A warm forming process for advanced martensitic high-strength steel

Abstract

This invention discloses a warm forming process for martensitic advanced high-strength steel, belonging to the field of sheet metal stamping technology. The invention includes the following steps: (1) heating the steel part to be used at a predetermined heating rate to a predetermined stamping temperature below the austenitization initiation temperature Ac1 of the steel and holding it at that temperature for a period of time to ensure uniform temperature; (2) rapidly transferring the sheet metal heated to the predetermined stamping temperature to a stamping die for stamping, stamping it to the required shape of the component at a predetermined stamping speed; (3) subjecting the stamped steel part to pressure quenching in the die. By setting appropriate heating rates and holding times, this invention heats martensitic advanced high-strength steel to a certain temperature for stamping, achieving good stamping performance without significantly affecting the original high strength of the martensitic advanced high-strength steel, thus reducing the difficulty of stamping; it also effectively reduces springback after forming and improves the precision of the final product.

Description

Technical Field

[0001] This invention belongs to the field of sheet metal stamping technology, and relates to a warm forming process for advanced high-strength martensitic steel. Background Technology

[0002] Over the past two decades, my country's automobile industry has flourished and has now become a vital pillar of the national economy. With the global energy crisis and environmental problems becoming 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 costs and performance requirements, advanced high-strength steel has become the preferred material for reducing automobile weight. 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 stamping to be manufactured into automotive parts. Currently, the mainstream processes are cold stamping and hot stamping. Hot stamping operates at temperatures above the austenitizing temperature of the material, resulting in low stamping resistance, minimal springback, and high dimensional accuracy. However, its application is limited, suitable only for a small number of steel grades, and it 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. In cold stamping, martensitic advanced high-strength steels, due to their high strength, are more difficult to stamp, exhibiting significant springback issues after stamping, and are prone to surface wrinkles and sheet metal breakage, affecting the final product's appearance and quality. With the development of automotive technology, the stamping precision requirements for body panels are increasingly stringent. In this industrial context, traditional cold stamping technology can no longer meet production needs.

[0004] In recent years, some scholars have referenced the stamping processes of metals such as magnesium and aluminum, proposing a warm stamping process that controls the stamping temperature between room temperature and the material's recrystallization temperature. Warm stamping is expected 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. Therefore, proposing a warm stamping process suitable for martensitic advanced high-strength steels, which improves their stamping formability and final product precision without affecting their high strength, is of great significance for the widespread application of advanced high-strength steels. Summary of the Invention

[0005] The purpose of this invention is to provide a warm forming process for martensitic advanced high-strength steel. Compared with traditional cold stamping forming processes, this invention can effectively improve its forming performance, reduce springback after forming, improve the precision of the final product, and retain the high strength characteristics of advanced high-strength steel to the maximum extent.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0008] (1) Heat the steel part to be used to the austenitizing start temperature A of the steel. c1 The following stamping temperature is maintained to ensure uniform temperature, resulting in pre-treated sheet material;

[0009] (2) The pretreated sheet metal is quickly transferred to a stamping die for stamping and forming. The sheet metal is stamped to the required shape of the component at a predetermined stamping speed to obtain the stamped steel part.

[0010] (3) The stamped steel part is subjected to pressure holding and quenching in the mold to obtain the formed steel part.

[0011] Further, the chemical composition of the martensitic advanced high-strength steel plate material described in step (1) is as follows by weight percentage: C: 0.05%~0.77%; Si: 0~4.0%; Mn: 0~7.0%; Cr: 0~3.0%; Ni: 0~3.0%; V: 0~0.6%; Mo: 0~2.0%; Nb: 0~0.5%; with the balance being Fe and unavoidable impurities.

[0012] Further, the steel part to be used in step (1) is a martensitic advanced high-strength steel raw material with a tensile strength ≥780 MPa and a martensite content of 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.

[0013] Furthermore, the heating rate in step (1) is ≥5℃ / min, and the heating method includes, but is not limited to, induction heating, salt bath heating, heating furnace heating, etc.

[0014] Further, the stamping temperature in step (1) is 150℃~A c1 The heat preservation time is ≤60 min.

[0015] Furthermore, the predetermined stamping speed in step (2) is ≥5mm / s.

[0016] Furthermore, in step (2), the temperature of the sheet material decreases by no more than 150°C during the process of transferring the pretreated sheet material to the stamping die.

[0017] Furthermore, in step (3), the mold is a water-cooled or other cooling medium-cooled mold.

[0018] Furthermore, in step (3), the pressure during pressure holding is ≥5 tons and the pressure holding time is ≤10 min.

[0019] Furthermore, the tensile strength of the formed steel part decreases by no more than 150 MPa compared to that before stamping.

[0020] The beneficial effects of this invention are:

[0021] (1) This invention heats the advanced high-strength steel based on martensitic material to an appropriate temperature for stamping, which can not only ensure that the advanced high-strength steel sheet has good stamping performance and reduce the difficulty of stamping, but also effectively reduce springback after forming and improve the precision of the final product.

[0022] (2) By setting appropriate heating rates and holding times, this invention can prevent excessive tempering of martensite in martensitic advanced high-strength steel, resulting in minimal strength loss after stamping and improving its plasticity to a certain extent. Furthermore, this process is applicable to almost all martensitic advanced high-strength steels and has the advantages of low energy consumption, high production efficiency, and low cost, making it suitable for industrial production. Attached Figure Description

[0023] Figure 1 This is a diagram illustrating the method for measuring the rebound angle in the embodiment;

[0024] Figure 2 These are actual images of the cold-stamped products from Examples 1 and 2.

[0025] Figure 3 This is a photograph of the actual product after temperature stamping in Example 1;

[0026] Figure 4 This is a photograph of the actual product after temperature stamping in Example 2;

[0027] Figure 5 The mechanical property curves of the steel in Example 1 after warm stamping and cold stamping are shown. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. The present invention will be further described in detail below with reference to specific implementation examples. However, the scope of protection of the present invention is not limited to the specific implementation methods.

[0029] In its specific implementation, this invention proposes a warm forming process for martensitic-based advanced high-strength steel, the specific steps of which are as follows:

[0030] (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%) sheet to the required stamping size, and heat to 150℃-A at a heating rate of ≥5℃ / min. c1 The stamping temperature (the austenitizing start temperature of the steel) is held at the temperature for a period of time (the holding time shall not exceed 60 min) to make the temperature uniform.

[0031] (2) Quickly transfer the sheet metal, heated to the predetermined stamping temperature, into the stamping die. During the transfer from the heating and holding position to the stamping position, the sheet metal temperature should not drop by more than 150°C. Then, stamp the part to the required shape at a stamping speed of ≥5 mm / s.

[0032] (3) The stamped steel parts are subjected to pressure quenching in the mold. The mold used is a water-cooled or other cooling medium-cooled mold.

[0033] In view of the above technical solutions, the implementation scheme of the present invention is as follows:

[0034] Example 1

[0035] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0036] The martensitic advanced high-strength steel selected in this embodiment is a 0.9 mm thick Q&P steel with a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. Its chemical composition, by weight percentage, is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, with the remaining elements being Fe and unavoidable impurities.

[0037] (1) Process the sheet metal to the required stamping size (170 mm × 80 mm), heat it to 300℃ in a muffle furnace at a heating rate of 5℃ / min and hold it for 60 min to make its temperature uniform;

[0038] (2) Quickly transfer the sheet metal heated to 300°C into the stamping die. During the transfer from the heating and heat preservation position to the stamping position, the sheet metal temperature should not drop by more than 50°C. Then, stamp it into a U-shaped part at a stamping speed of 10 mm / s. The actual stamped part is shown in the following image. Figure 3 .

[0039] (3) The stamped steel parts are held under pressure of 8 tons for 3 minutes in the mold. The mold used is a water-cooled mold.

[0040] (4) Cold stamping was performed at room temperature using the same stamping and holding process. Compared with the warm stamping process, the actual product image after stamping is as follows. Figure 2 .

[0041] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 7.05°, and the springback angle after room temperature stamping was 11.35°. Then, dog-bone shaped tensile specimens with a gauge length of 6 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1203 MPa, total elongation: 29.8%; tensile strength after cold stamping: 1233 MPa, total elongation: 23.6%. Mechanical property curves are shown below. Figure 5 .

[0042] Example 2

[0043] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0044] The martensitic advanced high-strength steel selected in this embodiment is a 1.1 mm thick Q&P steel with a martensite content of 36%, a tensile strength of 1208 MPa, and an elongation of 20.7%. Its chemical composition, by weight percentage, is as follows: C: 0.18%, Si: 1.51%, Mn: 2.48%, Nb: 0.41%, with the remaining elements being Fe and unavoidable impurities.

[0045] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 500℃ at a heating rate of 10℃ / s and hold it for 10 s to make its temperature uniform;

[0046] (2) Quickly transfer the sheet metal heated to 500°C into the stamping die. During the transfer from the heating and holding position to the stamping position, the sheet metal temperature should not drop by more than 50°C. Then, stamp it into a U-shaped part at a stamping speed of 50 mm / s. The actual stamped part is shown in the following image. Figure 4 .

[0047] (3) The stamped steel parts are held under pressure of 20 T for 1 min in the mold. The mold used is a water-cooled mold.

[0048] (4) Cold stamping was performed at room temperature using the same stamping and holding process. Compared with the warm stamping process, the actual product image after stamping is as follows. Figure 2 .

[0049] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 5.85°, and the springback angle after room temperature stamping was 11.35°. Then, dog-bone shaped tensile samples with a gauge length of 6 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1131 MPa, total elongation: 30.5%; tensile strength after cold stamping: 1233 MPa, total elongation: 21.6%.

[0050] Example 3

[0051] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0052] The martensitic advanced high-strength steel selected in this embodiment is a 1.7 mm thick Q&P steel with a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. Its chemical composition, by weight percentage, is as follows: C: 0.20%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, with the remaining elements being Fe and unavoidable impurities.

[0053] (1) Process the sheet metal to the required stamping size (170 mm × 80 mm), heat it to 400℃ at a heating rate of 10℃ / min and hold it for 30 min to make its temperature uniform;

[0054] (2) Quickly transfer the sheet metal heated to 400°C into the stamping die. The sheet metal temperature shall not drop by more than 50°C during the transfer from the heating and heat preservation position to the stamping position. Then stamp it into a U-shaped part at a stamping speed of 80 mm / s.

[0055] (3) The stamped steel parts are held under pressure of 30 tons for 30 seconds in the mold. The mold used is a water-cooled mold.

[0056] (4) Perform cold stamping at room temperature using the same stamping and holding process to compare with the warm stamping process.

[0057] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 6.45°, and the springback angle after room temperature stamping was 16.90°. Then, dog-bone shaped tensile samples with a gauge length of 10 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1131 MPa, total elongation: 16.2%; tensile strength after cold stamping: 1242 MPa, total elongation: 13.8%.

[0058] Example 4

[0059] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0060] The martensitic advanced high-strength steel selected in this embodiment is a 1.7 mm thick Q&P steel with a martensite content of 42%, a tensile strength of 1423 MPa, and an elongation of 12.5%. Its chemical composition, by weight percentage, is as follows: C: 0.20%, Si: 1.51%, Mn: 2.48%, Mo: 0.25%, V: 0.38%, with the remaining elements being Fe and unavoidable impurities.

[0061] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 300℃ at a heating rate of 20℃ / s and hold it for 100 s to make its temperature uniform;

[0062] (2) Quickly transfer the sheet metal heated to 200°C into the stamping die. The sheet metal temperature shall not drop by more than 50°C during the transfer from the heating and heat preservation position to the stamping position. Then stamp it into a U-shaped part at a stamping speed of 100 mm / s.

[0063] (3) The stamped steel parts are held under pressure of 50 tons for 20 seconds in the mold. The mold used is a water-cooled mold.

[0064] (4) Perform cold stamping at room temperature using the same stamping and holding process to compare with the warm stamping process.

[0065] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 10.85°, and the springback angle after room temperature stamping was 16.90°. Then, dog-bone shaped tensile samples with a gauge length of 10 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1203 MPa, total elongation: 14.6%; tensile strength after cold stamping: 1242 MPa, total elongation: 13.8%.

[0066] Example 5

[0067] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0068] The martensitic advanced high-strength steel selected in this embodiment is a DP steel with a thickness of 2.2 mm, a martensite content of 76%, a tensile strength of 1720 MPa, and an elongation of 9.2%. Its chemical composition, by weight percentage, is as follows: C: 0.68%, Si: 0.51%, Mn: 0.98%, Cr: 0.5%, with the remaining elements being Fe and unavoidable impurities.

[0069] (1) Process the sheet metal to the required size for stamping (170 mm × 80 mm), heat it to 400℃ at a heating rate of 100℃ / s and hold it for 10 s to make its temperature uniform;

[0070] (2) Quickly transfer the sheet metal heated to 200°C into the stamping die. The sheet metal temperature shall not drop by more than 50°C during the transfer from the heating and heat preservation position to the stamping position. Then stamp it into a U-shaped part at a stamping speed of 50 mm / s.

[0071] (3) The stamped steel parts are held under pressure of 25 tons for 10 seconds in the mold. The mold used is a common mold.

[0072] (4) Perform cold stamping at room temperature using the same stamping and holding process to compare with the warm stamping process.

[0073] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 9.60°, and the springback angle after room temperature stamping was 18.95°. Then, dog-bone shaped tensile samples with a gauge length of 10 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1645 MPa, total elongation: 9.6%; tensile strength after cold stamping: 1736 MPa, total elongation: 9.5%.

[0074] Example 6

[0075] A warm forming process for martensitic advanced high-strength steel includes the following steps:

[0076] The martensitic advanced high-strength steel selected in this embodiment is a DP steel with a thickness of 2.9 mm, a martensite content of 65%, a tensile strength of 1386 MPa, and an elongation of 10.3%. Its chemical composition, by weight percentage, is as follows: C: 0.35%, Si: 0.52%, Mn: 1.05%, Cr: 0.52%, with the remaining elements being Fe and unavoidable impurities.

[0077] (1) Process the sheet metal to the required stamping size (170 mm × 80 mm), heat it to 300℃ at a heating rate of 10℃ / min and hold it for 5 min to make its temperature uniform;

[0078] (2) Quickly transfer the sheet metal heated to 300°C into the stamping die. The sheet metal temperature shall not drop by more than 50°C during the transfer from the heating and heat preservation position to the stamping position. Then stamp it into a U-shaped part at a stamping speed of 50 mm / s.

[0079] (3) The stamped steel parts are held under pressure of 50 tons for 30 seconds in the mold. The mold used is a common mold.

[0080] (4) Perform cold stamping at room temperature using the same stamping and holding process to compare with the warm stamping process.

[0081] (5) According to Figure 1 The method shown indicates that the springback angle after warm stamping was 7.65°, and the springback angle after room temperature stamping was 15.85°. Then, dog-bone shaped tensile samples with a gauge length of 10 mm were cut from the stamped U-shaped part. The properties obtained are as follows: tensile strength after warm stamping: 1303 MPa, total elongation: 10.6%; tensile strength after cold stamping: 1395 MPa, total elongation: 10.1%.

Claims

1. A warm forming process for advanced martensitic high-strength steel, characterized in that, Includes the following steps: (1) Heat the steel part to be used to the austenitizing start temperature A of the steel. c1 The following stamping temperatures are maintained to ensure uniform temperature, resulting in pretreated sheet material; the heating rate is ≥5℃ / min, the stamping temperature is 150℃~Ac1, and the holding time is ≤60 min; the steel material to be used is advanced high-strength martensitic steel raw material with a tensile strength ≥780 MPa and a martensite content of not less than 20%; the steel material to be used is hot-rolled or cold-rolled steel sheet with a thickness of 0.5 mm~5 mm; (2) The pretreated sheet metal is transferred to a stamping die for stamping and forming. The sheet metal is stamped to the required shape of the component at a predetermined stamping speed to obtain the stamped steel part. During the process of transferring the pretreated sheet to the stamping die, the sheet temperature drops by no more than 150°C. (3) The stamped steel part is subjected to pressure holding and quenching in the mold to obtain the formed steel part.

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

3. The warm forming process for an advanced martensitic high-strength steel according to claim 1, characterized in that, The heating methods described in step (1) include induction heating, salt bath heating, and furnace heating.

4. The warm forming process for martensitic-based advanced high-strength steel according to claim 1, characterized in that, The predetermined stamping speed in step (2) is ≥5 mm / s.

5. The warm forming process for an advanced martensitic high-strength steel according to claim 1, characterized in that, The mold mentioned in step (3) is a water-cooled cooling mold.

6. The warm forming process for an advanced martensitic high-strength steel according to claim 1, characterized in that, The pressure for holding pressure in step (3) is ≥5 tons and the holding time is ≤10 min.

7. The warm forming process for an advanced martensitic 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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