components.
[0024] In some embodiments of the present invention, the pre-coating layer is aluminum or an aluminum alloy, the pre-coating layer is disposed on at least one surface of the pre-coated steel plate, and the initial thickness of the pre-coating layer on each surface is less than 36 μm.
[0025] In some specific embodiments, the pre-coating layer is an Al-Si alloy coating, typically containing 8-11% Si, 2-4% Fe, 85-90% Al, and unavoidable impurities by weight. Al primarily provides the coating with high-temperature stability and oxidation resistance; however, it is detrimental because Al alloys with the steel substrate at high temperatures, forming brittle intermetallic compounds. Si can inhibit the growth of these intermetallic compounds, reducing their harmful effects on the steel's fracture performance.
[0026] As an example, commercially available 22MnB5 steel with a pre-coated Al-Si layer can be used. The 22MnB5 steel substrate has a C content of 0.20-0.23% (by weight), a Mn content of 0.9-1.4%, and a Si content of 0.20-0.28%. Furthermore, the thickness of the pre-coated Al-Si layer is approximately 25 μm.
[0027] In a preferred embodiment, when the pre-coated layer thickness is less than 20 μm, the heating and holding step is performed at 850-920°C for 7 to 15 minutes; when the pre-coated layer thickness is greater than or equal to 20 μm, the heating and holding step is performed at 920-960°C for 5 to 10 minutes. In some embodiments, the pre-coated layer is an Al-Si coating; when the Al-Si coating thickness is less than 20 μm, the holding temperature range is 870-915°C for a total time of 2-7 minutes; when the Al-Si coating thickness is greater than or equal to 20 μm, the holding temperature range is 890-935°C for a total time of 4-9 minutes.
[0028] The selection of the heating and holding process in the pre-treatment stage is mainly to ensure the alloying of the coating and to obtain a suitable intermetallic compound structure. When the pre-coated layer is aluminum or an aluminum alloy, the intermetallic compound includes a FeAl phase with a volume fraction of not less than 60%. Compared with other intermetallic compounds, the FeAl phase has advantages such as a higher melting point and better fracture toughness, which can reduce mold wear, reduce coating cracks, and improve the fracture performance and corrosion resistance of the steel plate.
[0029] Furthermore, the adverse effects of austenite growth in the steel sheet substrate during the pre-heat treatment process can be eliminated or mitigated by the martensite or bainite structure obtained after cooling. This martensite or bainite structure can provide more austenite nucleation sites during subsequent hot stamping, refining the austenite structure and improving the fracture properties of the steel sheet substrate. Therefore, the pre-heat treatment process described in this invention has a wider range than existing hot stamping processes, which is beneficial for improving process stability.
[0030] The cooling rate during pre-heat treatment depends primarily on the hardenability of the steel substrate. It is important to note that a cooling rate of approximately 5°C / s cannot completely prevent carbide formation, nor can it yield a microstructure entirely composed of martensite or bainite. However, the inventors have discovered that a small amount of martensite or bainite microstructure (not less than 30%, or the sum of their volume fractions not less than 30%) can also refine austenite grains during subsequent hot stamping. Simultaneously, uniformly distributed carbides can provide numerous austenite nucleation sites during heating, further refining the austenite grains.
[0031] Preferably, the cooling rate is not less than 10°C / s, in which case the martensite volume fraction after cooling can reach more than 50%, pearlite will not form, and the carbide particle size is not greater than 0.1 μm. More preferably, the cooling rate is not less than 25°C / s, in which case the martensite volume fraction after cooling is close to 100%. However, when the cooling rate is increased to 50°C / s, the effect of further refining the austenite grains is not significant, and the fracture performance after subsequent hot stamping is hardly improved. Therefore, most preferably, the cooling rate is between 25°C / s and 50°C / s.
[0032] In actual production, cooling the pre-heat-treated steel plate to room temperature takes a long time, especially below 200°C, where the cooling rate slows down significantly. To improve production efficiency, as a preferred method, after the pre-heat treatment step, the steel plate is cooled to below 300°C but above 200°C before being transferred to a heating furnace for reheating. In this case, the martensitic transformation during the cooling process is complete or largely complete, which is sufficient to refine the austenite grains and improve the final fracture performance of the steel plate.
[0033] Preferably, the steel plate can be deformed using a stamping die during the cooling process of the pre-heat treatment. On the one hand, this reduces the amount of deformation in the subsequent hot stamping process, lowers the temperature requirements of the subsequent hot stamping process, and allows for a larger heating and heat preservation process window in the subsequent hot stamping process. On the other hand, the heating and heat preservation in the subsequent hot stamping process can fill the coating cracks caused by the pre-deformation. At the same time, the reduced deformation required for the subsequent hot forming process reduces the number and depth of coating cracks, thereby improving the corrosion resistance of the pre-coated layer and reducing the adverse effects of coating cracks on fracture performance.
[0034] Preferably, the steel plate undergoes several preliminary heat treatments, with the same or different process parameters used between each treatment. The alternating transformation between high-temperature austenite and room-temperature martensite improves grain refinement. Furthermore, using different process parameters between the preliminary heat treatments provides more possibilities for the design of the Fe-Al diffusion layer and intermetallic compound layer in the pre-coating layer (such as an Al-Si coating). Therefore, after multiple preliminary heat treatments, the chemical composition and microstructure of the steel plate substrate are more uniform, the grains are better refined, and the alloying degree of the pre-coating layer is also higher.
[0035] Furthermore, the adverse effects of austenite growth in the steel substrate during the pre-heat treatment process can be eliminated or mitigated by the martensite or bainite structure obtained after cooling. This martensite or bainite structure can provide more austenite nucleation sites during subsequent hot stamping, refining the austenite structure and improving the fracture properties of the steel substrate. Therefore, the pre-heat treatment process described in this invention has a wider range than existing hot stamping processes, which is beneficial for improving process stability.
[0036] According to the hot stamping forming method for pre-coated steel sheets provided by the present invention, the microstructure of the pre-coated steel sheet substrate after pre-heat treatment has the following characteristics:
[0037] (a) The volume fraction of martensite or bainite, or the sum of both, is not less than 30%;
[0038] (b) There are no spherical carbide particles with a diameter greater than 0.5 μm, and no lamellar candlesticks with a diameter greater than 1 μm.
[0039] The microstructure of the pre-coated layer after the pre-heat treatment consists of one or more intermetallic compounds and ferrite, wherein the FeAl phase accounts for not less than 60% of the volume fraction of all intermetallic compounds.
[0040] In some embodiments of the present invention, the hot stamping forming method further includes: after the pre-heat treatment, reheating the pre-coated steel plate to 780-940°C and holding it for 1 to 7 minutes to fully austenitize the pre-coated steel plate substrate; then, while maintaining the temperature above 500°C, transferring the pre-coated steel plate to a mold for hot stamping forming.
[0041] Preferably, the hot stamping forming method further includes: after the preheating treatment, reheating the pre-coated steel plate to 830-900°C and holding it for 1 to 4 minutes to fully austenitize the pre-coated steel plate substrate; then, maintaining the temperature above 600°C, transferring the pre-coated steel plate to a mold for hot stamping forming.
[0042] Preferably, in the reheating step, the temperature range is 830-900℃, and the total heating and holding time is 1-4 minutes.
[0043] More importantly, as the austenite grains in hot-stamped steel are refined, the martensitic lath bundles, lath blocks, and laths will also be refined, ultimately improving the fracture performance of hot-stamped components.
[0044] In some embodiments of the present invention, the microstructure of the pre-coated steel plate substrate after hot stamping is a fully martensitic structure.
[0045] In other embodiments of the present invention, the microstructure of the pre-coated steel sheet substrate after hot stamping is a mixed microstructure consisting of two or more phases of ferrite, bainite, martensite, and austenite. In this case, the strength of the hot-stamped component decreases while its fracture performance increases.
[0046] Furthermore, the original austenite grain size in the microstructure of the pre-coated steel sheet substrate after hot stamping does not exceed 18 μm. As a preferred embodiment, the original austenite grain size in the microstructure of the hot-stamped component does not exceed 10 μm. At this point, due to the higher degree of grain refinement, the fracture performance of the hot-stamped steel is better.
[0047] The microstructure of the hot-stamped steel sheet substrate is usually entirely martensitic, in which case the strength of the hot-stamped steel can reach 1.5 GPa.
[0048] In some embodiments of the present invention, after the pre-heat treatment, the steel plate is transferred to a heating furnace for reheating and hot stamping. As a preferred embodiment, after the pre-heat treatment step, the steel plate is cooled to below 300°C and above 200°C before being transferred to the heating furnace for reheating. In this case, the martensitic transformation during the cooling process is complete or largely complete, sufficient to refine the austenite grains and improve the final fracture performance of the steel plate. Transferring the steel plate to the heating furnace at above 200°C improves production efficiency and saves energy.
[0049] As another preferred option, after the pre-heat treatment step, the steel plate is cooled to below 200°C and above 100°C, then transferred to a heating furnace for reheating. Compared to the aforementioned options, this option is more suitable for situations where the pre-coated steel plate substrate has a high alloy content and a low martensitic transformation temperature.
[0050] The reason why the hot stamping forming method of the present invention can refine the original austenite grains is mainly related to the following factors: (A) During the heating and holding process of the preheating heat treatment, the pre-coating layer has been alloyed to form an intermetallic compound layer and a diffusion layer. During the hot stamping forming process, it is not necessary to take the alloying of the pre-coating layer as the main consideration. Therefore, a lower heating and holding temperature and a shorter heating and holding time can be used to suppress the growth of austenite grains; (B) During the cooling process of the preheating heat treatment, the steel plate substrate forms a complete or partial martensite or bainite structure. These structures have the characteristics of fine grains and high defect density. In addition, the carbide distribution in the steel plate substrate is finer and more uniform. During the reheating process of the hot stamping forming process, more austenite nucleation sites are provided, which ultimately refines the original austenite grains.
[0051] The reasons for improving fracture performance using this invention are as follows: The hot stamping forming technology described in this invention can refine the original austenite grains, martensite lath bundles, martensite lath blocks, and martensite laths in the microstructure of hot-stamped components. On the one hand, this grain refinement can reduce the enrichment of impurity elements such as phosphorus and sulfur, as well as hydrogen, at grain boundaries, mitigating the adverse effects of impurity elements and hydrogen on grain boundary fracture, thereby suppressing brittle intergranular fracture. On the other hand, through grain refinement, cracks need to penetrate more large-angle grain boundaries during propagation, including original austenite grain boundaries, martensite lath bundle interfaces, and martensite lath block interfaces. Therefore, crack propagation requires more energy and is more difficult, ultimately improving the fracture performance of hot-stamped components.
[0052] Therefore, according to the hot stamping forming method provided by the present invention, the pre-heat treatment can improve the alloying degree of the pre-coated layer, reduce the carbide structure in the steel substrate, and obtain a martensitic and / or bainitic substrate structure, thereby reducing the wear of the pre-coated layer on the stamping die during high-temperature stamping deformation, refining the grains, and improving the fracture performance of the final hot stamped component.
[0053] The hot stamping forming method of the present invention can be used for automotive safety structural parts, reinforcing structural parts, wheel components, high-strength and tough automotive structural parts, including but not limited to A-pillars, B-pillars, and roof beams of automotive bodies. Attached Figure Description
[0054] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:
[0055] Figure 1 This is a schematic diagram of an existing hot stamping forming method;
[0056] Figure 2 This is a schematic diagram of the hot stamping forming method of the present invention;
[0057] Figure 3This is a diagram showing the pre-coated layer and the original microstructure of the 22MnB5 steel plate used in the embodiments;
[0058] Figure 4 This is a schematic diagram of the microstructure of the Al-Si coating after preheating treatment in Examples 1-3 of the present invention;
[0059] Figure 5 This describes the change in the ratio of the total integral of FeAl phase to intermetallic compound in the pre-coated layer after preheating in Examples 1-3 of the present invention as a function of holding time.
[0060] Figure 6 This is a microstructure diagram of the steel plate substrate after preheat treatment in Embodiment 2 of the present invention.
[0061] Figure 7 This is a microstructure diagram of the steel plate substrate after hot stamping in Embodiment 4 of the present invention;
[0062] Figure 8 This is an inverse polarimetric diagram of the microstructure of the steel plate substrate after hot stamping in Embodiment 4 of the present invention;
[0063] Figure 9 It is an inverse polarimetric diagram of the microstructure of a steel plate substrate formed by hot stamping using existing technology;
[0064] Figure 10 This is a comparison of the three-point bending test results of the pre-coated hot-stamped steel sheet obtained by embodiment 4 of the present invention with those obtained by prior art. Detailed Implementation
[0065] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.
[0066] To specifically illustrate the hot stamping forming method of the present invention, Figure 1 and Figure 2 The present invention provides a summary of the hot stamping forming process method and an embodiment of a conventional hot stamping forming method. It should be noted that the present invention is not limited to the embodiments shown in the figures; the slopes in the figures are only used to illustrate the average heating or cooling rate and do not represent actual heating or cooling curves.
[0067] The pre-coated Al-Si layer and the original microstructure of the 22MnB5 steel plate used in the examples are as follows: Figure 3As shown, the pre-coating layers, starting from the steel substrate side, consist of: a Si-rich intermetallic compound layer and an Al layer. This pre-coated Al-Si layer structure is typical of the pre-coated layers of commercially available 22MnB5 steel. The steel substrate mainly consists of ferrite, pearlite, and carbides. The lamellar pearlite particles can reach sizes of 2 μm or more, and the spherical carbide particles can reach sizes of 0.5 μm or more.
[0068] Examples 1-3
[0069] The pre-coated 22MnB5 steel plate underwent the following preliminary heat treatment:
[0070] (1) The three pre-coated steel plates were heated to 930°C and held for 1 minute (Example 1), 5 minutes (Example 2) and 10 minutes (Example 3) respectively to austenitize the pre-coated steel plate substrate and alloy the pre-coated layer;
[0071] (2) After the heating and heat preservation step is completed, the steel plate is cooled to room temperature at a cooling rate of 300℃ / s.
[0072] After the preliminary heat treatment, the Al-Si coating in the above steel plate undergoes alloying, and its microstructure is as follows: Figure 4 As shown, starting from the steel substrate side, the layers are sequentially: an Fe-Al diffusion layer mainly composed of ferrite, and an intermetallic compound layer mainly composed of FeAl and Fe2Al5. The thickness of the Fe-Al diffusion layer is 4 to 30 μm, and the thickness of the intermetallic compound layer is 15 to 40 μm.
[0073] Figure 5 The variation of the total volume fraction ratio of FeAl phase to intermetallic compounds with holding time is presented. Compared to Fe2Al5 phase, FeAl has a higher melting point and fracture toughness, which can reduce the adverse effects of furnace roller scaling and mold wear. In this embodiment, the volume fraction of FeAl phase in all Fe-Al intermetallic compounds is not less than 60%.
[0074] The microstructure of the steel plate substrate after the pre-heat treatment in Example 2 is as follows: Figure 6 As shown, there are no spherical cementite particles with a particle size greater than 0.5 μm, and no lamellar pearlite fields with a particle size greater than 1 μm; the volume fraction of martensite or bainite, or the sum of the two, is not less than 30%.
[0075] Numerous parallel experiments have shown that although the cooling rate and temperature in this embodiment are outside the optimal range of the present invention, it does not affect the microstructure and conclusions in this embodiment. In other words, the microstructure and conclusions obtained within the optimal cooling rate and temperature ranges of the present invention are almost identical to those in the above embodiments. However, those optimal ranges are more conducive to industrial production.
[0076] Example 4
[0077] The steel sheet obtained in Example 2 was reheated and hot-stamped.
[0078] (3) The steel plate is reheated to 850°C and held for a total time of 1.5 minutes to make the steel plate completely austenitized;
[0079] (4) Transfer the heated steel plate to the mold, ensuring that the temperature of the heated steel plate is above 500°C when it is transferred to the mold;
[0080] (5) The heated steel plate is hot-stamped in the mold to obtain the final hot-stamped steel component.
[0081] Figure 7 This refers to the microstructure of the steel substrate after the hot stamping process. After hot stamping, the steel substrate acquires a fully martensitic structure, and the original austenite grains in the microstructure are significantly refined, with the grain size decreasing from 15-25 μm after pre-heat treatment to 3-8 μm after hot stamping. Simultaneously, because a relatively low temperature and short holding time are used in the hot stamping process, the alloying degree and microstructure of the Al-Si coating no longer change significantly.
[0082] Figure 8 This is an inverse pole figure of the microstructure of the steel sheet substrate after the hot stamping process. The figure allows for a clearer observation of the original austenite grain size within the microstructure of the steel sheet substrate, compared to... Figure 7 The observed original austenite grain size of 3-8 μm is consistent with this.
[0083] As a comparison Figure 9 Is adopted Figure 2The diagram shows the inverse pole figure of the microstructure of a steel sheet substrate obtained by a conventional hot stamping forming technique (i.e., without pre-heat treatment). The original austenite grain size is approximately 20 μm. The conventional hot stamping forming process, without pre-heat treatment, requires high heating and holding temperatures and long holding times, for example, 900-940°C for 5-10 minutes. Furthermore, the initial microstructure of the conventional hot stamping forming process consists of ferrite + lamellar pearlite + spherical carbides, which cannot refine the austenite grains by producing martensite or bainite. Therefore, the original austenite grain size in the steel sheet substrate obtained by the conventional hot stamping forming process is much larger than that of the hot stamping forming process described in this invention.
[0084] Figure 10 The results of three-point bending tests on hot-stamped steel sheets obtained by existing hot stamping methods and the method of this invention were compared. The three-point bending test results of the hot-stamped steel sheets can reflect the fracture performance of the hot-stamped components. The three-point bending test results show that, compared with existing hot stamping technologies, the hot-stamped steel sheets obtained by the method of this invention can achieve greater bending loads and larger bending angles. Specifically, the method of this invention can achieve a maximum bending load of approximately 1100 N and a bending angle of approximately 58.4°, while the existing technology can only achieve a maximum bending load of approximately 1000 kN and a bending angle of approximately 52.2°, meaning the maximum bending load is increased by 10%, and the bending angle is increased by 11.9%. Therefore, the hot-stamped steel sheets obtained by the method of this invention have better three-point bending performance, and correspondingly, the fracture performance of the hot-stamped components is also better.
[0085] As can be seen from the above embodiments, the method of the present invention can simultaneously improve the alloying degree of the pre-coated layer and the fracture performance of the steel plate substrate. The reasons for improving the fracture performance of the steel plate substrate are as follows: The hot stamping forming technology described in this invention can make the carbon element more uniformly distributed in the substrate and refine the original austenite grains, martensite lath bundles, martensite lath blocks, and martensite laths in the microstructure of the hot stamped forming component. On the one hand, the above grain refinement can reduce the enrichment of impurity elements such as phosphorus and sulfur, and hydrogen elements at grain boundaries, mitigating the adverse effects of impurity elements and hydrogen elements on grain boundary fracture, thereby suppressing brittle intergranular fracture. On the other hand, through grain refinement, cracks need to pass through more large-angle grain boundaries during propagation, including original austenite grain boundaries, martensite lath bundle interfaces, and martensite lath block interfaces. Therefore, crack propagation requires more energy and is more difficult, ultimately improving the fracture performance of the hot stamped forming component.
[0086] The above embodiments are provided for illustrative and descriptive purposes. It should be understood by those skilled in the art that the present invention is not limited to these embodiments or experimental data. The data and various parameters described in the embodiments are merely exemplary and do not constitute a limitation of the present invention.
Claims
1. A method for hot stamping forming of pre-coated steel sheet, the method comprising hot stamping forming the pre-coated steel sheet using a die at a hot stamping temperature to obtain a hot stamped component, characterized in that... The method further includes a pre-heat treatment of the pre-coated steel sheet prior to hot stamping, the pre-heat treatment comprising: (1) Heating and heat preservation: The pre-coated steel plate is heated to 850-920℃ and kept for 7 to 15 minutes, or heated to 920-960℃ and kept for 5 to 10 minutes, so as to make the pre-coated steel plate substrate austenitized and the pre-coated layer alloyed. (2) Cooling: After the heating and heat preservation step is completed, the pre-coated steel plate is cooled to below 300°C at a cooling rate of not less than 5°C / s; and (3) Optionally repeat the above heating, heat preservation, and cooling steps once or more. The microstructure of the pre-coated layer after pre-heat treatment consists of one or more intermetallic compounds and ferrite, wherein the FeAl phase accounts for not less than 60% of the volume fraction of all intermetallic compounds, and The microstructure of the pre-coated steel plate substrate after preheat treatment satisfies that the volume fraction of martensite or bainite or the sum of both is not less than 30%.
2. The hot stamping forming method according to claim 1, wherein, In addition to iron, the pre-coated steel substrate also contains the following components expressed in weight percent: carbon 0.2-0.4%; manganese 0.5-1.5%; boron 0-0.005%; not exceeding 1% of one or more alloying elements selected from aluminum, silicon, chromium, molybdenum, niobium, and vanadium; and other unavoidable impurity elements; or The pre-coated steel plate substrate also contains the following components expressed in weight percent: carbon 0.3-0.5%; manganese 0.5-2.5%; boron 0-0.005%; and no more than 3% of one or more alloying elements selected from aluminum, silicon, chromium, molybdenum, niobium, and vanadium. And other unavoidable impurity elements.
3. The hot stamping forming method according to claim 1, wherein, The thickness of the pre-coated steel plate substrate is 0.8-2.5 mm.
4. The hot stamping forming method according to claim 1, wherein, The pre-coating layer is aluminum.
5. The hot stamping forming method according to claim 1, wherein, The pre-coating layer is made of aluminum alloy.
6. The hot stamping forming method according to claim 1, wherein, The pre-coating layer is an Al-Si alloy.
7. The hot stamping forming method according to claim 1, wherein, The thickness of the pre-coated layer is 6-36 μm.
8. The hot stamping forming method according to claim 1, wherein, The thickness of the pre-coated layer is 15-27 μm.
9. The hot stamping forming method according to any one of claims 1 to 8, wherein, When the thickness of the pre-coated layer is less than 20 μm, the heating and heat preservation step is: 850-920℃ for 7 to 15 minutes; when the thickness of the pre-coated layer is greater than or equal to 20 μm, the heating and heat preservation step is: 920-960℃ for 5 to 10 minutes.
10. The hot stamping forming method according to any one of claims 1 to 8, wherein, The cooling rate is not less than 10℃ / s.
11. The hot stamping forming method according to claim 10, wherein, The cooling rate is not less than 25°C / s.
12. The hot stamping forming method according to claim 11, wherein, The cooling rate is from 25°C / s to 50°C / s.
13. The hot stamping forming method according to any one of claims 1 to 8, wherein, During the cooling process in step (2), the pre-coated steel plate is deformed by a stamping die.
14. The hot stamping forming method according to any one of claims 1 to 8, wherein, The microstructure of the pre-coated steel plate substrate after pre-heat treatment has the following characteristics: There are no spherical carbide particles with a diameter greater than 0.5 μm, and no lamellar candlesticks with a diameter greater than 1 μm.
15. The hot stamping forming method according to any one of claims 1 to 8, wherein, The method further includes: after the pre-heat treatment, reheating the pre-coated steel plate to 780-940°C and holding it for 1 to 7 minutes to fully austenitize the pre-coated steel plate substrate; then, while maintaining the temperature above 500°C, transferring the pre-coated steel plate to a mold for hot stamping.
16. The hot stamping forming method according to any one of claims 1 to 8, wherein, The method further includes: after the pre-heat treatment, reheating the pre-coated steel plate to 830-900°C and holding it for 1 to 4 minutes to fully austenitize the pre-coated steel plate substrate; then, maintaining the temperature above 600°C, transferring the pre-coated steel plate to a mold for hot stamping.
17. The hot stamping forming method according to any one of claims 1 to 8, wherein, The microstructure of the pre-coated steel sheet substrate after hot stamping is a fully martensitic structure; or The microstructure of the pre-coated steel sheet substrate after hot stamping is a mixed structure consisting of two or more phases of ferrite, bainite, martensite, and austenite.
18. The hot stamping forming method according to any one of claims 1 to 8, wherein, The original austenite grain size of the pre-coated steel plate substrate after hot stamping does not exceed 18μm.
19. The hot stamping forming method according to claim 18, wherein, The original austenite grain size of the pre-coated steel plate substrate after hot stamping does not exceed 10 μm.