Warm forming process of martensite-based advanced high-strength steel

Abstract

The invention discloses a warm forming process of martensite-based advanced high-strength steel, and belongs to the technical field of plate stamping forming. The method comprises the following steps: (1) heating a to-be-used steel part at a preset heating speed to a certain preset stamping temperature below a steel austenitizing starting temperature Ac1, and keeping the temperature for a period of time, so that the temperature is uniform; (2) the plate heated to the preset stamping temperature is rapidly transferred into a stamping die to be subjected to stamping forming, and the plate is stamped into the shape needed by the component at the preset stamping speed; and (3) carrying out pressure-maintaining quenching on the steel part subjected to stamping forming in a mold. According to the method, the martensite-based advanced high-strength steel is heated to a certain temperature for stamping forming by setting the proper heating rate and heat preservation time, so that the martensite-based advanced high-strength steel has good stamping forming performance on the basis that the original high strength of the martensite-based advanced high-strength steel is not influenced basically, and the stamping forming difficulty is reduced; and rebound after forming can be effectively reduced, and the precision of a final product is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of plate stamping and forming, and relates to a warm forming process of martensite-based advanced high-strength steel. Background Art

[0002] Over the past two decades, my country's automotive industry has flourished and has become a key pillar of the national economy. As the global energy crisis and environmental issues become increasingly severe, national regulations have placed higher demands on automobiles in terms of energy conservation and emission reduction. These factors have driven the automotive industry towards energy conservation, environmental protection, and safety. Taking into account both material cost and performance requirements, advanced high-strength steels have become the preferred material for automotive weight reduction. Martensitic-based advanced high-strength steels, such as martensitic steel, DP steel, and Q&P steel, are widely used in industrial production due to their combination of low cost and high strength.

[0003] Advanced high-strength steel sheets require a stamping process to be fabricated into automotive parts. Currently, the mainstream processes are cold stamping and hot stamping. Hot stamping operates at a temperature above the material's austenitization temperature, resulting in low stamping resistance, minimal springback after stamping, and relatively high dimensional accuracy. However, its application is limited to a few steel grades. It also suffers from significant workpiece oxidation, complex heat treatment processes, short die life, and high production costs, making it unsuitable for the stamping of most martensitic advanced high-strength steels. Cold stamping of martensitic advanced high-strength steels is challenging due to their high strength, resulting in significant springback after stamping and a high risk of surface wrinkling and sheet cracking, impacting the appearance and quality of the final product. With the advancement of automotive technology, the demand for stamping precision in body panels is becoming increasingly stringent. In this industrial context, traditional cold stamping technology is no longer sufficient to meet production needs.

[0004] In recent years, some scholars have proposed a warm stamping process, based on the stamping processes of metals such as magnesium and aluminum, in which the stamping temperature is controlled between room temperature and the material's recrystallization temperature. This warm stamping process promises to combine the advantages of both hot and cold stamping, effectively improving the formability of advanced high-strength steels while maintaining their original high-strength properties. Therefore, a warm stamping process suitable for martensitic advanced high-strength steels has been proposed. This process, while maintaining the high strength of the steel, improves its stamping capability and final product precision, which is of great significance for the promotion and application of advanced high-strength steels. Summary of the Invention

[0005] The present invention aims to provide a warm forming process for martensitic advanced high-strength steel. Compared with conventional cold stamping processes, this method can effectively improve forming performance, reduce post-forming springback, and enhance final product precision, while maximally retaining the high-strength properties of the advanced high-strength steel.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: A warm forming process for martensitic advanced high-strength steel comprises the following steps: (1) Heat the steel to be used to the austenitization start temperature A c1 The following stamping temperatures are maintained to make the temperature uniform and obtain pre-treated sheets; (2) quickly transferring the pretreated sheet to a stamping die for stamping, and stamping the sheet into the desired shape of the component at a predetermined stamping speed to obtain a stamped steel part; (3) The stamped steel part is subjected to pressure-maintaining quenching in a die to obtain a formed steel part.

[0007] Furthermore, the chemical composition of the martensitic advanced high-strength steel sheet material described in step (1) is as follows by weight: C: 0.05% to 0.77%; Si: 0 to 4.0%; Mn: 0 to 7.0%; Cr: 0 to 3.0%; Ni: 0 to 3.0%; V: 0 to 0.6%; Mo: 0 to 2.0%; Nb: 0 to 0.5%; the remainder being Fe and unavoidable impurities.

[0008] Furthermore, the steel part to be used in step (1) is a martensite-based advanced high-strength steel raw material, whose tensile strength is ≥780 MPa and martensite content is not less than 20%; the steel part to be used is a hot-rolled or cold-rolled steel plate with a thickness of 0.5 mm to 5 mm.

[0009] Furthermore, the heating rate in step (1) is ≥5°C / min, and the heating methods include but are not limited to induction heating, salt bath heating, heating furnace heating and other heating methods.

[0010] Furthermore, the stamping temperature in step (1) is 150°C to c1 , the insulation time is ≤60 min.

[0011] Furthermore, the predetermined punching speed in step (2) is ≥5 mm / s.

[0012] Furthermore, in the step (2), the temperature of the plate does not drop by more than 150° C. during the process of transferring the pretreated plate to the stamping die.

[0013] Furthermore, in step (3), the mold is a water-cooled or other cooling medium cooling mold.

[0014] Furthermore, in step (3), the holding pressure is ≥5 tons and the holding time is ≤10 minutes.

[0015] Furthermore, the tensile strength of the steel part after forming is reduced by no more than 150 MPa compared with that before stamping.

[0016] Beneficial effects of the present invention: (1) The present invention heats the martensitic advanced high-strength steel to an appropriate temperature for stamping, which not only ensures that the advanced high-strength steel sheet has good stamping performance and reduces the difficulty of stamping, but also effectively reduces springback after forming and improves the precision of the final product; (2) By setting an appropriate heating rate and holding time, the present invention can prevent excessive tempering of martensite in the structure of martensite-based advanced high-strength steel, resulting in minimal strength loss of the steel after stamping, and can also improve its plasticity to a certain extent. This process is basically applicable to all martensite-based advanced high-strength steels and has the advantages of low energy consumption, high production efficiency, and low cost, making it suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a diagram showing the method for measuring the rebound angle in the embodiment; Figure 2 This is a picture of the objects after cold stamping in Example 1 and Example 2; Figure 3 This is a physical picture of Example 1 after warm stamping; Figure 4 This is a real picture of the product after warm stamping in Example 2; Figure 5 The mechanical property curves of the steel in Example 1 after warm stamping and cold stamping. DETAILED DESCRIPTION

[0018] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. The present invention is further described in detail below with reference to specific implementation cases. However, the scope of protection of the present invention is not limited by the specific implementation methods.

[0019] In the specific implementation process, the present invention proposes a warm forming process for martensitic advanced high-strength steel, and the specific steps are as follows: (1) Process 0.5 mm to 5 mm hot-rolled or cold-rolled martensitic advanced high-strength steel (tensile strength ≥780 MPa, martensite content not less than 20%) sheets to the required size for stamping, and heat them to 150°C-A at a heating rate of ≥5°C / min. c1 Keep the stamping temperature (steel austenitization start temperature) for a period of time (holding time not exceeding 60 minutes) to make the temperature uniform; (2) The sheet metal, which has been heated to the predetermined stamping temperature, is quickly transferred to the stamping die. The temperature of the sheet metal does not drop by more than 150°C during the transfer from the heating and insulation position to the stamping position. The sheet metal is then stamped to the desired shape of the component at a stamping speed of ≥5 mm / s; (3) The steel parts after stamping are subjected to pressure quenching in the mold. The mold used is a water-cooled or other cooling medium cooling mold.

[0020] In view of the above technical solutions, the implementation scheme of the present invention is as follows:

[0021] Example 1

[0022] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is Q&P steel with a thickness of 0.9 mm. It has a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. Its chemical composition, by weight, is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, with the remainder being Fe and unavoidable impurities.

[0023] (1) The sheet metal was processed to the required size for stamping (170 mm × 80 mm), and heated to 300 °C in a muffle furnace at a heating rate of 5 °C / min for 60 min to make the temperature uniform; (2) The sheet heated to 300°C is quickly transferred to the stamping die. The sheet temperature does not drop more than 50°C during the transfer from the heating and insulation position to the stamping position. Then, a U-shaped part is stamped at a stamping speed of 10 mm / s. The actual picture after stamping is as follows: Figure 3 .

[0024] (3) The stamped steel parts are quenched in the mold at a pressure of 8 tons for 3 minutes. The mold used is a water-cooled mold.

[0025] (4) Cold stamping is performed at room temperature using the same stamping and holding process, and compared with the warm stamping process, the actual picture after stamping is as follows: Figure 2 .

[0026] (5) According to Figure 1 Using the method shown, the springback angle measured after warm stamping was 7.05°, and the springback angle after room temperature stamping was 11.35°. A dog-bone tensile specimen with a gauge length of 6 mm was then cut from the stamped U-shaped part. The resulting properties were as follows: 1203 MPa tensile strength and 29.8% total elongation after warm stamping; 1233 MPa tensile strength and 23.6% total elongation after cold stamping. Mechanical property curves are shown in [1]. Figure 5 .

[0027] Example 2

[0028] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is Q&P steel with a thickness of 1.1 mm. It has a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. Its chemical composition, by weight, is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, with the remainder being Fe and unavoidable impurities.

[0029] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 500°C at a heating rate of 10°C / s and keep it at that temperature for 10 s to make it uniform; (2) The sheet heated to 500℃ is quickly transferred to the stamping die. The temperature of the sheet does not drop more than 50℃ during the transfer from the heating and heat preservation position to the stamping position. Then it is stamped into a U-shaped part at a stamping speed of 50 mm / s. The actual picture after stamping is as follows Figure 4 .

[0030] (3) The stamped steel parts are quenched in the die at a pressure of 20 T for 1 min. The die used is a water-cooled die.

[0031] (4) Cold stamping is performed at room temperature using the same stamping and holding process, and compared with the warm stamping process, the actual picture after stamping is as follows: Figure 2 .

[0032] (5) According to Figure 1 Using the method shown, the springback angle measured after forming was 5.85° after warm stamping and 11.35° after room temperature stamping. Dog-bone tensile specimens with a gauge length of 6 mm were then cut from the stamped U-shaped part. The resulting properties were as follows: 1131 MPa tensile strength and 30.5% total elongation after warm stamping; 1233 MPa tensile strength and 21.6% total elongation after cold stamping.

[0033] Example 3

[0034] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is Q&P steel with a thickness of 1.7 mm. It has a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. Its chemical composition, by weight, is as follows: C: 0.20%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, and the remaining elements are Fe and unavoidable impurities.

[0035] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 400°C at a heating rate of 10°C / min and keep it at that temperature for 30 min to make it uniform; (2) The sheet heated to 400°C is quickly transferred to the stamping die. The sheet temperature does not drop by more than 50°C during the transfer from the heating and holding position to the stamping position. The U-shaped part is then stamped at a stamping speed of 80 mm / s.

[0036] (3) The stamped steel parts are quenched in the die at a pressure of 30 tons for 30 seconds. The die used is a water-cooled die.

[0037] (4) Cold stamping is performed at room temperature using the same stamping and holding process for comparison with the warm stamping process.

[0038] (5) According to Figure 1 Using the method shown, the springback angle measured after forming was 6.45° after warm stamping and 16.90° after room temperature stamping. Dog-bone tensile specimens with a gauge length of 10 mm were then cut from the stamped U-shaped part. The resulting properties were as follows: 1131 MPa tensile strength and 16.2% total elongation after warm stamping; 1242 MPa tensile strength and 13.8% total elongation after cold stamping.

[0039] Example 4

[0040] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is Q&P steel with a thickness of 1.7 mm. It has a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. Its chemical composition, by weight, is as follows: C: 0.20%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, and the remaining elements are Fe and unavoidable impurities.

[0041] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 300°C at a heating rate of 20°C / s and keep it at that temperature for 100 s to make it uniform; (2) The sheet heated to 200°C is quickly transferred to the stamping die. The sheet temperature does not drop by more than 50°C during the transfer from the heating and holding position to the stamping position. Then, a U-shaped part is stamped at a stamping speed of 100 mm / s.

[0042] (3) The stamped steel parts are quenched in a die at a pressure of 50 tons for 20 seconds. The die used is a water-cooled die.

[0043] (4) Cold stamping is performed at room temperature using the same stamping and holding process for comparison with the warm stamping process.

[0044] (5) According to Figure 1 Using the method shown, the springback angle measured after forming was 10.85° after warm stamping and 16.90° after room temperature stamping. Dog-bone tensile specimens with a gauge length of 10 mm were then cut from the stamped U-shaped part. The resulting properties were as follows: 1203 MPa tensile strength and 14.6% total elongation after warm stamping; 1242 MPa tensile strength and 13.8% total elongation after cold stamping.

[0045] Example 5

[0046] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is 2.2 mm thick DP steel, with a martensite content of 76%, a tensile strength of 1720 MPa, and an elongation of 9.2%. Its chemical composition, by weight, is as follows: C: 0.68%, Si: 0.51%, Mn: 0.98%, Cr: 0.5%, with the remainder being Fe and unavoidable impurities.

[0047] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 400°C at a heating rate of 100°C / s and keep it at that temperature for 10 s to make it uniform; (2) The sheet heated to 200°C is quickly transferred to the stamping die. The sheet temperature does not drop by more than 50°C during the transfer from the heating and holding position to the stamping position. Then, a U-shaped part is stamped at a stamping speed of 50 mm / s.

[0048] (3) The stamped steel parts are quenched in a die at a pressure of 25 tons for 10 seconds. The die used is a common die.

[0049] (4) Cold stamping is performed at room temperature using the same stamping and holding process for comparison with the warm stamping process.

[0050] (5) According to Figure 1 Using the method shown, the springback angle measured after forming was 9.60° after warm stamping and 18.95° after room temperature stamping. Dog-bone tensile specimens with a gauge length of 10 mm were then cut from the stamped U-shaped part. The resulting properties were as follows: 1645 MPa tensile strength and 9.6% total elongation after warm stamping; 1736 MPa tensile strength and 9.5% total elongation after cold stamping.

[0051] Example 6

[0052] A warm forming process for martensitic advanced high-strength steel comprises the following steps: The martensitic advanced high-strength steel used in this example is 2.9 mm thick DP steel with a martensite content of 65%, a tensile strength of 1386 MPa, and an elongation of 10.3%. Its chemical composition, by weight, is as follows: C: 0.35%, Si: 0.52%, Mn: 1.05%, Cr: 0.52%, with the remainder being Fe and unavoidable impurities.

[0053] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 300°C at a heating rate of 10°C / min and keep it at that temperature for 5 min to make it uniform; (2) The sheet heated to 300°C is quickly transferred to the stamping die. The sheet temperature does not drop more than 50°C during the transfer from the heating and holding position to the stamping position. The U-shaped part is then stamped at a stamping speed of 50 mm / s.

[0054] (3) The stamped steel parts are quenched in a die at a pressure of 50 tons for 30 seconds. The die used is a common die.

[0055] (4) Cold stamping is performed at room temperature using the same stamping and holding process for comparison with the warm stamping process.

[0056] (5) According to Figure 1 Using the method shown, the springback angle measured after forming was 7.65° after warm stamping and 15.85° after room temperature stamping. Dog-bone tensile specimens with a gauge length of 10 mm were then cut from the stamped U-shaped part. The resulting properties were as follows: 1303 MPa tensile strength and 10.6% total elongation after warm stamping; 1395 MPa tensile strength and 10.1% total elongation after cold stamping.

Claims

1. A warm forming process for martensitic advanced high-strength steel, characterized in that: The following steps are involved: (1) Heat the steel to be used to the austenitization start temperature A c1 The following stamping temperatures are maintained to make the temperature uniform and obtain pre-treated sheets; (2) transferring the pretreated plate to a stamping die for stamping, and stamping the plate into the desired shape of the component at a predetermined stamping speed to obtain a stamped steel part; (3) The stamped steel part is subjected to pressure-maintaining quenching in a die to obtain a formed steel part.

2. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: The chemical composition weight percentage of the steel to be used in step (1) is: C: 0.05% to 0.77%; Si: 0 to 4.0%; Mn: 0 to 7.0%; Cr: 0 to 3.0%; Ni: 0~3.0%; V: 0~0.6%; Mo: 0~2.0%; Nb: 0~0.5%; the balance is Fe and inevitable impurities.

3. The warm forming process of martensite-based advanced high-strength steel according to claim 1, characterized in that: The steel piece to be used in step (1) is a martensite-based advanced high-strength steel raw material with a tensile strength of ≥780 MPa and a martensite content of not less than 20%; the steel piece to be used is a hot-rolled or cold-rolled steel plate with a thickness of 0.5 mm to 5 mm.

4. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: The heating rate in step (1) is ≥5°C / min, and the heating methods include but are not limited to induction heating, salt bath heating, heating furnace heating and other heating methods.

5. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: The stamping temperature in step (1) is 150°C~A c1 , the insulation time is ≤60 min.

6. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: The predetermined punching speed in step (2) is ≥5 mm / s.

7. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: In the step (2), the temperature of the plate does not drop by more than 150° C. during the process of transferring the pre-treated plate to the stamping die.

8. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: In the step (3), the mold is a water-cooled or other cooling medium cooling mold.

9. The warm forming process of martensitic advanced high strength steel according to claim 1, characterized in that: In the step (3), the holding pressure is ≥5 tons and the holding time is ≤10 minutes.

10. The warm forming process of martensite-based advanced high-strength steel according to claim 1, characterized in that: The tensile strength of the steel part after forming is reduced by no more than 150 MPa compared with that before stamping.

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