A mechanical connection method of hot-formed steel and light alloy

Abstract

The present invention discloses a method for mechanically connecting hot-formed steel and light alloy, which relates to the technical field of connecting different materials in vehicles, and solves the technical problem in the related art that the hardness of hot-formed parts is high, which leads to difficulty in piercing when mechanically connected through connectors, and affects the structural strength of the connectors. It includes: acting on the preset position of the steel material through a cooling device, and acting on other positions of the steel material except the preset position through a heating device; moving the steel material to a hot-forming device to obtain hot-formed steel; overlapping the part of the hot-formed steel corresponding to the preset position with the part to be connected of the light alloy and mechanically connecting them through a connector. The method can smoothly carry out the mechanical connection between the hot-formed part and the light alloy, and no upsetting damage occurs when the connector penetrates the hot-formed part, and the joint has good mechanical properties and energy absorption characteristics.

Description

Technical Field

[0001] The invention relates to the technical field of vehicle dissimilar material connection, and in particular to a mechanical connection method of hot-formed steel and light alloy. Background Art

[0002] In order to meet the need for lightweight car bodies, the application of high-strength steel and lightweight materials has become a development trend, and high-strength steel includes hot-formed steel plates. In car body manufacturing, in order to connect lightweight materials such as automotive steel and aluminum alloys, the traditional resistance spot welding method is difficult to achieve due to the formation of intermetallic compounds between dissimilar metals; mechanical connection methods are chosen to achieve effective connection of dissimilar materials in the car body, such as self-piercing riveting SPR, hot melt self-tapping FDS, etc.

[0003] Mechanical connections such as steel and aluminum, because the strength of the parts after hot forming is close to that of the connecting parts, it becomes very difficult to pierce the hot-formed steel. For example, in the process of using high-strength rivets to pierce the hot-formed steel in the SPR process, quality problems such as rivet leg upsetting and rivet cracking will occur. Summary of the invention

[0004] The present application provides a method for mechanically connecting hot-formed steel and light alloy, which solves the technical problem in the related art that the high hardness of hot-formed parts leads to difficulty in piercing during mechanical connection through connecting pieces, thus affecting the structural strength of the connecting pieces.

[0005] The present application provides a method for mechanically connecting hot-formed steel and light alloy, comprising: acting on a preset position of the steel material through a cooling device, and acting on other positions of the steel material except the preset position through a heating device; moving the steel material after being acted upon by the cooling device and the heating device to a hot-forming device to obtain hot-formed steel; overlapping the portion of the hot-formed steel corresponding to the preset position with the portion of the light alloy to be connected and mechanically connecting them through a connecting piece.

[0006] Optionally, the lightweight alloy comprises an aluminum alloy.

[0007] Optionally, moving the steel material to the hot forming device includes: moving the steel material out of the heating device, moving the cooling device along with the steel material from the heating device to the hot forming device, moving the cooling device out of the steel material adjacent to the hot forming device, and moving the steel material into the hot forming device.

[0008] Optionally, the cooling device acts on a preset position of the steel material, and the heating device acts on other positions of the steel material except the preset position, including forming a cooling zone at the preset position of the steel material, a transition zone adjacent to the cooling zone, and a heating zone outside the transition zone;

[0009] The temperature of the cooling zone is lower than the austenitizing temperature AC1, and the temperature of the heating zone is greater than or equal to the austenitizing temperature AC3.

[0010] Optionally, the temperature of the cooling zone is less than or equal to 400°C.

[0011] Optionally, the cooling device comprises:

[0012] The cooling head contacts the steel material at a preset position;

[0013] A cooling tube, wherein the inner wall of the tube forms a liquid inlet channel;

[0014] The cooling shell is sleeved outside the cooling tube at intervals, the shell of the cooling shell and the outer wall of the cooling tube are enclosed to form a liquid return channel, and the liquid return channel is connected with the liquid inlet channel at the side of the cooling head away from the steel material;

[0015] a cooling pool communicating with the cooling pipe and the cooling shell; and

[0016] The power pump is used to pump the coolant from the cooling pool into the liquid inlet channel of the cooling pipe.

[0017] Optionally, the side of the cooling head away from the steel material is arranged in a concave shape.

[0018] Optionally, the cooling device acts on a preset position of the steel material, including:

[0019] The cooling head is brought into contact with a preset position of the steel material, and pressure is applied so that the pressure between the cooling head and the steel material is greater than or equal to 10 MPa.

[0020] Optionally, the mechanical connection in the mechanical connection through the connector includes self-piercing riveting or hot-melt self-tapping connection.

[0021] Optionally, the cooling device acts on a preset position of the steel material, and the heating device acts on other positions of the steel material except the preset position, including: connecting the cooling device to the steel material, placing the steel material equipped with the cooling device into the heating device and keeping it warm.

[0022] The beneficial effects of the present application are as follows: The present application provides a method for mechanically connecting hot-formed steel and light alloy, which realizes regional temperature differentiation of the heated steel material by cooperating with a cooling device and a heating device, and then sends the steel material into a hot forming device for hot forming. The hardness of the obtained hot-formed steel at a preset position is significantly lower than that at other positions. The preset position is advantageous for piercing the connecting piece, and is advantageous for mechanically connecting the connecting piece to the light alloy. The method is used to smoothly carry out mechanical connection between the hot-formed parts and the light alloy, and no upsetting damage occurs during the penetration of the connecting piece into the hot-formed parts. The method has good joint mechanical properties and energy absorption characteristics, which solves the problem that the hot-formed steel has high strength but is difficult to mechanically connect with other aluminum alloys. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for describing the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention.

[0024] Figure 1 A schematic flow chart of a method for mechanically connecting hot-formed steel and light alloy provided in this application;

[0025] Figure 2 A schematic diagram of the structure of a cooling device involved in the mechanical connection method provided in this application;

[0026] Figure 3 A schematic diagram of the cooling device and the heating device acting on the steel material in the mechanical connection method provided in the present application;

[0027] Figure 4 A schematic structural diagram of a thermoforming device involved in the mechanical connection method provided in this application;

[0028] Figure 5 A schematic diagram of a connecting piece connecting hot-formed steel and a light alloy involved in the mechanical connection method provided in the present application.

[0029] Figure markings: 100-hot forming steel, 110-cooling zone, 120-transition zone, 130-heating zone, 200-light alloy, 300-connecting part, 310-rivet rod, 320-rivet die, 400-cooling device, 410-cooling head, 420-cooling pipe, 421-liquid inlet channel, 430-cooling shell, 431-liquid return channel, 500-heating device, 600-hot forming device, 610-upper mold, 611-upper mold cooling pipeline, 620-lower mold, 621-lower mold cooling pipeline. DETAILED DESCRIPTION

[0030] The embodiment of the present application provides a mechanical connection method between hot-formed steel and light alloy, thereby solving the technical problem in the related art that the hardness of hot-formed parts is high, which leads to difficulty in piercing during mechanical connection through connecting pieces and affects the structural strength of the connecting pieces.

[0031] The technical solution in the embodiment of the present application is to solve the above technical problems, and the overall idea is as follows:

[0032] A method for mechanically connecting hot-formed steel and light alloy comprises: acting on a preset position of the steel material through a cooling device, and acting on other positions of the steel material except the preset position through a heating device; moving the steel material to a hot-forming device to obtain hot-formed steel; overlapping the portion of the hot-formed steel corresponding to the preset position with the portion of the light alloy to be connected and mechanically connecting them through a connecting piece.

[0033] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0034] Example 1

[0035] Please refer to Figure 1 The present embodiment discloses a method for mechanically connecting a hot-formed steel 100 and a light alloy 200, comprising: acting on a preset position of the steel material through a cooling device 400, and acting on other positions of the steel material except the preset position through a heating device 500; moving the steel material to a hot-forming device 600 to obtain the hot-formed steel 100; overlapping the portion of the hot-formed steel 100 corresponding to the preset position with the portion to be connected of the light alloy 200 and mechanically connecting them through a connecting piece 300.

[0036] Specifically, the cooling device 400 cooperates with the heating device 500 to achieve regional temperature differentiation of the heated steel material, and then the steel material is sent to the hot forming device 600 for hot forming. The hardness of the obtained hot-formed steel at the preset position is significantly lower than that at other positions. The preset position is advantageous for the piercing of the connector 300, and is advantageous for the mechanical connection with the light alloy 200 through the connector 300. The method is used to smoothly carry out the mechanical connection between the hot-formed part and the light alloy 200. The connector 300 does not suffer from upsetting damage during the penetration of the hot-formed part, and has good joint mechanical properties and energy absorption characteristics, thereby solving the problem that the hot-formed steel 100 has high strength but is difficult to mechanically connect with other aluminum alloys.

[0037] The hot-formed steel 100 in this embodiment includes uncoated hot-formed steel 100 , AlSi-coated hot-formed steel 100 , and galvanized hot-formed steel 100 .

[0038] The light alloy 200 in this embodiment includes aluminum alloy, magnesium alloy and titanium alloy.

[0039] Regarding the above-mentioned cooling device 400 acting on the preset position of the steel material, and the heating device 500 acting on other positions of the steel material except the preset position, please refer to Figure 2 and Figure 3 The steel material forms a cooling zone 110 at a preset position, a transition zone 120 adjacent to the cooling zone 110, and a heating zone 130 outside the transition zone 120, wherein the temperature of the transition zone 120 continuously changes between the temperature of the cooling zone 110 and the temperature of the heating zone 130. It can be understood that the cooling zone 110 portion of the steel material is used for mechanical connection after hot forming.

[0040] Regarding the above-mentioned cooling device 400 acting on the preset position of the steel material, and the heating device 500 acting on the steel material at other positions other than the preset position, the preliminary result is to form differential temperatures in the cooling zone 110, the transition zone 120, and the heating zone 130. It is widely understood in this method that the order of cooling and heating is not limited. However, considering that all areas of the steel material are fully heated and then partially cooled, there are defects that affect the structure and are difficult to control. This method preferably adopts the following solution: first turn on the cooling device 400 and connect the cooling device 400 to the steel material; then place the steel material equipped with the cooling device 400 into the heating device 500, such as from Figure 2 to Figure 3 process.

[0041] Regarding the heating device 500 acting on other positions of the steel material except the preset position, the steel material and the cooling device 400 are kept warm in the heating device 500 for a period of time until the temperatures of the heating zone 130 and the cooling zone 110 meet the relevant requirements.

[0042] Regarding the cooling zone 110, transition zone 120 and heating zone 130, in certain possible implementation schemes, the temperature of the cooling zone 110 is limited to be lower than the austenitizing temperature AC1, and a softening zone with low hardness is subsequently formed; the temperature of the heating zone 130 is greater than or equal to the austenitizing temperature AC3, and a hardening zone with high hardness is subsequently formed to achieve the purpose of improving strength. This ensures that the hot forming is carried out smoothly, and the preset position, that is, the corresponding part of the cooling zone 110 in the hot-formed steel 100 has a smaller hardness, laying the foundation for the subsequent mechanical connection.

[0043] Optionally, the temperature of the cooling zone 110 is less than or equal to 400° C., so that the hardness of the preset position, ie, the corresponding portion of the cooling zone 110 in the hot-formed steel 100, is substantially the same as the hardness of the original parent material.

[0044] On the other hand, by minimizing the temperature of the cooling zone 110, the width of the transition zone 120 is less than or equal to 5 mm, so that the mechanical connection between the hot-formed material and the lightweight alloy 200 can be achieved, and the influence of local softening on the overall structure can be controlled to a minimum level.

[0045] Considering that the temperature difference of the steel material must be maintained until the moment before entering the hot forming device 600, the mechanical connection method of this embodiment can be adopted: moving the steel material to the hot forming device 600 specifically includes: moving the steel material out of the heating device 500, moving the cooling device 400 along with the steel material from the heating device 500 to the hot forming device 600, moving the cooling device 400 out of the steel material near the hot forming device 600, and moving the steel material into the hot forming device 600. Through this method, the disadvantageous problem that the cooling zone 110 is affected again by the heating zone 130 before hot forming and the temperature rises is improved.

[0046] It should be noted that the method involves the movement of the steel material and even the cooling device 400. In some possible implementation schemes, a clamping device is configured to provide movement support. The clamping device is broadly understood to be a separate device setting, or to give the cooling device 400 a clamping function, that is, the clamping device is integrated with the cooling device 400, etc.

[0047] Similar to keeping warm in the heating device 500, during the action stage of the hot forming device 600, for example, the steel material is taken out from the heating device 500, quickly moved to the hot forming device 600 together with the cooling device 400, and hot forming is performed after the cooling device 400 is quickly removed. After keeping warm for a certain period of time, the steel material is taken out to achieve the purpose of low hardness in the cooling zone 110, high hardness in the heating zone 130, and a narrow transition zone 120.

[0048] Regarding the thermoforming device 600, in certain embodiments, as Figure 4 As shown, it includes an upper mold and a lower mold, and is respectively provided with an upper mold cooling pipeline 611 and a lower mold cooling pipeline 621, and the cooling pipeline 420 has a coolant. Figure 4 As shown, the working surfaces of the upper die and the lower die are both arranged in a plane.

[0049] As for the heating device 500, a heating furnace may be selected.

[0050] The mechanical connection method of the connector 300 includes self-piercing riveting or hot-melt self-tapping connection. Figure 5 As shown, the portion corresponding to the preset position in the hot-formed steel 100 is overlapped with the portion to be connected of the light alloy 200, and is mechanically connected through a connecting piece 300. A rivet rod 310 is arranged above the connecting piece 300, and a rivet mold 320 is arranged on the other side of the light alloy 200 to assist in the smooth installation of the connecting piece 300.

[0051] The opposite surface of the rivet die 320 and the light alloy 200 may be optionally provided with a flat surface; or optionally provided with a groove, wherein the groove adapts the connector 300 to extend into the light alloy 200 and deform, resulting in a local volume change, surface or protrusion of the light alloy 200.

[0052] Example 2

[0053] Based on the mechanical connection method of hot-formed steel 100 and light alloy 200 provided in Example 1, this embodiment further describes the cooling device 400 in the mechanical connection method. Generally speaking, the cooling device 400 reduces the temperature by taking away heat through a cooling medium.

[0054] This embodiment provides a cooling device 400, such as Figure 2As shown, it includes a cooling head 410, a cooling pipe 420, a cooling shell 430, a cooling pool and a power pump. The cooling head 410 contacts the preset position of the steel material, the inner wall of the cooling pipe 420 forms a liquid inlet channel 421, the cooling shell 430 is sleeved outside the cooling pipe 420 at intervals, the shell of the cooling shell 430 and the outer wall of the cooling pipe 420 are enclosed to form a liquid return channel 431, the liquid return channel 431 is connected to the liquid inlet channel 421 at the side of the cooling head 410 away from the steel material, the cooling pool is connected to the cooling pipe 420 and the cooling shell 430, and the power pump is used to pump the coolant in the cooling pool into the liquid inlet channel 421 of the cooling pipe 420.

[0055] In detail, the coolant in the cooling pool is pumped into the cooling pipe 420 through the power pump, and then delivered to the preset position of the steel material through the liquid inlet channel 421, so as to realize the temperature reduction control of the preset position of the steel material; and then the coolant flows back to the cooling pool along the liquid return channel 431.

[0056] It can be understood that the temperature of the heating device 500 is greater than the temperature of the coolant in the return liquid channel 431, and the temperature of the coolant in the return liquid channel 431 is greater than the temperature of the coolant in the inlet channel 421, so that the return liquid channel 431 has the effect of separating the inlet channel 421 from the external heating device 500, further realizing the "isolation" of the inlet channel 421, avoiding direct heat exchange between the coolant and the high-temperature heating device 500 before entering the preset position of the steel material, thereby improving the cooling efficiency.

[0057] The coolant may consist of water.

[0058] Optionally, the outside of the cooling shell 430 is covered with a heat insulating material, and / or the outside of the cooling pipe 420 is covered with a heat insulating material to reduce heat exchange between the cooling shell 430 and the outside.

[0059] From the perspective of thermal energy, a heat sink may be optionally installed in the cooling pool; the cooling pool may drain the original coolant or add new coolant according to the temperature of the coolant therein.

[0060] Optionally, the temperature of the coolant in the cooling pool is controlled to be less than 30°C.

[0061] like Figure 2 As shown, optionally, the side of the cooling head 410 away from the steel material is recessed to form a groove-like configuration, and the groove can also be understood as a part of the coolant circulation loop. The recessed configuration is helpful in increasing the contact area between the coolant and the steel material, which is beneficial to the cooling efficiency.

[0062] Optionally, the side wall of the groove of the cooling head 410 is in the shape of a truncated cone with a large opening end and a small middle, and is used for mating and connecting with the cooling shell 430 .

[0063] Optionally, the contact surface between the cooling head 410 and the steel material is circular, and the diameter of the cooling head 410 is controlled to be greater than or equal to 6 mm. The cooling head 410 is made of a material with high thermal conductivity and high melting point, such as a CuCrZr alloy, which has a softening temperature of 550°C and a thermal conductivity of 330 W / m·k. The corresponding cooling shell 430 and the cooling pipe 420 are made of a material with low thermal conductivity, which can improve the disadvantage that the coolant is overheated before being transported to the cooling head 410.

[0064] Optionally, the cooling device 400 acts on the preset position of the steel material, including: contacting the cooling head 410 with the preset position of the steel material, and applying pressure, the pressure being such that the pressure between the cooling head 410 and the steel material is greater than or equal to 10 MPa. Thus, the cooling head 410 and the steel material are attached under a certain pressure, and the heat conduction between the two is sufficient to ensure the cooling effect.

[0065] Alternatively, if Figure 2 As shown, the cooling devices 400 are symmetrically arranged on both sides of the preset position of the steel material, which is beneficial to increase the cooling effect.

[0066] In combination with the cooling device 400 provided in this embodiment, the method for mechanically connecting the hot-formed steel 100 and the light alloy 200 includes: using the cooling device 400 to act on a preset position of the steel material, and using the heating device 500 to act on other positions of the steel material except the preset position, including:

[0067] First, the cooling device 400 is turned on, and the coolant is circulated repeatedly; then, the cooling device 400 is brought into contact with a preset position of the steel material, and a certain pressure is applied; the steel material with the cooling device 400 installed is placed in the heating device 500 and kept warm.

[0068] Example 3

[0069] Based on the mechanical connection method of the hot-formed steel 100 and the light alloy 200 in Embodiment 1 and Embodiment 2, this embodiment provides a comparative example for illustration.

[0070] Test example:

[0071] The steel material is 22MnB5, with a thickness of 1.2 mm, and both the upper and lower surfaces have aluminum silicon coatings.

[0072] Turn on the cooling device 400, the coolant flow rate is 6L / min, the coolant flows through the cooling pipe 420, the cooling head 410, and the cooling shell 430 and then flows back to the cooling pool, and the cycle is repeated. The temperature of the coolant in the cooling pool is less than 30°C;

[0073] The cooling devices 400 arranged in pairs are brought into contact with the preset positions of the steel material and a certain pressure of 4 kN is applied;

[0074] The cooling device 400 is also a clamping device. The steel material equipped with the cooling device 400 is placed in the heating device 500 and kept warm for 5 minutes until the temperature of the heating zone 130 of the steel material is ≥ the austenitizing temperature AC3, the temperature of the cooling zone 110 is < the austenitizing temperature AC1, and the temperature of the transition zone 120 changes continuously between the temperature of the cooling zone 110 and the temperature of the heating zone 130; the heating device 500 is a heating furnace, and the furnace temperature is set to 930°C; due to the presence of the cooling device 400, the temperature of the cooling zone 110 is ≤400°C during the heating process;

[0075] Under the clamping of the cooling device 400, the steel material is taken out from the heating device 500, and quickly moved with the cooling device 400 to between the upper die and the lower die of the hot forming device 600. The time from the heating device 500 to the delivery to the die is about 10s; the cooling device 400 is quickly removed and hot forming is performed, and the steel material is taken out after being kept warm for a certain period of time; at this time, the cooling zone 110 has low hardness, the heating zone 130 has high hardness, and the transition zone 120 is narrow; after hot forming, the hardness of the heating zone 130 is about 500HV, the hardness of the cooling zone 110 is about 260HV, the hardness of the transition zone 120 gradually changes between 500HV and 260HV, and the width of the transition zone 120 is 4.5mm;

[0076] The cooling zone 110 of the hot-formed steel material is overlapped with the part to be connected of the light alloy 200 for mechanical connection; the light alloy 200 is a 5082 aluminum alloy with a thickness of 2.0 mm, and the connection method adopted is self-piercing riveting;

[0077] Through the tensile test of the riveted joint, the maximum tensile force is 7.5kN and the energy absorption is 30J;

[0078] Through metallographic observation of the riveted joints, it was found that the rivets had no upsetting phenomenon.

[0079] Comparative Example:

[0080] The steel material is still 1.2 mm 22MnB5, and compared with the embodiment of the present invention, except that there is no cooling device 400, the rest is the same;

[0081] Therefore, the steel material after hot forming has no cooling zone 110 and transition zone 120, but only a heating zone 130, and its hardness is 500 HV;

[0082] Similarly, self-piercing riveting is performed with 2.0 mm thick 5082 aluminum alloy, and the riveting equipment is the same;

[0083] Through metallographic observation, the rivet was upset during the riveting process;

[0084] Through the tensile test, the maximum tensile force of the joint is 7.0kN and the energy absorption is 25J.

[0085] By comparing the test examples with the comparative examples, the maximum tensile force and energy absorption of the local soft zone joint obtained by the present invention are significantly higher than those of the direct riveting process, and the rivet upsetting failure phenomenon will not occur.

[0086] In the above embodiments, only the self-piercing riveting case is discussed, but based on the same implementation principle, it is also applicable to mechanical connection methods such as hot-melt self-tapping that need to pierce ultra-high-strength steel.

[0087] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0088] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for mechanically connecting hot-formed steel and light alloy, characterized in that: The mechanical connection method comprises: The cooling device acts on a preset position of the steel material, and the heating device acts on other positions of the steel material except the preset position; Moving the steel material after being acted upon by the cooling device and the heating device to a hot forming device to obtain the hot formed steel; Overlapping the portion of the hot-formed steel corresponding to the preset position with the portion of the light alloy to be connected and mechanically connecting them through a connecting piece; Wherein, the cooling device comprises: A cooling head in contact with the preset position of the steel material; A cooling tube, wherein the inner wall of the tube forms a liquid inlet channel; A cooling shell is sleeved outside the cooling tube at intervals, the shell of the cooling shell and the outer wall of the cooling tube are enclosed to form a liquid return channel, and the liquid return channel is connected with the liquid inlet channel at the side of the cooling head away from the steel material; a cooling pool in communication with the cooling pipe and the cooling shell; and A power pump is used to pump the coolant in the cooling pool into the liquid inlet channel of the cooling pipe.

2. The mechanical connection method according to claim 1, characterized in that: The light alloy includes an aluminum alloy.

3. The mechanical connection method according to claim 1, characterized in that: The step of moving the steel material to a hot forming device comprises: The steel material is moved out of the heating device, and the cooling device is moved along with the steel material from the heating device to the hot forming device. The cooling device is moved out of the steel material adjacent to the hot forming device, and the steel material is moved into the hot forming device.

4. The mechanical connection method according to claim 1, characterized in that: The cooling device acts on the preset position of the steel material, and the heating device acts on other positions of the steel material except the preset position, including forming a cooling zone at the preset position of the steel material, a transition zone adjacent to the cooling zone, and a heating zone outside the transition zone; The temperature of the cooling zone is lower than the austenitizing temperature AC1, and the temperature of the heating zone is greater than or equal to the austenitizing temperature AC3.

5. The mechanical connection method according to claim 4, characterized in that: The temperature of the cooling zone is less than or equal to 400°C.

6. The mechanical connection method according to claim 1, characterized in that: The cooling head is arranged in a concave shape on the side away from the steel material.

7. The mechanical connection method according to claim 1, characterized in that: The cooling device acts on the preset position of the steel material, comprising: The cooling head is brought into contact with the preset position of the steel material, and pressure is applied thereto, wherein the pressure between the cooling head and the steel material is greater than or equal to 10 MPa.

8. The mechanical connection method according to claim 1, characterized in that: The mechanical connection in the mechanical connection through the connecting piece includes self-piercing riveting or hot-melt self-tapping connection.

9. The mechanical connection method according to claim 1 or 4, characterized in that: The cooling device acts on a preset position of the steel material, and the heating device acts on other positions of the steel material except the preset position, including: The cooling device is connected to the steel material, and the steel material equipped with the cooling device is placed in the heating device and kept warm.

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