A laser welding process control method for acid pickling and rolling process of 2000mpa grade hot stamping steel

Abstract

The application discloses a kind of 2000MPa grade hot stamping steel acid rolling process laser welding process control method, using acid rolling combined unit production, the tail of front coil strip steel and the head of rear coil strip steel are welded using laser when, after welding heating power is 30~40kW, laser power is 11.0~12.0kW, laser focal length-2mm~‑3mm, focal length compensation-2mm~‑3mm, welding speed 4.5~5.4m / min, laser head pressure 16~18kN.Laser welding is used to cooperate before and after heating process, form the tempering martensite structure with good deformation capacity, greatly improve production speed and weld quality;Cup convex experiment does not appear along weld cracking phenomenon, under different reduction rate, the fracture position is in base material area after tensile test, and the transition effect of the mechanical properties of weld and base material is good, and no strip breakage caused by weld quality occurs after drawing and rolling process, which meets the stable production demand in batches.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hot stamping steel, and particularly relates to a 2000MPa-grade hot stamping steel pickling and rolling process laser welding process control method. BACKGROUND

[0002] Consumers have increasingly high requirements for automobile safety. Under the premise of ensuring automobile safety, the strength of automobile steel is improved, and automobile weight reduction is realized under the premise of ensuring automobile crash performance through part thinning, which is an effective way for energy saving and emission reduction of the automobile industry. Therefore, in recent years, new vehicle models have continuously optimized advanced high-strength steel and advanced production processes, among which the large use of hot stamping steel is one of the most typical representatives. Hot stamping forming steel is combined with hot forming process technology to solve the contradiction between strength and formability by separating forming and strengthening into two steps, and to produce ultra-high strength automobile parts. Compared with cold forming, the hot stamping forming steel has the advantages of high part strength, good formability, high part size precision, etc., and is widely used in reinforcement parts for preventing deformation during automobile collision, such as door impact beams, bumper beams, side beams, A-pillar, B-pillar reinforcement parts and central passages. At present, hot forming steel for automobiles mainly uses 1500MPa steel grade, and develops towards 1800MPa grade and above, with the highest reaching 2000MPa. Hot stamping steel products are designed with a carbon content of 0.20% to 0.40%, and a large amount of alloying elements are added. After cold rolling and continuous annealing, the products are delivered for use. During the whole production process, the control of the pickling and rolling process is difficult, and there is a phenomenon of strip breakage during pickling and rolling due to high carbon equivalent, which affects the production efficiency and the like. SUMMARY

[0003] In order to overcome the defects of the prior art, the application proposes a pickling and rolling process control method and a pickling and rolling laser welding joint structure regulation method to change the structure form of the laser welding joint, temper the brittle structure of twinned martensite formed in the high-temperature heating and rapid cooling process of high-carbon steel, obtain tempered martensite, solve the strip breakage phenomenon of the laser welding joint under cold deformation due to the increase of deformation amount, and realize stable and continuous production of 2000MPa-grade hot stamping steel products.

[0004] To achieve the above-mentioned objectives, this invention provides a laser welding process control method for the pickling and rolling process of 2000MPa grade hot stamping steel. The method uses a combined pickling and rolling unit for production. When the tail end of the front coiled strip and the head end of the rear coiled strip are laser-welded, the laser welding process parameters are as follows: heating power before welding is 0kW, heating power after welding is 30-40kW, laser power is 11.0-12.0kW, laser focal length is -2mm to -3mm, focal length compensation is -2mm to -3mm, welding speed is 4.5-5.4m / min, and laser head pressure is 16-18kN.

[0005] In the above technical solution, the width of the hot-rolled substrate is 1000-1500mm and the thickness is 2.0-4.5mm.

[0006] Further, the laser welding process is as follows: the tail of the previous coil and the head of the next coil enter the welding machine. After passing through the four alignment points of the welding machine, the head and tail are aligned along the longitudinal centerline of the strip. The welding machine's inlet and outlet clamps clamp the strip. The welding machine's double shears cut off 30-70mm from the head and tail respectively. At the same time, a hole is punched in the middle of the tail 50mm away from the weld seam for weld seam tracking. The welding machine's inlet and outlet clamps butt the head and tail of the strip together, with a butt gap between 0.02-0.1mm. The welding machine's laser head and the pressing wheel drop down, and welding preparation begins. The welding machine carriage begins to move from the operating side to the drive side. The preheating process involves contacting the strip steel first, with a maximum preheating power of 40kW. Under the protection of a protective gas (helium) on both sides of the strip steel, the laser is emitted from the laser head with a maximum laser power of 120,000 kW. After welding, the weld is annealed by post-heating of the welding machine, with a maximum annealing power of 40kW (only high-strength steel requires post-heating). After welding, the weld quality is judged by a weld quality inspection system. Qualified welds can be released, while unqualified welds are re-welded, and the above steps are repeated. For welds that require edge trimming, crescent shears are located at the weld machine exit section to remove the defective areas at the weld edges.

[0007] Furthermore, during the production process, when welding 2000MPa grade hot stamping steel with the same steel grade, the thickness difference of the connecting raw materials must be controlled within 0.4mm, and the width difference must be controlled within 50mm.

[0008] Furthermore, when welding 2000MPa grade hot-formed steel with other steel grades, if the width and thickness change simultaneously, it is necessary to ensure that the coil with the smaller thickness has a larger width, and the coil with the larger thickness has a smaller width.

[0009] Furthermore, when connecting 2000MPa grade hot-formed steel to different steel grades, 1500MPa hot-formed steel should be selected for the connection.

[0010] Further, when the 2000MPa grade hot forming steel is overlapped with the 1500MPa grade hot forming steel, the principle of thin-to-thick and narrow-to-wide should be ensured.

[0011] A laser welding process control method for a 2000MPa grade hot stamping steel pickling process, the chemical composition of the hot stamping steel is C: 0.20-0.40%, Si: 0.05-1.50%, Mn: 1.20-2.00%, P≤0.030%, S≤0.005%, Al≥0.010%, Ti: 0.010-0.030%, Nb: 0.01-0.03, V: 0.10-0.30%, N≤0.0100%, B: 0.0010-0.0030%, the rest is Fe and inevitable impurities.

[0012] Compared with the prior art, the beneficial effects of the present application are:

[0013] ①The laser welding is combined with the pre and post heating process, which avoids the formation of difficult-to-deform and brittle twin martensite in the laser weld of the 2000MPa grade hot stamping steel product pickling, and forms the tempered martensite structure with good deformation capacity, greatly improves the production speed and weld quality, realizes continuous and stable production of the same steel grade, and does not produce the strip breaking phenomenon.

[0014] ②The weld obtained by the laser welding method has no cracking phenomenon along the weld in the cup convex experiment, the fracture position is in the base material region after the tensile experiment under different reduction rates, the mechanical property transition effect of the weld and the base material is good, the hardness of the base material region is 275-385HV, the hardness of the weld region is 399-470HV, the hardness difference between the base material region and the weld region is about 124-195HV, and there is no strip breaking phenomenon caused by the weld quality after the straightening and rolling process, which meets the batch stable production demand. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is the microstructure (50X) photo of the upper surface of the weld of Example 1;

[0016] Figure 2 It is the microstructure (50X) photo of the middle part of the weld of Example 1;

[0017] Figure 3 It is the microstructure (50X) photo of the lower surface of the weld of Example 1;

[0018] Figure 4 It is the microstructure (200X) photo of the weld part of Example 1;

[0019] Figure 5 It is the microstructure (200X) photo of the heat affected zone of the weld of Example 1;

[0020] Figure 6 Figure 1 - Macro view of Example 1 fracture area (1 mm)

[0021] Figure 7 Figure 2 - Macro view of Example 1 fracture area (200 um)

[0022] Figure 8 Figure 3 - Macro view of Example 1 fracture area (90 um)

[0023] Figure 9 Figure 4 - Inclusion view in Example 1 fracture dimple

[0024] Figure 10 Figure 5 - Example 1 fracture area view

[0025] Figure 11 Figure 6 - Example 2 weld top surface microstructure (50X) photo

[0026] Figure 12 Figure 7 - Example 2 weld middle microstructure (50X) photo

[0027] Figure 13 Figure 8 - Example 2 weld bottom surface microstructure (50X) photo

[0028] Figure 14 Figure 9 - Example 2 weld area microstructure (200X) photo

[0029] Figure 15 Figure 10 - Example 2 weld heat affected zone microstructure (200X) photo

[0030] Figure 16 Figure 11 - Macro view of Example 2 fracture area (1 mm)

[0031] Figure 17 Figure 12 - Macro view of Example 2 fracture area (200 um)

[0032] Figure 18 Figure 13 - Pre-patent weld microstructure photo

[0033] Figure 19 Figure 14 - Post-patent weld microstructure photo

[0034] Figure 20 Figure 15 - Pre-patent weld cupping view

[0035] Figure 21 Figure 16 - Post-patent weld cupping view

[0036] Figure 22 Figure 17 - 60% deformation weld quality macro view

[0037] Figure 23 Microhardness of the welded joint at different reductions. DETAILED DESCRIPTION

[0038] The application will be further described in conjunction with specific examples, but in no way limit the application. For the sake of brevity, the raw materials in the following examples are all commercially available products unless otherwise specified, and the methods used are all conventional methods unless otherwise specified.

[0039] A laser welding process control method for pickling and rolling process of 2000MPa hot stamping steel, the laser welding process of Example 1 and Example 2 is shown in Table 1.

[0040] Table 1 Laser welding process parameters of examples

[0041] Number Heating power Laser power Laser focal length Focal length compensation Welding speed Laser head pressure Example 1 40KW 12KW -2mm -3mm 5.0m / min 18KN Example 2 35KW 11.0KW -2mm -3mm 4.8m / min 16KN

[0042] Example 1

[0043] In Example 1, 2000MPa hot stamping steel of the same steel grade is selected for lap welding, i.e. 2000+2000, the chemical composition is C: 0.3%, Si: 0.4%, Mn: 1.6%, Ti: 0.02%, Nb: 0.02%, V: 0.2%, B: 0.0025, and the rest is Fe element and inevitable impurities. The size specification is 2.8mm×1300mm lap 3.2mm×1250mm.

[0044] The laser welding process is shown in Table 1. The microstructure of the weld is shown in Table 4.

[0045] The metallographic photograph of the weld structure of Example 1 is shown in Figure 1-5 The weld is full and has no obvious welding defects. The mechanical property transition effect of the weld and the base material is good.

[0046] Table 2 Energy spectrum analysis results of inclusion morphology in the dimple on the fracture of Example 1 Figure 9

[0047] Spectrum C O Al Si Ti V Mn Fe Sum Figure 9 - spectrum Figure 1 8.72 36.09 2.82 4.70 0.49 1.52 17.73 27.93 100.00 Figure 9 - spectrum Figure 2 8.66 41.40 3.22 6.97 0.66 0.32 23.51 15.24 100.00 Figure 9 - spectrum Figure 3 5.54 6.54 0.78 1.34 31.96 53.84 100.00 Figure 9 - spectrum Figure 4 8.60 30.78 2.54 5.90 0.29 0.66 27.33 23.90 100.00 Figure 9 - spectrum Figure 5 26.57 25.20 11.87 36.35 100.00 Figure 9 - spectrum Figure 6 1.78 4.54 0.34 0.89 5.11 87.34 100.00 Maximum 26.57 41.40 3.22 6.97 0.78 1.52 31.96 87.34 Minimum 1.78 4.54 0.34 0.89 0.29 0.32 5.11 15.24

[0048] The low-magnification morphology of the fracture local area of Example 1 is shown in Figure 6-8 The morphology and energy spectrum analysis of the cup convex fracture are shown in Figure 9-10 Figure 9 The inclusion morphology in the dimple on the fracture of Example 1 and the energy spectrum analysis results are shown in Table 2. Figure 10 The morphology of the fracture local area of Example 1 and the energy spectrum analysis results are shown in Table 3; the morphology is mainly characterized by dimples, and small range of inclusion aggregation can be seen near the edge of the convex position.

[0049] Table 3 Morphology of the fracture local area of Example 1 and energy spectrum analysis results Figure 10 ​​Energy spectrum analysis result of fracture local area morphology

[0050] Spectrum C O Mn Fe Sum Figure 10 - spectrum Figure 1 2.73 1.61 1.49 94.17 100.00 Figure 10 - spectrum Figure 2 8.89 1.60 1.83 87.69 100.00 Figure 10 - spectrum Figure 3 10.27 8.48 2.54 78.70 100.00 Figure 10 - spectrum Figure 4 2.03 1.54 1.20 95.23 100.00 Maximum 10.27 8.48 2.54 95.23 Minimum 2.03 1.54 1.20 78.70

[0051] The strip steel produced after the above welding process has good weld quality after the cold rolling process of the stretcher leveler, multiple tension rolls, bending and large deformation, and no strip breakage phenomenon occurs in the weld. The laser weld forms a tempered martensite structure with good deformation capacity, and no twin martensite with large brittleness and difficult deformation is formed, which greatly improves the weld quality and production efficiency, and realizes stable production of the same steel grade.

[0052] Example 2

[0053] In example 2, the 2000MPa grade hot stamping steel is selected for heterogeneous lap welding, i.e. 1500MPa grade product is lap welded with 2000 grade product, and the chemical composition remains unchanged. The size specification is: 1500MPa grade product: 3.2mm x 1200mm, 2000 grade product: 3.6mm x 1200mm.

[0054] The laser welding process is shown in Table 1. The microstructure of the weld is shown in Table 4.

[0055] The fracture local area macroscopic morphology of example 2 is shown in Figure 16-17 According to the cup convex fracture morphology and energy spectrum analysis, the morphology is mainly characterized by toughness, and small range inclusions can be seen near the edge of the convex position.

[0056] According to the weld microstructure metallographic chart of example 2, as shown in Figure 11-15 The weld has good fullness and no obvious welding defects. The mechanical property transition effect of the weld and the base material is good. The weld also forms a tempered martensite structure inside, and no strip breakage phenomenon occurs during rolling production, realizing stable production of 2000MPa grade hot stamping steel heterogeneous steel grade lap welding and meeting the needs of enterprises.

[0057] Table 4: Microstructure of weld of example

[0058]

[0059] The laser welding process of the present application is combined with the heating process before and after, which avoids the formation of twin martensite with large brittleness and difficult deformation in the laser weld of 2000MPa grade hot stamping steel product, and forms a tempered martensite structure with good deformation capacity, which greatly improves the production speed and weld quality, realizes continuous and stable production of the same steel grade, and does not produce strip breakage phenomenon. The weld microstructure morphology before the implementation of the patent is shown in Figure 18 The weld microstructure morphology after the implementation of the patent is shown in Figure 19 .

[0060] The weld obtained using the laser welding method described in this invention did not exhibit cracking along the weld during the cupping test (the cupping morphology of the weld before patent implementation is shown in the figure). Figure 20 As shown, the weld cup convex shape after patent implementation is as follows: Figure 21 As shown in the figure, under different reduction rates, the fracture location after tensile testing was always in the base metal region. The transition effect of mechanical properties between the weld and the base metal was good. The hardness of the base metal region was 275-385 HV, and the hardness of the weld region was 399-470 HV. The hardness difference between the base metal region and the weld region was approximately 124-195 HV. After tensile straightening and rolling processes, no strip breakage due to weld quality was observed, meeting the requirements for stable mass production. A macroscopic diagram of the weld quality with 60% deformation is shown in the figure. Figure 22 As shown, the microhardness of the welded joint under different reduction amounts is as follows: Figure 23 As shown.

[0061] For anyone skilled in the art, many possible variations and modifications can be made to the technical solutions of this invention, or equivalent embodiments can be modified based on the disclosed technical content, without departing from the scope of the technical solutions of this invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this invention without departing from the content of the technical solutions of this invention should still fall within the protection scope of the technical solutions of this invention.

Claims

1. A method of controlling a laser welding process of a pickling and rolling process of a 2000 MPa grade hot-stamped steel, characterized in that, The method is produced by using an acid pickling combined unit, when the tail of the front coil strip and the head of the rear coil strip are welded by laser, the laser welding process parameters are as follows: the heating power before welding is 0 kW, the heating power after welding is 30-40 kW, the laser power is 11.0-12.0 kW, the laser focal length is -2 mm to -3 mm, the focal length compensation is -2 mm to -3 mm, the welding speed is 4.5-5.4 m / min, and the laser head pressure is 16-18 kN; The hot-rolled base plate has a width of 1000-1500 mm and a thickness of 2.0-4.5 mm; The laser welding process is as follows: the tail of the upper coil and the head of the lower coil enter the welding machine, the head and the tail are aligned along the longitudinal center line of the strip through the four centers of the welding machine, the welding machine entry and exit clamps clamp the strip, the welding machine double cutting shears cut off 30-70 mm of the head and the tail respectively, and a hole is punched at the middle of the tail 50 mm away from the weld; the welding machine entry and exit clamps butt joint the head and the tail of the strip together, the butt joint gap is between 0.02-0.1 mm, the welding machine laser head falls down, the rolling wheel falls down, the welding preparation starts, the welding machine trolley starts to run from the operation side to the transmission side, the welding machine front heating first contacts the strip, under the action of the protective gas above and below the strip, the laser is emitted from the laser head; after welding, the weld is annealed by the welding machine rear heating; In the production process, when the 2000 MPa hot stamping steel is welded with the same steel, the thickness difference of the connecting raw material is controlled within 0.4 mm, and the width difference is controlled within 50 mm; When the 2000 MPa hot stamping steel is welded with the same steel, when the width and thickness change at the same time, the width of the strip coil with smaller thickness is larger, and the width of the strip coil with larger thickness is smaller; When the 2000 MPa hot stamping steel is connected with different steel, the 1500 MPa hot stamping steel is selected for connection; When the 2000 MPa hot stamping steel is connected with the 1500 MPa hot stamping steel, the principle of thin connection thick and narrow connection wide is ensured; The chemical composition of the 2000 MPa hot stamping steel, in terms of mass percentage, is C: 0.20%-0.40%, Si: 0.05%-1.50%, Mn: 1.20%-2.00%, P≤0.030%, S≤0.005%, Al≥0.010%, Ti: 0.010%-0.030%, Nb: 0.01%-0.03%, V: 0.10%-0.30%, N≤0.0100%, B: 0.0010%-0.0030%, and the rest is Fe and inevitable impurities.

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