A hot forming method of a zinc-based plated hot forming steel sheet using air-cooling die forming

Abstract

The present application relates to the technical field of steel plate hot forming, and particularly relates to a hot forming method of zinc-based plating layer hot forming steel plate by air-cooled die forming; after the zinc-based plating layer hot forming steel plate is heated by a heating furnace, the zinc-based plating layer hot forming steel plate is cooled in an air-cooled stamping die with air blowing cooling function to completely solidify the plating layer, and then is formed by the air-cooled stamping die. The heated steel plate is cooled by the stamping die with the air cooling function, the temperature of the zinc-based plating layer hot forming steel plate can be accurately controlled, the surface plating layer is solidified, the liquid phase brittleness is inhibited, the additional H element is reduced, and the risk of delayed cracking is reduced.

Description

Technical Field

[0001] This invention relates to the field of hot forming technology for steel plates, and more particularly to a hot forming method for zinc-based coated steel plates using air-cooled die forming. Background Technology

[0002] Under the combined requirements of lightweighting and safety in automobiles, the application of hot-formed steel is becoming increasingly widespread. By heating steel sheets to above the austenitic region and quenching them in a mold, high-strength parts with martensitic structures and strengths exceeding 1500 MPa can be obtained. Traditional bare steel sheets have a dense oxide layer on their surface, requiring shot peening before proper painting. Furthermore, the oxide scale causes significant wear on the mold, and the thick decarburized layer on the steel sheet surface affects the final strength. To overcome these shortcomings of bare steel sheets, the industry uses aluminum-silicon coated sheets for hot pressing. However, the process cost of hot-dip aluminum-silicon coated sheets is high, and the Al-Fe-Si coating after hot forming is brittle, with micro-cracks visible everywhere on the part surface, essentially losing its corrosion resistance.

[0003] In recent years, pre-plating Zn-based coatings on steel plates has become a mainstream technology. At high temperatures, the Zn layer preferentially forms a dense ZnO layer, effectively preventing substrate oxidation and the evaporation of zinc at high temperatures. Furthermore, the formed Zn-Fe phase and residual Zn both possess anodic sacrificial properties; even with cracks within the coating, the Zn-based phase can provide corrosion protection. However, Zn's low melting point means that the coating completely melts when heated above 900℃, even exhibiting vaporization (the melting point of Zn-Fe alloys is approximately 780–785℃ in equilibrium). During mold forming, this easily triggers liquid metal embrittlement (LME), where molten Zn-based coating penetrates along the austenite grain boundaries of the substrate, inducing microcracks under forming stress, leading to a sharp decrease in the strength of parts under impact and other conditions.

[0004] To address these issues, early forming methods involved cooling the material completely below the zinc layer's solidification point (416°C). However, at low temperatures, the material's strength increases while its ductility decreases, making it unsuitable for complex components. Furthermore, high-strength steel plates with high martensite content could not be obtained for the substrate. Chinese invention patent CN105492134B discloses "A method for manufacturing hot-pressed components and hot-pressed components," the core of which involves heating the steel plate to the austenitic region and then cooling the surface to below 400°C, while maintaining the core temperature above 500°C, under which hot pressing is performed. Chinese patent application CN120169908A discloses "A processing control method and related equipment for hot-formed steel parts," which uses air cooling to completely solidify the zinc layer before stamping. Other methods include water bath cooling or mist cooling to solidify the coating, the core idea being to ensure complete solidification of the coating before hot stamping to prevent liquid phase embrittlement during stamping.

[0005] However, existing precooling methods, whether air cooling, water bath cooling, or mist cooling, all face a significant problem: uneven overall part temperature, especially a noticeable temperature difference between the edges and center of the part. This leads to uneven overall part performance and can even cause micro-cracks due to uneven residual stress. Furthermore, various cooling media are difficult to quantify and control. Automotive parts are diverse in shape and size, making it difficult to adjust the amount of either gas or liquid in real time, and achieving uniform plate temperature control is even more challenging. Currently, water bath cooling is widely used in China. However, during the water bath process, the high temperature of the steel plate causes the liquid water to vaporize instantly. The resulting bubbles can easily seep back into the plate, increasing the hydrogen content of the part and significantly impacting its resistance to delayed cracking. Moreover, the process requires additional water bath tanks and places certain requirements on the site.

[0006] In summary, for the hot stamping forming process of zinc-based coated steel sheets, it is extremely important to develop a method that can precisely control the cooling effect after heating. The goal is to achieve controllable cooling parameters, effectively suppress LME (liquid metal brittleness), and reduce the risk of delayed cracking. Summary of the Invention

[0007] This invention provides a hot forming method for zinc-based coated hot-formed steel sheets using air-cooled molding. The method employs a stamping die with air-cooling function to cool and reduce the temperature of the heated steel sheet, which can precisely control the temperature of the zinc-based coated hot-formed steel sheet, ensuring that the surface coating is cured while inhibiting the occurrence of liquid phase embrittlement, and reducing the infiltration of additional H elements, thereby reducing the risk of delayed cracking.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A hot forming method for zinc-based coated hot-formed steel sheets using air-cooled molding involves heating the zinc-based coated hot-formed steel sheet in a heating furnace, then cooling it in an air-cooled stamping die with a blowing cooling function until the coating is completely solidified, and finally stamping it into shape by the air-cooled stamping die.

[0009] The zinc-based coated hot-formed steel sheet has a double-sided zinc coating thickness of 40–200 g / m. 2 .

[0010] The zinc-based coated hot-formed steel sheet is heated at 800–910°C in the heating furnace for 3–10 minutes.

[0011] The time for the zinc-based coated hot-formed steel sheet to be transferred from the heating furnace to the air-cooled stamping die is controlled within 5 to 15 seconds (this process has a short-term air-cooling effect).

[0012] The air-cooled stamping die consists of an upper die and a lower die, with multiple air vents evenly arranged inside the upper and lower dies; during air cooling, the zinc-based coated hot-formed steel plate is suspended between the upper and lower dies.

[0013] After the zinc-based coated hot-formed steel sheet exits the heating furnace, it is moved by a robot to the air-cooled stamping die and suspended between the upper and lower dies. After cooling until the coating is completely solidified, the robot places the zinc-based coated hot-formed steel sheet on the lower die and then moves it away. The upper die moves down and cooperates with the lower die to perform hot stamping on the zinc-based coated hot-formed steel sheet.

[0014] The blowing pressure of the air-cooled stamping die is 10-500 mbar, and the air-cooling time is 2-10 s; the temperature of the zinc-based coated hot-formed steel sheet after air cooling is 500-700℃.

[0015] The stamping temperature of the zinc-based coated hot-formed steel sheet is 500–700℃, and the holding time is 5–30s.

[0016] Compared with the prior art, the beneficial effects of the present invention are: Using a stamping die with air cooling function to cool down the heated steel plate can precisely control the temperature of the zinc-based coated hot-formed steel plate, ensuring that the surface coating is cured while inhibiting liquid phase embrittlement, and reducing the infiltration of additional H elements, thus reducing the risk of delayed cracking. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the process of a hot forming method for zinc-based coated hot forming steel sheet using air-cooled molding as described in this invention.

[0018] In the picture: 1. Heating furnace; 2. Zinc-based coated hot-formed steel sheet; 3. Air outlet; 4. Air-cooled stamping die. Detailed Implementation

[0019] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings: like Figure 1 As shown, the present invention describes a hot forming method for zinc-based coated hot-formed steel sheets using air-cooled molding. After the zinc-based coated hot-formed steel sheet 2 is heated in a heating furnace 1, it enters an air-cooled stamping die 4 with a blowing cooling function to cool until the coating is completely solidified, and then is stamped by the air-cooled stamping die 4.

[0020] The zinc-based coated hot-formed steel sheet 2 has a double-sided zinc coating thickness of 40–200 g / m. 2 .

[0021] The zinc-based coated hot-formed steel sheet 2 is heated at a temperature of 800–910°C in the heating furnace 1 for a time of 3–10 minutes.

[0022] The time for the zinc-based coated hot-formed steel sheet 2 to be transferred from the heating furnace 1 to the air-cooled stamping die 4 is controlled within 5 to 15 seconds.

[0023] The air-cooled stamping die 4 consists of an upper die and a lower die, with multiple air vents 3 evenly arranged inside the upper and lower dies; during air cooling, the zinc-based coated hot-formed steel plate 2 is suspended between the upper and lower dies.

[0024] After the zinc-based coated hot-formed steel sheet 2 exits the heating furnace 1, it is moved by a robot to the air-cooled stamping die 4 and suspended between the upper and lower dies. After cooling until the coating is completely solidified, the robot places the zinc-based coated hot-formed steel sheet 2 on the lower die and then moves it away. The upper die moves down and cooperates with the lower die to perform hot stamping on the zinc-based coated hot-formed steel sheet 2.

[0025] The blowing pressure of the air-cooled stamping die 4 is 10-500 mbar, and the air-cooling time is 2-10 s; the temperature of the zinc-based coated hot-formed steel sheet after air cooling is 500-700℃.

[0026] The stamping temperature of the zinc-based coated hot-formed steel sheet is 500–700℃, and the holding time is 5–30s.

[0027] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0028]

Example

[0029] Table 1

[0030] Table 2

[0031] Table 3

[0032] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for hot forming zinc-based coated steel sheets using air-cooled die forming, characterized in that, After being heated in a heating furnace, the zinc-based coated hot-formed steel sheet is cooled in an air-cooled stamping die with a blowing cooling function until the coating is completely cured, and then stamped into shape by the air-cooled stamping die.

2. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, The zinc-based coated hot-formed steel sheet has a double-sided zinc coating thickness of 40–200 g / m. 2 .

3. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, The zinc-based coated hot-formed steel sheet is heated at 800–910°C in the heating furnace for 3–10 minutes.

4. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, The time for the zinc-based coated hot-formed steel sheet to be transferred from the heating furnace to the air-cooled stamping die is controlled within 5 to 15 seconds.

5. A hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1 or 4, characterized in that, The air-cooled stamping die consists of an upper die and a lower die, with multiple air vents evenly arranged inside the upper and lower dies; during air cooling, the zinc-based coated hot-formed steel plate is suspended between the upper and lower dies.

6. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, After the zinc-based coated hot-formed steel sheet exits the heating furnace, it is moved by a robot to the air-cooled stamping die and suspended between the upper and lower dies. After cooling until the coating is completely solidified, the robot places the zinc-based coated hot-formed steel sheet on the lower die and then moves it away. The upper die moves down and cooperates with the lower die to perform hot stamping on the zinc-based coated hot-formed steel sheet.

7. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, The blowing pressure of the air-cooled stamping die is 10-500 mbar, and the air-cooling time is 2-10 s; the temperature of the zinc-based coated hot-formed steel sheet after air cooling is 500-700℃.

8. The hot forming method for zinc-based coated hot-formed steel sheets using air-cooled die forming according to claim 1, characterized in that, The stamping temperature of the zinc-based coated hot-formed steel sheet is 500–700℃, and the holding time is 5–30s.

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