Method for laser cladding and laser quenching partition strengthening die hot stamping plated steel sheet

Abstract

The application discloses a method for laser cladding and laser quenching partition strengthening die hot stamping plated steel plate, simulates and analyzes three-dimensional contact and stress of a plated steel plate, a male die and a female die of a die, determines a laser cladding area, a laser quenching area and a geometric transition area connecting the two, mills the laser quenching area, makes a milled surface of the laser quenching area consistent with the shape and size of a forming die surface, pre-mills the laser cladding area and the geometric transition area, adopts a six-axis manipulator with a cladding head and a two-axis shifter to perform laser curved surface cladding on the pre-milled area, performs secondary milling on the laser cladding layer to obtain a well-cladded pre-milled area, quenches the laser quenching area after the milling, assembles the male die and the female die into a hot stamping die after laser cladding and laser quenching to perform hot stamping on the plated steel plate, reduces the friction coefficient between the die surface and the plated steel plate by means of the self-lubricating property of the laser cladding layer, and reduces the generation of powdering and micro-cracks of the plated steel plate.

Description

Technical Field

[0001] This invention pertains to mold surface strengthening and hot stamping technology for steel plates. Specifically, it describes a method for improving the hot stamping forming quality of steel plates by using surface-strengthened molds as tools, particularly a method for hot stamping ultra-high strength coated steel plates using hot stamping molds with large profile dimensions and complex curved shapes. Background Technology

[0002] Hot stamping of ultra-high strength steel sheets is one of the main methods for manufacturing high-safety, lightweight parts that has emerged in recent years. It involves heating ultra-high strength steel sheets to 900–950°C and then forming them under high-temperature conditions using hot stamping dies to obtain structural parts with tensile strengths exceeding 1500 MPa. During hot stamping, the high temperature and high load conditions complicate the friction and wear between the steel sheet and the die, and this friction and wear behavior directly affects the quality of the stamped parts and the service life of the die.

[0003] Currently, laser cladding and laser hardening technologies are widely used in the surface strengthening of stamping dies. Laser cladding involves adding a coating material to the die surface, which is then irradiated with a laser to melt simultaneously with a thin layer on the die surface. After rapid solidification, a surface coating with extremely low dilution and metallurgical bonding with the die is formed, significantly improving the wear resistance and heat resistance of the die surface. Laser hardening is a quenching technique that uses a laser to heat the die surface above its phase transformation point. As the material cools, the die surface rapidly cools to below the martensitic phase transformation point, thereby hardening the material surface.

[0004] For hot stamping dies with large dimensions and complex curved shapes, such as those used in automobiles, it is still difficult to strengthen the curved surfaces of these large-sized hot stamping dies using conventional laser cladding and laser hardening. For example, the die surface size of a two-cavity hot stamping die is about 1400mm×1400mm. If such a large die surface is only treated with laser cladding technology, although the strengthening quality is good, the cost is high, the efficiency is low, and the subsequent finishing work is large. On the other hand, using laser hardening, which has a high strengthening efficiency, alone is difficult to meet the high strengthening requirements of the key parts of the die that are prone to wear.

[0005] Chinese patent application number 200810155785.4, entitled "A Method and Apparatus for Surface Modification by Laser Cladding Combined with Laser Shot Peening," proposes combining laser cladding with shot peening to improve alloy properties. Chinese patent application number 202210521535.8, entitled "A Method for Repairing Rail Surfaces," proposes a two-stage process of first laser cladding the rail surface and then laser quenching to obtain a coating with fine microstructure and high performance. Chinese patent application number 201810006295.1, entitled "A Method for Laser Cladding Wear-Resistant and Impact-Resistant Coating on Cast Iron Surfaces," employs a strengthening process of "laser quenching of cast iron surface + cladding metal alloy underlayer + cladding metal alloy wear-resistant and impact-resistant layer." All three patented technical solutions involve laser cladding followed by laser quenching, or laser quenching followed by laser cladding, or laser cladding combined with other laser strengthening methods. None of them have solved the problems of high cost and low efficiency in cladding of large-size curved mold surfaces.

[0006] Because the surface of hot stamping dies is a complex curved surface, a six-axis robot with a laser cladding head or quenching head must work in eight-axis linkage with a two-axis positioner to meet the requirements of laser curved surface cladding. For laser curved surface cladding, the axis of the cladding head and the normal of the curved surface should be as coaxial as possible to ensure that the variation in the size of the laser spot is controlled, thus ensuring the laser irradiation area. Due to the weight of the metal powder, it may deviate from the trajectory during powder feeding, and flow after laser heating and melting, causing a deviation between the powder beam convergence point and the laser spot position. Similarly, laser curved surface quenching also requires eight-axis linkage between the robot and the positioner, but because it does not involve cladding powder, the equipment technology and process requirements are relatively simple. Chinese Patent Application No. 201910932636.2, entitled "A Laser Pre-cladding Assisted Plasma Additive Manufacturing Equipment," discloses a six-axis welding robot working in conjunction with a two-axis positioner. This provides a large working range, enabling rapid prototyping or repair of large-sized parts and the manufacturing of complex parts with complex spatial curved surfaces. The two-axis positioner offers an unrestricted C-axis rotation range and an A-axis rotation range of ±110°. Chinese Patent Application No. 202210444829.5, entitled "A Laser Cladding Path Planning Method and System for Aero-engine Blade Surfaces," discloses obtaining the laser cladding path planning diagram by inversely solving the normal vectors of each processing point. Chinese patent application No. 2020 10334162.4, entitled "A Method for Ultra-High-Speed ​​Laser Cladding of Complex Curved Surfaces of Rotation," discloses a method for efficiently controlling the relative motion between an ultra-high-speed laser cladding head and a high-speed rotating complex body of rotation. This method ensures cladding quality while stabilizing cladding efficiency, resulting in a cladding layer with good metallurgical bonding, density, and uniform thickness. However, the aforementioned patent technology does not study the linkage of the three angles (α, β, γ) of the robot and positioner during the curved surface cladding process, nor does it study the eight-axis linkage laser curved surface cladding process.

[0007] The method disclosed in Chinese patent application No. 201911066862.3, entitled "A method for preparing a high-entropy cladding coating to improve the surface strength and toughness of mold steel", involves mixing 1.2-1.4% WC, 2.5-4% Cr, 0.3-0.7% graphene by mass, with the remainder being powder of the same composition as the mold steel to be coated, to form a cladding material. The method employs induction cladding, vacuum heating, low-temperature tempering, nitriding, and surface micro-treatment to improve the surface strength and toughness of the mold steel, but it fails to reduce the surface friction coefficient or improve friction performance. The method disclosed in Chinese Patent Application No. 201811256725.1, entitled "A Solid Self-Lubricating Wear-Resistant and Corrosion-Resistant Composite Coating and Its Preparation Method," uses laser cladding to clad a composite material (4-8 wt% molybdenum disulfide, 30-50 wt% tungsten carbide, and 40-70 wt% nickel-chromium-boron-silicon alloy) onto the surface of a hot-work die steel substrate to obtain a solid self-lubricating wear-resistant and corrosion-resistant composite coating. The method disclosed in Chinese Patent Publication No. CN106501120A, entitled "A Rapid Detection Method for Powdering of Hot-Dip Galvanized Alloyed Steel Sheets," to a certain extent detects the degree of powdering of the formed coated steel sheets. The method disclosed in Chinese Patent Application No. 202110299994.1, entitled "A Hot Stamping Method and System for Zinc-Based Coated Hot-Stamped Steel," establishes a correlation between room temperature and safe cooling time to reduce the generation of coating cracks during pressure holding and quenching. Therefore, although technologies such as laser cladding strengthening, self-lubricating cladding improving friction performance, and coating steel sheet pulverization and microcracks have emerged, they are all one-sided technologies. There is no evidence that self-lubricating laser cladding is used on hot stamping die surfaces, nor is there evidence of using self-lubricating cladding to control coating steel sheet pulverization and microcracks during hot stamping. Summary of the Invention

[0008] The purpose of this invention is to address all the shortcomings of the existing technology by proposing a method for hot stamping coated steel sheets using laser cladding and laser quenching for partitioned strengthening of the die. Based on stress analysis and strengthening zone division, the curved hot stamping die is subjected to self-lubricating laser cladding and laser quenching for partitioned composite strengthening. Using this composite-strengthened die as a tool, the coated steel sheet is hot stamped, thereby improving the friction and wear performance of the hot stamping die surface and extending the service life of the die, resulting in ultra-high strength parts that meet the requirements of surface morphology and product strength.

[0009] To achieve the above objectives, the method for hot stamping coated steel plates using laser cladding and laser quenching partitioned strengthening molds described in this invention employs the following steps:

[0010] Step 1): Simulate and analyze the three-dimensional contact and stress of the convex and concave dies of the coated steel plate and curved mold to determine the laser cladding zone, laser quenching zone and the geometric transition zone connecting the two, and select the mold substrate and cladding powder.

[0011] Step 2): Mill the laser hardening zone to make the milled surface of the laser hardening zone consistent with the shape and size of the forming mold surface; pre-mill the laser cladding zone and the geometric transition zone to obtain a pre-milled zone; use a six-axis robot with a laser cladding head and a two-axis positioner to perform laser cladding on the pre-milled zone to obtain a laser cladding layer, the thickness of the laser cladding layer being greater than the thickness of the pre-milled zone;

[0012] Step 3): Perform secondary milling on the laser cladding layer. The thickness of the secondary milling is less than the thickness of the pre-milling area to obtain the clad pre-milling area. The mold surface of the clad pre-milling area has the same shape and size as the forming mold surface and smoothly transitions to the laser quenching area through the geometric transition area.

[0013] Step 4): Quench the laser-hardened zone after milling;

[0014] Step 5): Assemble the punch and die after laser cladding and laser quenching into a hot stamping die, and perform hot stamping on the coated steel sheet.

[0015] Preferably, in the simulation analysis of step 1), the raised covering surface of the punch in contact with the coated steel plate and the raised covering surface of the die in contact with the coated steel plate are taken as the stress area. The outer edge of each of the raised covering surfaces is extended outward by 10mm to form a boundary. The area inside the boundary is determined as the laser cladding area. The area outside the laser cladding area is the laser quenching area and the geometric transition area.

[0016] Preferably, in step 1), the mold substrate is hot work die steel H13, and the cladding powder is composed of Co-based alloy, Cr3C2 and WS2 powder, with the weight percentages of the three being 75%, 15% and 10%, respectively. WS2 is decomposed at 470°C, and the decomposed elements react with Cr3C2 to generate self-lubricating CrS. The undecomposed WS2 and CrS lubricate and resist wear in the cladding layer.

[0017] Preferably, from the laser cladding zone through the geometric transition zone to the laser hardening zone, the milling thickness decreases linearly from the pre-milling thickness to 0 mm in the laser hardening zone, and the inclination angle of the thickness line in the geometric transition zone is 30°.

[0018] Preferably, in step 2), the sum of the following three angles—the angle α between the axis of the current laser cladding head and the Z-axis, the angle β between the axis of the current laser cladding head and the normal of the current curved surface, and the adjustment angle γ of the positioner—is equal to the angle between the initial normal of the curved surface and the Z-axis.

[0019] Preferably, the coefficient of friction between the self-lubricating laser cladding layer (10) and the coated steel plate (1) during hot stamping is 0.25–0.30.

[0020] The beneficial effects of adopting the above technical solution in this invention are:

[0021] (1) Based on stress analysis and reinforcement area division, the present invention can achieve the most reasonable composition of laser cladding area and quenching area of ​​the die surface through laser cladding and laser quenching partition composite reinforcement, and obtain a high cost-performance (good reinforcement quality, low cost and high efficiency) hot stamping die surface reinforcement scheme, thereby realizing hot stamping die surface reinforcement and service life extension.

[0022] (2) This invention uses DynaForm software to perform full three-dimensional, full-process dynamic simulation of sheet metal, punch, and die contact and stress during hot stamping to accurately determine whether the die surface is under stress and whether it slides relative to the sheet metal during the forming process. For the die surface with stress and sheet metal sliding, the boundary formed by extending 10mm beyond the outer surface of the cladding surface of the protruding part of the die surface is the key area of ​​friction and wear under stress, which needs to be strengthened by laser cladding. The area outside the boundary is the laser hardening area where the die surface is not under stress and there is no relative sliding of the sheet metal. Only when the die is finally closed does the punch, die, and sheet metal come into contact and compact. This achieves the most reasonable composition of cladding and hardening areas.

[0023] (3) The self-lubricating laser cladding layer can effectively improve the friction between the die surface and the coated steel plate. The self-lubricating properties of the laser cladding layer can ensure that the friction coefficient between the cladding die surface and the coated steel plate is 0.25 to 0.30. By reducing the number of micro-protrusion joints on the coating surface that are compressed and broken during dynamic friction in the hot stamping process, the powdering and peeling of the coating can be reduced. By weakening the tangential stress of the cladding die surface on the coated steel plate, the generation of micro-cracks in the coated steel plate can be reduced. By reducing the powdering and peeling of the coating and reducing the generation of micro-cracks in the coating, the protective performance of the coating can be improved, and the surface quality of the parts obtained by hot stamping coated steel plate can be improved.

[0024] (4) Laser cladding of curved surfaces is performed through the linkage of a six-axis robot and a two-axis positioner. Specifically, three angles are controlled (angle α between the axis of the laser cladding head and the Z-axis, angle β between the axis of the cladding head and the normal to the curved surface, and adjustment angle γ of the positioner) to meet the comprehensive requirements of the metal powder's self-weight, spot area, and powder beam area. The resulting curved surface laser cladding layer has uniform thickness, small secondary processing allowance, and can guarantee the roughness and precision requirements of the final mold surface, resulting in good comprehensive mechanical properties of the cladding layer. Attached Figure Description

[0025] The present invention will be further described in detail below with reference to the accompanying drawings:

[0026] Figure 1 This is a schematic diagram showing the state where the punch is not in contact with the coated steel sheet during the hot stamping process;

[0027] Figure 2 for Figure 1 A schematic diagram of the three-dimensional force analysis when the punch moves downward to press the coated steel sheet;

[0028] Figure 3 for Figure 2 A schematic diagram of the three-dimensional force analysis when the punch continues to move downwards to press the coated steel sheet;

[0029] Figure 4 for Figure 2 A schematic diagram showing the enlarged structure of part M in the middle section, the division of the reinforced area, and some dimension annotations;

[0030] Figure 5 This is a schematic diagram illustrating the process of laser cladding and laser quenching in the reinforced area.

[0031] Figure 6 for Figure 5 Enlarged view of the structure of part N in the middle section, as well as cladding and milling dimension markings;

[0032] Figure 7 This is a schematic diagram of the initial position when using a six-axis robot and a two-axis positioner for cladding processing.

[0033] Figure 8 for Figure 7 A schematic diagram of the angular linkage relationship between a six-axis robotic arm and a two-axis positioner.

[0034] In the figure: 1. Coated steel plate; 2. Punch; 3. Die; 4. Pressure ring; 5. Laser hardening zone; 6. Geometric transition zone; 7. Laser cladding zone; 8. Forming die surface; 9. Pre-milling zone; 10. Laser cladding layer; 11. Positioner; 12. Laser hardening layer. Detailed Implementation

[0035] Reference Figure 1 The hot stamping curved surface die shown includes a punch 2, a die 3, and a blank holder 4, and has the geometric features of a hot stamping die such as a blank holder surface, die fillet, sidewalls, punch fillet, and bottom. A coated steel plate 1 is fixed between the blank holder 4 and the die 3. The punch 2 and die 3 used in this invention are both relatively large and have complex curved surfaces; the die surface dimensions of the punch 2 and die 3 can reach 1400mm × 1400mm or more.

[0036] Based on the performance requirements of the cladding layer to resist high temperature and high pressure, and alternating hot and cold temperatures on hot stamping die surfaces, and considering the thermal conductivity requirements of the die surface during the cladding and quenching processes, this invention uses hot work die steel H13 (4Cr5MoSiV1) as... Figure 1The base material of the hot stamping die shown, namely the punch 2 and the die 3, is made of hot work die steel H13 (4Cr5MoSiV1).

[0037] Using DynaForm software Figure 1 In the hot stamping process, the three-dimensional contact and stress of the coated steel sheet 1, punch 2, and die 3 are simulated and analyzed to determine the stress-bearing surface of the protruding part of the die surface, and to divide the reinforcement area according to whether it is subjected to stress during the forming process. Figure 2 and Figure 3 As shown, the raised surfaces I, II, III, and IV in the punch 2 that contact the coated steel plate 1 are the stress zones, and the raised surfaces A, B, C, and D in the die 3 that contact the coated steel plate 1 are also stress zones. The outer edges of these raised surfaces I, II, III, IV, A, B, C, and D are extended outwards by 10mm to form the boundaries of the raised surfaces. Figure 4 As shown. Figure 4 Taking raised surface I as an example, the method for forming the boundaries of other raised surfaces II, III, IV, A, B, C, and D is the same. The area within the boundary of each raised surface I, II, III, IV, A, B, C, and D is considered a key area of ​​friction and wear that is stressed and slides relative to the coated steel plate 1. These key areas of friction and wear need to be strengthened with laser cladding to ensure excellent overall strengthening performance of the die surface; this area is defined as laser cladding zone 7. The area outside laser cladding zone 7 is laser hardening zone 5 and geometric transition zone 6. Geometric transition zone 6 exists between laser cladding zone 7 and laser hardening zone 5, connecting them. Geometric transition zone 6, like laser cladding zone 7, is also strengthened using laser cladding. The laser hardening zone 5 is a region where the mold surface is not subjected to force and there is no relative sliding with the coated steel plate 1. Only when the mold is finally closed does the punch 2, die 3 and coated steel plate 1 come into contact and compact. This region is strengthened by laser hardening and is thus identified as the laser hardening zone 5.

[0038] Before laser cladding, the cladding powder must first be selected. Considering the requirements of the mold surface for good high-temperature strength, high toughness, thermal fatigue resistance, and self-lubrication, the cladding material is composed of Co-based alloy Stellite 6, Cr3C2, and WS2 powder, with weight percentages of 75%, 15%, and 10%, respectively. A Stellite 6-Cr3C2-WS2 composite coating with a self-lubricating phase is prepared by cladding. The added WS2 decomposes at 470℃, and the decomposed elements react with Cr3C2 to generate CrS, which has self-lubricating capabilities. A small amount of undecomposed WS2 and CrS play a lubricating and anti-wear role in the cladding layer.

[0039] The laser hardening zone 5, geometric transition zone 6, and laser cladding zone 7 are milled respectively. When milling the laser hardening zone 5, it is necessary to ensure that the milled surface of the laser hardening zone 5 has the same shape and dimensions as the final forming die surface, i.e., consistent with... Figure 5 The forming mold surface 8 shown has a consistent shape and size. During milling of the laser cladding zone 7 and the geometric transition zone 6, as... Figure 5 As shown in (a), for the forming die surface 8 of the laser cladding zone 7 and the geometric transition zone 6, it is necessary to pre-mill the forming die surface 8 to obtain the pre-milled zone 9. The surface of the pre-milled zone 9 is the pre-milled die surface, so that the pre-milled die surface achieves the surface roughness Ra1.6 required for laser cladding, ensuring the required surface roughness for laser cladding. The pre-milling thickness h is determined based on the magnitude of the force and friction wear on the die surface during hot stamping, as well as the wear resistance, hardness, toughness, and bonding degree between the cladding layer and the die substrate H13. It also depends on the selected laser cladding metal powder and cladding process parameters, combined with... Figure 6 As shown.

[0040] Next, the secondary milling thickness h' is determined according to the requirement of surface roughness Ra1.25 after secondary milling. The sum of the pre-milling thickness h and the secondary milling thickness h' is taken as the total thickness H of the initial cladding layer H = h + h'. Laser cladding is then performed on the pre-milled area 9 based on the total thickness H of the initial cladding layer to obtain the laser cladding layer 10, as shown below. Figure 5 As shown in (b), the thickness of the laser cladding layer 10 is H, which is greater than the pre-milling thickness h. After laser cladding, the surface of the laser cladding layer 10 is subjected to secondary milling. The secondary milling thickness is h', which is less than the pre-milling thickness h, to obtain the final clad mold surface area, as shown in [example image]. Figure 5 As shown in (c), the shape and dimensions of the clad mold surface are consistent with those of the forming mold surface 8, that is, only the clad pre-milled area 9 is retained. The thickness of the clad pre-milled area 9 is h, ensuring that the mold surface of the clad pre-milled area 9 obtained after the second milling smoothly transitions to the laser hardening area 5 through the geometric transition area 6. That is, the shape and dimensions of the mold surfaces of the laser cladding area 7 and the laser hardening area 5 are smoothly connected, such as... Figure 5 As shown in (d).

[0041] Combination Figure 4 and Figure 6 As shown, the preferred pre-milling thickness of this invention is h = 0.8 mm. This pre-milling thickness h is the remaining thickness of the laser cladding layer 10 after secondary milling, which is also the required thickness of the cladding reinforcement layer on the mold surface. Preferably, the secondary milling thickness h' is less than the pre-milling amount h, preferably h' = 0.6 mm. Therefore, the total thickness H of the laser cladding layer 10 is H = 0.8 + 0.6 = 1.4 mm.

[0042] During pre-milling, the forming die surface 8 is sequentially rough-milled, semi-finish-milled, and finish-milled according to the pre-milling thickness h. For the preferred pre-milling thickness h = 0.8 mm, the rough milling thickness is 0.5 mm, the semi-finish milling thickness is 0.2 mm, and the finish milling thickness is 0.1 mm, resulting in a surface roughness Ra1.6 after finish milling. Then, based on the required surface roughness Ra1.25 for the forming die surface 8, the secondary milling thickness h' is determined, preferably h' = 0.6 mm. During secondary milling, a 0.5 mm thickness is first rough-milled, followed by a 0.1 mm thickness finish milled, resulting in a die surface roughness Ra1.25 after the secondary milling, ensuring the surface roughness required for hot stamping.

[0043] Combination Figure 4 and Figure 5 In the pre-milling zone 9, the thickness decreases linearly from the laser cladding zone 7 through the geometric transition zone 6 to the laser hardening zone 5, from the pre-milling zone h = 0.8 mm to the laser hardening zone 5 at 0 mm. The inclination angle of the thickness line in the geometric transition zone 6 is 30°.

[0044] like Figure 7 and Figure 8 As shown, when laser cladding the pre-milled area 9, the present invention employs a laser cladding head. six axis robotic arms and Two shafts The positioner 11 performs laser cladding on the material to obtain a laser cladding layer 10. Eight-axis linkage laser surface cladding is performed using different powder ratios and combinations of laser cladding process parameters to obtain a self-lubricating Stellite 6-Cr3C2-WS2 surface cladding layer. The laser cladding process parameters include laser power W, powder feed rate Q, laser scanning speed V, and spot diameter. The laser power W is 2000W, the laser scanning speed V is 450mm / min, the powder feed rate Q is 10g / min, the spot diameter is 5mm, and the protective gas is nitrogen. During laser cladding, a linkage is established between the laser cladding head and the Z-axis, the laser head and the surface normal, and the positioner's adjustment angle. This primarily involves controlling three angles: the angle α between the current laser cladding head axis a2 and the Z-axis, the angle β between the current laser cladding head axis a2 and the current surface normal b2, and the adjustment angle γ of the two-axis positioner 11. This linkage is achieved by a six-axis robot and the two-axis positioner to meet the comprehensive requirements of the metal powder's weight, spot area, and powder beam area. The initial angle β between the initial laser cladding head axis a1 and the initial surface normal b1, and the adjustment angle γ of the two-axis positioner 11, equal to the angle between the initial surface normal b1 and the current surface normal b2, are all considered. The sum of α + β + γ equals the angle between the initial surface normal b1 and the Z-axis. The maximum value of α is α_max. max At 30°, the maximum value of β is β. max The maximum value of γ is 15°. maxIt is 45°.

[0045] During the cladding process, the angle between the tangent of the mold surface at the laser beam irradiation point and the inclination of the horizontal plane is (α+β). Compared with the laser cladding process parameters for cladding downward along the inclined plane, the laser power and powder feeding amount for cladding upward along the inclined plane are reduced by (α+β)% respectively. Other cladding process parameters (cladding rate, spot size, defocusing amount, overlap rate, etc.) remain unchanged. This ensures that the thickness of the laser cladding layer 10 obtained by the upward and downward cladding paths is consistent, that is, the surface roughness is as consistent as possible.

[0046] like Figure 5 As shown in (d), after a second milling of the surface of the laser cladding layer 10 to obtain the pre-milled area 9 with good cladding, the laser quenching area 5, which has the same shape and size as the forming die surface 8, is subjected to laser surface quenching to obtain the laser quenched layer 12. The surface of the laser quenched layer 12 is smoothly connected to the surface of the pre-milled area 9 through the surface of the geometric transition area 6. The laser quenching area 5 is quenched by an eight-axis linkage of a robot arm with a quenching head and a positioner. Since it does not involve cladding powder, the equipment technology and process requirements are relatively simple.

[0047] Finally, the punch 2 and die 3, after laser cladding and laser hardening, are assembled into a single unit. Figure 1 The hot stamping die shown is used to hot stamp the coated steel sheet 1. During the hot stamping process, the friction coefficient between the self-lubricating laser cladding layer 10 and the coated steel sheet 1 is 0.25 to 0.30, which reduces the friction coefficient between the coated steel sheet 1 and the cladding die surfaces of the punch 2 and die 3 by 0.10 to 0.15. This reduces the tangential stress on the surface of the coated steel sheet 1, improves the protective performance of the coating by reducing the powdering and peeling of the coating and reducing the generation of microcracks in the coating, and improves the surface quality of the hot stamped coated steel sheet 1. After hot stamping, an ultra-high strength structural component with a tensile strength of over 1500 MPa can be obtained.

Claims

1. A method for hot stamping coated steel sheet for laser cladding and laser quenching partitioned strengthening molds, characterized by the following steps: Step 1): Simulate and analyze the three-dimensional contact and stress of the coated steel plate (1), the punch (2) and the die (3) of the curved mold, determine the laser cladding zone (7), the laser quenching zone (5) and the geometric transition zone (6) connecting the two, and select the mold substrate and cladding powder. Step 2): The laser hardening zone (5) is milled to make the milled surface of the laser hardening zone (5) consistent with the shape and size of the forming mold surface (8); the laser cladding zone (7) and the geometric transition zone (6) are pre-milled to obtain the pre-milled zone (9); the pre-milled zone (9) is laser clad using a six-axis robot with a laser cladding head and a two-axis positioner to obtain the laser cladding layer (10); the thickness of the laser cladding layer (10) is greater than the thickness of the pre-milled zone (9); Step 3): The laser cladding layer (10) is milled twice. The thickness of the second milling is less than the thickness of the pre-milled area (9) to obtain the clad pre-milled area (9). The mold surface of the clad pre-milled area (9) has the same shape and size as the forming mold surface (8) and smoothly transitions to the laser quenching area (5) through the mold surface of the geometric transition area (6). Step 4): Quench the laser-hardened zone (5) after milling; Step 5): Assemble the punch (2) and die (3) after laser cladding and laser quenching into a hot stamping die, and perform hot stamping on the coated steel plate (1).

2. The method for hot stamping coated steel plates of laser cladding and laser quenching partitioned strengthening molds according to claim 1, characterized in that: In step 1), during the simulation analysis, the raised covering surface of the punch in contact with the coated steel plate and the raised covering surface of the die in contact with the coated steel plate are taken as the stress area. The outer edge of each of the raised covering surfaces is extended outward by 10mm to form a boundary. The area inside the boundary is determined as the laser cladding area (7). The area outside the laser cladding area (7) is the laser quenching area (5) and the geometric transition area (6).

3. The method for hot stamping coated steel plates of laser cladding and laser quenching partitioned strengthening molds according to claim 1, characterized in that: In step 1), the mold substrate is hot work die steel H13, and the cladding powder is composed of Co-based alloy Stellite 6, Cr3C2 and WS2 powder, with the weight percentages of the three being 75%, 15% and 10%, respectively. WS2 is decomposed at 470°C, and the decomposed elements react with Cr3C2 to generate self-lubricating CrS. The undecomposed WS2 and CrS lubricate and resist wear in the cladding layer.

4. The method for hot stamping coated steel plates of laser cladding and laser quenching partitioned strengthening molds according to claim 3, characterized in that: The coefficient of friction between the self-lubricating laser cladding layer (10) and the coated steel plate (1) during hot stamping is 0.25 to 0.

30.

5. The method for hot stamping coated steel sheet of laser cladding and laser quenching partitioned strengthening mold according to claim 3, characterized in that: The laser cladding head has a laser power of 2000W, a scanning speed of 450mm / min, a powder feeding rate of 10g / min, a spot diameter of 5mm, and a protective gas of nitrogen.

6. The method for hot stamping coated steel sheet of laser cladding and laser quenching partitioned strengthening mold according to claim 1, characterized in that: From the laser cladding zone (7) through the geometric transition zone (6) to the laser hardening zone (5), the milling thickness decreases linearly from the pre-milling thickness to 0 mm in the laser hardening zone (5), and the inclination angle of the thickness line in the geometric transition zone (6) is 30°.

7. The method for hot stamping coated steel sheet for laser cladding and laser quenching partitioned strengthening molds according to claim 1, characterized in that: The pre-milling thickness is 0.8 mm, the secondary milling thickness is 0.6 mm, and the thickness of the laser cladding layer (10) is 1.4 mm.

8. The method for hot stamping coated steel sheet of laser cladding and laser quenching partitioned strengthening mold according to claim 7, characterized in that: The pre-milling thickness of 0.8mm is achieved by rough milling of 0.5mm, semi-finish milling of 0.2mm, and finish milling of 0.1mm. The surface roughness of the mold after pre-milling is Ra1.

6. The secondary milling thickness of 0.6mm is achieved by rough milling of 0.5mm and finish milling of 0.1mm. The surface roughness of the mold after secondary milling is Ra1.

25.

9. The method for hot stamping coated steel sheet of laser cladding and laser quenching partitioned strengthening mold according to claim 1, characterized in that: In step 2), the angle α between the axis of the current laser cladding head and the Z-axis, the angle β between the axis of the current laser cladding head and the normal of the current curved surface, and the adjustment angle γ of the positioner are controlled. The sum of these three angles is equal to the angle between the initial normal of the curved surface and the Z-axis. The maximum value of angle α is 30°, the maximum value of angle β is 15°, and the maximum value of angle γ is 45°.

10. The method for hot stamping coated steel sheet of laser cladding and laser quenching partitioned strengthening mold according to claim 9, characterized in that: During the cladding process, the angle between the tangent of the mold surface at the laser beam irradiation point and the inclination of the horizontal plane is (α+β). The laser power and powder feeding amount for cladding upward along the inclined plane are reduced by (α+β)% compared with the laser cladding process parameters for cladding downward along the inclined plane, while other cladding process parameters remain unchanged.

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