Hot-formed steel riveting die and pressing rivet connection method

Abstract

The invention provides a hot forming steel riveting die and a pressing rivet connection method, and relates to the technical field of hot forming steel riveting, the hot forming steel riveting die comprises a pressing rivet upper die, a rivet feeding device is arranged on the pressing rivet upper die, and the rivet feeding device is used for placing a pressing rivet nut on hot forming steel; a pressure device used for pressing the pressing rivet nut into the hot forming steel is arranged on the pressing rivet upper die, and a heating device used for heating the hot forming steel is arranged in the pressing rivet upper die. According to the hot-formed steel riveting die, hot-formed steel can be heated and located in an austenite single-phase area, and then a rivet nut is placed on the upper surface of the hot-formed steel through a rivet feeding device; when the hot-formed steel is austenitized, the pressure device presses the pressing rivet nut into the hot-formed steel, the problem that hot-formed steel parts cannot be riveted due to high hardness at normal temperature is solved, and metallurgical-mechanical composite connection of the hot-formed steel in a plastic state is realized.

Description

Technical Field

[0001] This invention relates to the technical field of hot-formed steel riveting, and in particular to a hot-formed steel riveting mold and a press-fit connection method. Background Technology

[0002] With the deep integration of hot forming technology and the automotive industry, its application in vehicle body has expanded from local structural components to the entire vehicle safety framework. Currently, the proportion of hot-formed steel in mainstream models has exceeded 30%. This technological evolution not only conforms to the trend of the automotive industry towards greater safety and efficiency, but also demonstrates enormous potential and challenges in its synergistic application with lightweight alloy materials such as magnesium alloys.

[0003] This technological evolution is primarily influenced by the following policies: (1) Under the dual-carbon strategy, the demand for lightweight vehicle bodies is growing exponentially. Using hot-formed components with a tensile strength of 1500-2000MPa can achieve a weight reduction of more than 40% compared with traditional steel. The introduction of lightweight alloy materials such as magnesium alloys can further enhance the lightweight effect. At the same time, this combination can also geometrically improve the collision safety performance of the vehicle body and meet increasingly stringent environmental and safety standards.

[0004] (2) In order to meet the stringent requirements of high-speed impact in the C-IASI 25% small offset crash test, thermoformed parts can provide core support for the vehicle body safety architecture with their ultra-low springback of 0.5-1.5mm, dimensional tolerance accuracy of ±0.2mm, and energy absorption characteristics of more than 5 times that of traditional soft steel.

[0005] However, hot-formed steel differs significantly from light alloys such as magnesium alloys and aluminum alloys in terms of physical and chemical properties. The melting point of hot-formed steel is typically around 1500℃, while that of magnesium alloys is about 650℃ and that of aluminum alloys is about 660℃. This huge difference in material properties makes it difficult for traditional joining processes to meet the reliable joining requirements of both. The existing hot-formed joining technology system has shown many limitations, as follows: (1) Technical barriers of hot forming projection welding process: 1. Pre-welding pretreatment requires laser cutting to process pre-drilled holes, which extends the processing cycle of a single piece by 35-45 seconds, resulting in a production line cycle time loss of more than 18%.

[0006] 2. Due to the high electrical resistance of martensitic structures during welding, an unconventional electrode pressure of 1.5-2.0 kN needs to be applied, resulting in an average monthly electrode tip wear rate as high as 27%.

[0007] 3. The welding quality dispersion reaches ±15%, and microcracks are prone to occur at the interface, which leads to an expansion of the fluctuation range of the shear strength of the weld point.

[0008] (2) Technical barriers of thermoforming riveting process: 1. During the punching process, the rivet piercing needs to overcome a yield strength of about 800-1200MPa, and conventional C-type rivet guns are difficult to achieve effective plastic deformation.

[0009] 2. During the connection process, local stress concentration can reach 120% of the tensile strength of the material body, which increases the probability of grain boundary cracks in the matrix.

[0010] The aforementioned technical barriers have severely restricted the application of hot forming technology. According to SAE research data, connection problems in hot forming technology can lead to a 12-18% loss in vehicle body stiffness, directly affecting the passenger compartment intrusion in a 25% offset collision, exceeding the standard by 30-50mm. With the application of integrated die-cast lightweight alloy materials for vehicle bodies, developing new methods to reliably connect hot-formed steel with lightweight alloys such as magnesium alloys is of paramount importance for overcoming the bottleneck of vehicle body lightweighting, improving vehicle safety performance and production efficiency, and has become a technical problem urgently needing to be solved by those skilled in the art. Summary of the Invention

[0011] The purpose of this invention is to provide a riveting mold and a press-fit connection method for hot-formed steel, so as to alleviate the technical problem that the physical and chemical properties of hot-formed steel are significantly different from those of light alloys such as magnesium alloys and aluminum alloys, and that traditional methods are difficult to meet the requirements for reliable connection.

[0012] The present invention provides a hot-formed steel riveting mold, including an upper riveting mold and a lower riveting mold. The upper end surface of the lower riveting mold is provided with a lower riveting cavity, and at least one material dropping hole cavity is provided in the lower riveting cavity. The upper riveting cavity is provided on the lower end face of the riveting mold. The upper riveting cavity and the lower riveting cavity are correspondingly provided to form a riveting cavity for placing hot-formed steel. A rivet feeding device is provided on the press-fit die, which is used to place the press-fit nut on the hot-formed steel. The upper riveting mold is equipped with a pressure device for pressing the riveting nut into the hot-formed steel, and the upper riveting mold is equipped with a heating device for heating the hot-formed steel.

[0013] In an optional embodiment, the upper riveting die is provided with at least one feeding channel, the lower end of which is connected to the upper riveting cavity, and the feeding channel is used to deliver the riveting nut to the upper surface of the hot-formed steel. In an optional embodiment, the pressure device includes at least one pressure component, with each feeding channel corresponding to one pressure component, the movable end of the pressure component being inserted into the feeding channel and pressing the rivet nut into the hot-formed steel.

[0014] In an optional embodiment, the blanking hole cavity on the riveting die is correspondingly provided with the riveting feeding channel.

[0015] In an optional embodiment, a wear-resistant layer is sprayed into the material discharge cavity; the wear-resistant layer has a thickness of 50 μm and is made of WC-Co.

[0016] In an optional implementation, the riveting depth is 1.2 times the thickness of the hot-formed steel.

[0017] In an optional embodiment, a positioning post is provided on the upper end face of the side wall of the lower riveting cavity of the riveting die. The lower end face of the side wall of the upper riveting cavity of the riveting mold is provided with positioning holes corresponding to the positioning pins. The hot-formed steel riveting mold provided by this invention can heat the hot-formed steel and place it in the austenitic single-phase region. Then, the riveting device places the rivet nut on the upper surface of the hot-formed steel. When the hot-formed steel is in the austenitic state, the pressure device presses the rivet nut into the hot-formed steel, solving the problem that hot-formed steel parts cannot be riveted due to their high hardness at room temperature. This achieves a metallurgical-mechanical composite connection of hot-formed steel in the plastic state. Subsequently, bolt holes can be pre-drilled on the integrated light alloy die casting to connect the two materials parts with rivet nuts and bolts, increasing the selection of lightweight materials for the vehicle body.

[0018] This invention provides a press-fit connection method based on the hot-formed steel riveting mold described in the foregoing embodiments, comprising the following steps: S1. Place the hot-formed steel into the hot-formed steel riveting mold; S2. Start heating and heat the hot-formed steel to the austenitic single-phase region and hold it at that temperature; S3. The rivet feeding device places the rivet nut into the designated position on the hot-formed steel. S4. The pressure device presses the rivet nut into the hot-formed steel. S5. After the mold is closed and cooled, the hot-formed steel is removed from the hot-formed steel riveting mold.

[0019] In an optional implementation, in step S2, the heat preservation time is calculated as 0.25×t^(1 / 2), where t is the thickness of the hot-formed steel in mm.

[0020] In an optional implementation, in step S5, the mold closing cooling process is divided into multiple cooling zones, each with a different cooling rate.

[0021] The press-fit connection method provided by this invention involves placing hot-formed steel into a hot-formed steel riveting mold and heating the hot-formed steel to the austenitic single-phase region and holding it at that temperature. While the hot-formed steel is in an austenitic state, a press-fit nut is pressed into it. Subsequently, bolt holes can be pre-drilled on the integrated lightweight alloy die-casting part to connect the two material parts using the press-fit nut and bolts, thus increasing the selection of lightweight materials for the vehicle body. Attached Figure Description

[0022] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of the hot-forming steel riveting mold provided in an embodiment of the present invention; Figure 2 for Figure 1 The diagram shows a longitudinal section of a hot-formed steel riveting die. Figure 3 This is a schematic flowchart of the press-fit connection method provided in an embodiment of the present invention.

[0024] Icons: 100-lower riveting die; 200-upper riveting die; 300-pressure device; 400-riveting device; 500-heating device; 600-lower riveting cavity; 700-discharge hole cavity; 800-positioning pin; 900-riveting channel; 110-positioning hole; 120-upper riveting cavity. Detailed Implementation

[0025] The terms “first,” “second,” “third,” etc., are used only for distinguishing descriptions and do not indicate a sequence number, nor should they be interpreted as indicating or implying relative importance.

[0026] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0027] In the description of this application, it should be noted that the terms "inner", "outer", "left", "right", "upper", "lower", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this application is in use. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0028] In the description of this application, unless otherwise expressly specified and limited, the terms “set up,” “install,” “connect,” and “link” shall be interpreted broadly, for example, as a fixed connection, a detachable connection, or an integral connection; as a mechanical connection or an electrical connection; as a direct connection or an indirect connection through an intermediate medium; or as a connection within two components.

[0029] The technical solution of this application will now be clearly and completely described with reference to the accompanying drawings.

[0030] Example Reference Figures 1-2 The present invention provides a hot-formed steel riveting mold, including an upper riveting mold 200 and a lower riveting mold 100. The upper end surface of the lower riveting mold 100 is provided with a lower riveting cavity 600, and at least one material dropping hole cavity 700 is provided in the lower riveting cavity 600. The upper riveting cavity 120 is provided on the lower end surface of the riveting mold 200. The upper riveting cavity 120 is correspondingly provided with the lower riveting cavity 600 and forms a riveting cavity for placing hot-formed steel. A rivet feeding device 400 is provided on the riveting mold 200, which is used to place the riveting nut on the hot-formed steel. A pressure device 300 for pressing the rivet nut into the hot-formed steel is provided on the upper rivet mold 200, and a heating device 500 for heating the hot-formed steel is provided inside the upper rivet mold 200.

[0031] In some embodiments, the upper riveting mold 200 of the hot-formed steel riveting mold is disposed at the upper end of the lower riveting mold 100, and the upper riveting cavity 120 of the upper riveting mold 200 and the lower riveting cavity 600 of the lower riveting mold 100 form a riveting cavity.

[0032] Hot-formed steel is assembled in the riveting cavity, and the heating device 500 is generally an embedded thermocouple. According to the material of the hot-formed steel, the thermodynamic parameters of the hot-formed steel parts are obtained by inputting the material composition of the parts into Thermo-Calc thermodynamic software, calculating its thermodynamic dynamic equilibrium phase diagram, and obtaining the temperature range of the austenite phase transformation point.

[0033] Hot-formed steel is in the austenitic phase transformation temperature range. The rivet feeding device 400 places a rivet nut on the upper end face of the hot-formed steel, and the pressure device 300 presses the rivet nut into the hot-formed steel, thereby fixing the rivet nut in the hot-formed steel. The hot-formed steel plate with the rivet nut can be connected to the integrated casting (magnesium or aluminum alloy casting) by bolts, realizing a stable connection of dissimilar material parts.

[0034] In an optional embodiment, the upper riveting mold 200 is provided with at least one rivet feeding channel 900, the lower end of the rivet feeding channel 900 is connected to the upper riveting cavity 120, and the rivet feeding channel 900 is used to feed the riveting nut to the upper end surface of the hot-formed steel.

[0035] In an optional embodiment, the pressure device 300 includes at least one pressure component, with each of the feeding channels 900 corresponding to one pressure component, the movable end of the pressure component being inserted into the feeding channel 900 and pressing the rivet nut into the hot-formed steel.

[0036] In an optional embodiment, the material discharge cavity 700 provided on the riveting die 100 is correspondingly provided with the riveting feeding channel 900.

[0037] Because the specifications and positions of the rivet nuts that need to be riveted are different, multiple rivet feeding channels 900 are set on the riveting upper mold 200, and each rivet feeding channel 900 corresponds to a pressure component. The pressure component can be a hydraulic cylinder or a pneumatic cylinder, etc. The movable end of the pressure component can penetrate into the rivet feeding channel 900 and press the rivet nut on the upper end face of the hot-formed steel into the hot-formed steel.

[0038] The riveting die 100 is provided with a material discharge cavity 700, and excess material enters into the material discharge cavity 700; each feeding channel 900 corresponds to a material discharge cavity 700.

[0039] In an optional embodiment, a wear-resistant layer is sprayed inside the discharge cavity 700; the wear-resistant layer has a thickness of 50 μm and is made of WC-Co.

[0040] A wear-resistant layer of WC-Co material with a thickness of 50μm is sprayed inside the blanking cavity 700; this ensures the bearing guidance for material flow during the riveting process.

[0041] The rivet feeding device 400 can move the press-fit screw into the corresponding rivet feeding channel 900; the rivet feeding device 400 can be a robot or other clamping component, which clamps the press-fit nut and puts it into the rivet feeding channel 900.

[0042] In an optional implementation, the riveting depth is 1.2 times the thickness of the hot-formed steel.

[0043] In an optional embodiment, a positioning post 800 is provided on the upper end surface of the side wall of the lower riveting cavity 600 of the riveting die 100. The lower end face of the side wall of the upper riveting cavity 120 of the riveting mold 200 is provided with a positioning hole 110 corresponding to the positioning post 800. To ensure precise assembly of the upper riveting die 200 and the lower riveting die 100, multiple positioning pins 800 are provided on the lower riveting die 100, and multiple positioning holes 110 are provided on the upper riveting die 200. By inserting the positioning pins 800 into the positioning holes 110, the upper riveting die 200 and the lower riveting die 100 are precisely assembled, ensuring the accurate pressing position of the subsequent riveting nut.

[0044] The hot-formed steel riveting mold provided by this invention can heat the hot-formed steel and place it in the austenitic single-phase region. Then, the rivet feeding device 400 places the rivet nut on the upper surface of the hot-formed steel. When the hot-formed steel is in the austenitic state, the pressure device 300 presses the rivet nut into the hot-formed steel, solving the problem that hot-formed steel parts cannot be riveted due to their high hardness at room temperature. This achieves a metallurgical-mechanical composite connection of hot-formed steel in the plastic state. Subsequently, bolt holes can be pre-drilled on the integrated light alloy die casting to connect the two material parts with rivet nuts and bolts, increasing the selection of lightweight materials for the vehicle body. During the austenitization stage of the hot forming process, when the material flow stress drops to 80-120 MPa, the system implements in-mold riveting via pressure device 300. This technology has achieved a breakthrough in solving the puncture problem of 500 HV martensitic matrix, reducing the riveting forming force from 25 kN in the traditional cold process to 1.8-2.5 kN. Metallographic analysis confirms that hot riveting can induce dynamic recrystallization of the matrix grains, forming an ultrafine grain structure of 5-8 μm, further enhancing the interfacial bonding strength.

[0045] Traditional projection welding requires pre-drilling holes in hot-formed steel using laser cutting to facilitate the welding of projection welding nuts. However, this invention eliminates the need for pre-drilling holes in hot-formed parts using laser cutting. The entire connection process is completed during hot forming, greatly simplifying the processing flow and saving costs.

[0046] Traditional connection methods for two materials result in unstable connection quality and insufficient compatibility in the connection area. By pressing rivet nuts into the hot-formed steel parts, the connection strength is improved. Subsequently, bolt holes can be pre-drilled in the integrated lightweight alloy die-cast part, allowing the two material parts to be connected to the bolts via rivet nuts, thus increasing the selection of lightweight materials for the vehicle body.

[0047] Reference Figure 3The present invention provides a press-fit connection method based on the hot-formed steel riveting mold described in the foregoing embodiments, comprising the following steps: S1. Place the hot-formed steel into the hot-formed steel riveting mold; S2. Start heating and heat the hot-formed steel to the austenitic single-phase region and hold it at that temperature; S3, The rivet feeding device 400 places the rivet nut into the designated position on the hot-formed steel; S4. Pressure device 300 presses the rivet nut into the hot-formed steel. S5. After the mold is closed and cooled, the hot-formed steel is removed from the hot-formed steel riveting mold.

[0048] In an optional implementation, in step S2, the heat preservation time is calculated as 0.25×t^(1 / 2), where t is the thickness of the hot-formed steel in mm.

[0049] In an optional implementation, in step S5, the mold closing cooling process is divided into multiple cooling zones, each with a different cooling rate.

[0050] The integrated processing within the mold throughout the entire process breaks through the bottleneck of hot-formed steel connection technology and eliminates the "laser hole cutting" in the traditional projection welding connection process: Under the traditional connection technology, after the steel is hot-formed, it needs to be placed on a laser cutting platform for processing to cut out the pre-made holes required for projection welding. With the existing solution, the steel plate is integrated with the rivet nut during the hot forming process, eliminating the laser cutting step, greatly shortening the processing process and saving processing costs.

[0051] Avoiding the heat-affected zone generated by traditional projection welding: Traditional projection welding creates HAZ areas, resulting in a 20-30% decrease in local hardness. In contrast, this technology has no heat input during the connection process, and the uniformity of hardness distribution on the substrate surface is improved by about 5%.

[0052] The projection weld nut of hot-formed steel plate is connected to the steel plate by welding, but the welding quality is unstable, resulting in uncontrollable overall connection quality. However, under the short-process press riveting technology, the connection quality of the riveted joint is more stable and the joint strength is higher. The shear strength of the press riveted joint reaches 325±15MPa, which is 73% smaller than the strength standard deviation of the projection welded joint (210±40MPa), and the coefficient of variation is reduced from 19% to 4.6%.

[0053] By heating to austenitize and soften the matrix, the press-fitting force can be reduced from 24 kN to 2.2 kN, a reduction of 90.8%. This solves the problem that hot-formed steel, due to its high matrix hardness, cannot be effectively press-fitted, greatly improving connection quality and enhancing safety. It also expands compatibility with multiple materials and breaks through the strength limits of traditional SPR processes.

[0054] Taking the in-mold riveting connection of the B-pillar reinforcement and its connection with the magnesium alloy side impact beam as an example, the material of the hot-formed steel riveting mold is H13 hot work die steel, which is surface treated, and K-type thermocouples are embedded at key points of the mold to detect the temperature change inside the mold in real time.

[0055] A 1.6mm Usibor® 1500 steel was placed in a hot-forming steel riveting mold, and then the steel was heated to the austenitic single-phase region (room temperature → 850±20℃, heating rate 10℃ / s; 850±20℃ → 930℃±10℃, heating rate 5℃ / s). The temperature range was determined by the complete dissolution of carbides in the matrix structure to complete austenitization. The temperature was then held for 126s to reduce the hardness of local areas.

[0056] After the steel softens, the conveying device is started. The rivet nut material is AlSi7Mg with a size of Φ5mm. The nut is fed to the softened steel surface. The pressure device 300 intervenes, the riveting temperature is 760±10℃, the pressing force is 2.2-2.8kN, the pressing speed is 22mm / s, and the holding time is 0.5s. The rivet nut is pressed into the softened steel to complete the riveting.

[0057] After riveting, closed-mold cooling is adopted, and the cooling medium is nitrogen-water mist mixture. The temperature drops from 930℃ to 500℃ at a cooling rate of 25℃ / s (to avoid bainite transformation). When the temperature drops from 500℃ to 80℃, the cooling rate is 15℃ / s (to suppress residual austenite). The hot-formed steel gradually cools down as the mold cools down, and finally a part with a press-fit nut integrated with the hot-formed steel is obtained.

[0058] Holes of a specific size are machined into the magnesium alloy side impact beam. Then, bolts of the corresponding size are inserted into the hot-formed steel B-pillar with press-fit nuts. Finally, the bolts pass through the holes on the magnesium alloy side impact beam and are connected to the B-pillar.

[0059] Compared to common projection welding, the materials in the embodiments of this invention undergo austenitizing heating, resulting in recrystallization and growth at the joint, leading to a finer and more uniform microstructure. Furthermore, the integrated production in the hot forming mold solves the problem of hot-formed steel parts being unable to be punctured at room temperature, thus realizing a short-process riveting process for hot-formed steel parts.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hot-forming steel riveting die, characterized in that, It includes an upper riveting mold (200) and a lower riveting mold (100). The lower riveting mold (100) has a lower riveting cavity (600) on its upper end surface, and at least one material dropping hole cavity (700) is provided in the lower riveting cavity (600). The upper riveting cavity (120) is provided on the lower end face of the riveting upper mold (200). The upper riveting cavity (120) is provided in correspondence with the lower riveting cavity (600) and forms a riveting cavity for placing hot-formed steel. A rivet feeding device (400) is provided on the press-fit mold (200), the rivet feeding device (400) being used to place the press-fit nut on the hot-formed steel; A pressure device (300) for pressing the rivet nut into the hot-formed steel is provided on the rivet upper mold (200), and a heating device (500) for heating the hot-formed steel is provided inside the rivet upper mold (200).

2. The hot-forming steel riveting die according to claim 1, characterized in that, The upper riveting mold (200) is provided with at least one feeding channel (900), the lower end of the feeding channel (900) is connected to the upper riveting cavity (120), and the feeding channel (900) is used to feed the riveting nut to the upper surface of the hot-formed steel.

3. The hot-forming steel riveting die according to claim 2, characterized in that, The pressure device (300) includes at least one pressure component, each of the feeding channels (900) corresponds to one pressure component, and the movable end of the pressure component is inserted into the feeding channel (900) and presses the rivet nut into the hot-formed steel.

4. The hot-forming steel riveting die according to claim 3, characterized in that, The blanking hole (700) provided on the riveting die (100) is correspondingly provided with the riveting feeding channel (900).

5. The hot-forming steel riveting die according to claim 4, characterized in that, The material discharge cavity (700) is coated with a wear-resistant layer; the wear-resistant layer has a thickness of 50μm and is made of WC-Co.

6. The hot-forming steel riveting die according to claim 4, characterized in that, The riveting depth is 1.2 times the thickness of the hot-formed steel.

7. The hot-formed steel riveting die according to claim 1, characterized in that, A positioning post (800) is provided on the upper end surface of the side wall of the lower riveting cavity (600) of the riveting mold (100). The lower end face of the side wall of the upper riveting cavity (120) of the riveting mold (200) is provided with a positioning hole (110) corresponding to the positioning post (800).

8. A press-fit connection method, based on the hot-formed steel riveting mold as described in claim 1, characterized in that, Includes the following steps: S1. Place the hot-formed steel into the hot-formed steel riveting mold; S2. Start heating and heat the hot-formed steel to the austenitic single-phase region and hold it at that temperature; S3, The rivet feeding device (400) places the rivet nut into the designated position on the hot-formed steel; S4, The pressure device (300) presses the rivet nut into the hot-formed steel; S5. After the mold is closed and cooled, the hot-formed steel is removed from the hot-formed steel riveting mold.

9. The press-fit connection method according to claim 8, characterized in that, In step S2, the heat preservation time is calculated as 0.25×t^(1 / 2), where t is the thickness of the hot-formed steel in mm.

10. The press-fit connection method according to claim 8, characterized in that, In step S5, the mold closing cooling process is divided into multiple cooling zones, each with a different cooling rate.

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