Laser welding joint and laser welding method for ultrahigh-strength steel

Abstract

The invention provides a laser welding method for tailor-welding hot forming steel with the tensile strength exceeding 1500 Mpa after hot forming before hot forming, and particularly relates to a method capable of avoiding cracking of a heat affected zone after welding. The hardness distribution of a heat affected zone is controlled during laser welding cooling, so that the hardness of the heat affected zone after welding does not exceed the highest hardness of a weld joint melting zone, and the highest hardness value does not exceed H / 3-20 HV; (H is the tensile strength of the material obtained after thermal forming); the minimum width of the heat affected zone is not less than 0.2 m; the method specifically comprises the steps that energy distribution of a laser welding heat source is controlled in the welding process, a laser beam forms a flat-top light source on the surface of a workpiece, a welding pool is formed when the light source irradiates the workpiece for welding, the welding pool has the length L and the width B, and L / B is larger than or equal to 3. A welded joint obtained through the laser welding method has excellent post-welding cracking resistance for welding of high-strength steel with the tensile strength exceeding 1500 Mpa after thermal forming.

Description

Technical Field

[0001] The present invention relates to the manufacture of welded parts of ultra-high strength steel, and particularly to a laser welded joint and a laser welding method of an ultra-high strength steel with a tensile strength exceeding 1500 Mpa grade hot forming steel. Background Art

[0002] In certain fields, especially in the manufacture of parts for automobiles and aerospace, there are increasingly higher requirements for lightweight in the use of materials, and the application demand for ultra-high strength steel as one of the most effective lightweight materials is increasing. Taking typical load-bearing components such as automotive A and B pillars as an example, ultra-high strength hot forming steel is increasingly used. In actual applications, laser welding is first performed on the spliced plates to form a welded structure, and then a hot stamping process is carried out on the overall welded joint to obtain a structural member with a tensile strength exceeding 1500 Mpa. However, when laser welding the spliced plates, cracking often occurs in the heat affected zone after welding, especially for materials with a strength reaching above 2000 Mpa. This is mainly because severe hardening occurs in the heat affected zone during the laser welding process, generating high-hardness martensite quenching, and its hydrogen embrittlement and cold cracking sensitivity are poor.

[0003] Patent CN202410625524.3 discloses a welding method suitable for preventing cold cracks in 1100 Mpa grade high strength steel. By opening a narrow gap groove and adding welding materials, and controlling the heat input of argon arc welding after preheating below 150°, the purpose of preventing cold cracks can be achieved without post-weld heat treatment. However, this method is only applicable to 1100 Mpa materials and is not applicable to higher strength materials. Moreover, preheating is still required, the operation is complex, and the cost is high. In addition, it only applies to argon arc welding, and its cooling rate is slower than that of laser welding itself, so it is not applicable to this application.

[0004] Patent CN202311842552.2 discloses a welding method for ultra-high strength steel with a yield strength exceeding 1400 Mpa. By opening a groove and controlling the heat input and interpass temperature during the arc welding process, the formation of post-weld cold cracks can be suppressed. Obviously, this method is only applicable to the arc welding process and is not applicable to laser welding with a higher cooling rate.

[0005] Patent CN116984716A discloses a submerged arc welding method for 690 Mpa grade high strength steel. By preheating the steel plate to 50 - 80°C before welding and controlling the heat input during the welding process, the cold cracking sensitivity can be reduced. Preheating treatment is required in this process, with low efficiency and high cost, and it only applies to the submerged arc welding process for 690 Mpa grade materials, which is not applicable to this application requirement either.

[0006] Patent CN202310750923.8 discloses a method for preventing cold cracks in laser welding of high-strength steel pipes. This method first uses laser cleaning and preheats to 400°C, adds Ni wire during the welding process, and finally needs to implement a heat preservation process after welding to avoid the occurrence of cold cracks. It can be seen that the process of this method is very complex, with low efficiency and high cost, and is not applicable to the welding of ultra-high-strength steel in this application.

[0007] Patent CN116079273A discloses a welding method for solving the cold crack problem in laser welding of 27SiMn materials. By preheating the material to 290 - 320°C before welding and heat preserving at 275 - 285°C after welding, the cooling rate of the welded joint is reduced, and the generation of hydrogen-induced cracks is reduced. It can be seen that this method requires overall preheating and post-weld heat treatment before and after welding, with low efficiency.

[0008] Patent CN114406463A discloses a welding method for laser welding ultra-high-strength steel using ultrasonic assistance. By introducing an ultrasonic energy field during the laser welding process, the solidification time of the molten pool is extended, thereby improving the cold crack tendency. This method requires the addition of a complex ultrasonic system, and the process control for the combination of ultrasonic and laser is also relatively complex, with a high cost for the entire system method.

[0009] Patent CN108672929A discloses a laser welding method for ultra-high-strength armor steel. By preheating 50 - 100°C before welding and maintaining a welding speed of 0.6 m / min, cold cracks can be avoided. This method also requires preheating, with a low welding speed, and the overall method has low efficiency and high cost.

[0010] Patent CN201511012666.X discloses a method for preventing cold cracks in laser welding of ultra-high-strength steel, which realizes the inhibition of cold cracks through overall annealing for up to 3 - 4 hours after welding. This method has very low efficiency.

[0011] In summary, currently, for the cold crack problem of high-strength steel, mainly methods such as preheating before welding or post-weld heat treatment are adopted to inhibit it; at the same time, the main application field is arc welding, and the strength of the materials is relatively low. Therefore, for the laser tailor welding of hot-formed steel exceeding 1500 Mpa grade, there is a lack of an efficient, low-cost, and simple method to achieve the inhibition of welding cold cracks. Summary of the Invention

[0012] The present invention provides a method for manufacturing a high-performance laser welding joint. By controlling the action range of multiple energy beams and multiple processes of heating and cooling during the laser welding process, especially realizing the control of the welding molten pool state, controlling the length and width of the slender molten pool, reducing the hardness of the post-weld heat-affected zone, obtaining a structure mainly composed of tempered martensite with self-tempering occurring, and finally obtaining a welding joint with excellent cold crack sensitivity resistance.

[0013] The present invention provides a laser welding joint of ultra-high strength steel, which includes a base metal area, a heat-affected zone, and a melting zone. Its characteristics are that the structure of the base metal area is composed of ferrite and pearlite, the heat-affected zone close to the base metal is a tempered martensite structure formed by austenitizing the base metal structure and then quenching with a certain degree of self-tempering, the Vickers hardness of the base metal area Hbm ≤ 300HV, the hardness of the melting zone Hfz ≤ H / 3 - 20HV, the maximum hardness of the heat-affected zone Hhaz ≤ H / 3 - 20HV, and the minimum width of the heat-affected zone is not less than 0.2mm; H is the tensile strength obtained after the hot forming of the material.

[0014] Furthermore, the structure at the highest hardness of the heat-affected zone is tempered martensite, and its carbon content by mass percentage is 0.25% to 0.4%.

[0015] Furthermore, the maximum hardness of the heat-affected zone Hhaz ≤ 630HV.

[0016] The present invention also provides a laser welding method for ultra-high strength steel, which is characterized by including the steps of:

[0017] Providing a pair of ultra-high strength steel workpieces and butting them to form an assembly;

[0018] Using a main laser heat source and a second heat source to act on the assembly together;

[0019] Forming a keyhole by the main laser heat source, and regulating the morphology of the molten pool by the second heat source, controlling the ratio of the length L to the width B of the molten pool to satisfy L / B ≥ 3 to extend the existence time of the molten pool;

[0020] After cooling and solidification, a laser welding joint as described in any one of claims 1 - 3 is formed.

[0021] Furthermore, the length L of the molten pool formed by laser welding is not less than 6mm.

[0022] Furthermore, the spot energy irradiated on the top surface of the assembly by the main laser heat source is evenly distributed, and under the action of the laser heat source, it is sufficient to completely melt the assembly in the thickness direction.

[0023] Furthermore, the second heat source is a laser heat source, and the energy ratio of it to the energy of the main laser heat source does not exceed 0.8;

[0024] The center distance d between the beam spots of the second laser heat source and the main laser heat source satisfies d > (d1 + d2) / 2, where d1 and d2 are the equivalent circle diameters of the irradiation areas of the main laser heat source and the second laser heat source on the surface of the assembly, respectively.

[0025] Furthermore, the equivalent circle diameters d1 and d2 of the irradiation areas of the main laser heat source and the second laser heat source on the surface of the assembly are both not less than 0.5 mm.

[0026] Furthermore, the second heat source is a laser heat source. The irradiation shape of the laser heat source on the top surface of the assembly is a rectangular morphology, and the energy is evenly distributed in this rectangular area. The length and width of the rectangle are L1 and B1 respectively, satisfying L1 ≥ 3 mm, B1 ≤ 1 mm, and L1 ≥ L.

[0027] Furthermore, the second heat source is an arc heat source. The arc heat source and the main laser heat source are arranged front and back along the welding direction to jointly melt the assembly to form a molten pool.

[0028] Furthermore, during the laser welding process, a welding wire is synchronously filled and melted into the molten pool.

[0029] The present invention obtains high-performance welded joints by simply controlling the distribution of the energy of multiple energy beams in the plate thickness direction on the butt joint assembly and the change process of the molten pool temperature over time. On the one hand, the obtained welded joints have high tensile strength and elongation. At the same time, they have high hydrogen embrittlement sensitivity, high corrosion resistance, and high surface wear resistance. At the same time, the welding method can be adapted to various environments with different gaps, with high adaptability. The entire process is intermittent, which can reduce internal residual stress, greatly reduce manufacturing costs, improve production efficiency, and obtain high-performance welded joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0031] Figure 1 Shown is a schematic diagram of the butt joint assembly of the workpiece during laser welding in the present invention.

[0032] Figure 2 Shown is a schematic side cross-sectional view during the laser welding process.

[0033] Figure 3 Shown is a schematic top view during the laser welding process.

[0034] Figure 4The figure shows a schematic diagram of the arrangement of two laser beams during the laser welding process.

[0035] Figure 5 The figure shows another schematic diagram of the arrangement of two laser beams during the laser welding process.

[0036] Figure 6 The figure shows another schematic diagram of the arrangement of three laser beams during the laser welding process.

[0037] Figure 7 The figure shows a schematic diagram of the cross-section of the weld seam obtained after laser welding.

[0038] Figure 8 The figure shows a schematic diagram of the present invention when arc-guided laser welding is used.

[0039] Figure 9 The figure shows a side schematic diagram of the present invention when laser filler wire welding is used.

[0040] Figure 10 The figure shows a schematic diagram of the cross-section of the weld seam obtained in the present invention.

[0041] Figure 11 The figure shows an example of the molten pool morphology during the laser welding process in the present invention.

[0042] Figure 12 The figure shows a schematic diagram of the cross-section of the weld seam obtained in the present invention.

[0043] Figure 13 The figure shows an example of the molten pool morphology during the laser welding process in the present invention.

[0044] Figure 14 The figure shows an example of the cross-section of the weld seam obtained by arc-guided laser welding in the present invention.

[0045] Figure 15 It is a cross-sectional view of a comparative example.

[0046] Figure 16 It is a cross-sectional view of another comparative example.

[0047] Reference numerals: 11 - first steel workpiece, 12 - second steel workpiece; 111 - coating on the upper surface of the first steel workpiece; 112 - coating on the lower surface of the first steel workpiece; 12 - second steel workpiece; B1 - butt gap; b - coating thickness; 3 - laser beam; 31 - first laser beam; 32 - second laser beam; d: distance between the beam centers of the first laser beam and the second laser beam; d1 - diameter of the first laser beam spot; d2 - diameter of the second laser beam spot; 4 - welding wire; 5 - molten pool; 6 - weld seam; h1 - depth of the laser keyhole formed by the first laser beam; 51 - molten metal on the front side of the laser keyhole in the molten pool; 52 - molten metal on the rear side of the laser keyhole in the molten pool; 511 - front wall of the molten pool; 512 - rear wall of the molten pool; 71 - laser keyhole; 711 - wall of the laser keyhole; T - thickness of the assembly; 12a - top surface of the alloy butt joint position; 12b - bottom surface of the assembly butt joint position; 33 - third laser beam spot; 41 - projected area of the welding wire on the top surface of the alloy; h2 - height of the intersection point of the welding wire and the laser beam from the top surface; L - length of the molten pool; B - width of the molten pool; L1 - length of the second beam spot; B1 - width of the second beam spot; 61 - base material; 62 - heat affected zone of the weld seam; 63 - melted zone of the weld seam; B bf- Width at the narrowest part of the heat affected zone; 8 - arc welding torch. Detailed implementation mode

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be further described below in combination with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, the drawings are schematic diagrams, so the devices and equipment of the present invention are not limited by the dimensions or proportions of the schematic diagrams.

[0049] It should be noted that in the claims and the description of this patent, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one" does not exclude the existence of additional identical elements in the process, method, article or device including the element. The final weld seam in the present invention refers to the laser welding joint.

[0050] The present invention provides a method for manufacturing a high-strength laser welded joint. It is generally divided into the following steps:

[0051] Now refer toFigure 1 , a pair of first and second metal workpieces 11 and 12 for welding, and a main laser beam 3 for laser welding are provided; the first and second metal workpieces are butted to form an assembly to be welded and a butting line; the laser beam travels along the direction of the butting line of the assembly to be welded to melt the butting positions of the first and second metal workpieces, and a weld seam is formed after cooling and solidification. Wherein the laser beam at least includes a main laser beam 31 and another energy source 32, wherein each energy source irradiates the top surface of the butting line, wherein the main laser beam forms a keyhole and molten metal surrounding the keyhole, and when the molten metal around the keyhole is still in a molten liquid state, the other energy source is irradiated on the molten metal outside the keyhole, as Figure 2 shown; under the irradiation of the main laser beam energy in the keyhole and the irradiation of the second energy source outside the keyhole, the welding area of the assembly is melted to form an integral elongated molten pool 5, the molten pool has a length L along the welding direction and a width B perpendicular to the welding direction, and the weld seam 6 is formed after the molten pool cools and solidifies, as Figure 3 shown.

[0052] Among them, the schematic cross-sectional views of the steel workpieces 11 and 12 are as Figure 2As shown in the figure; in the present invention, the steel workpiece mentioned refers to high-strength steel that can be transformed into hot-formed steel after hot stamping, which is well-known in the art; its base material generally consists of ferrite and pearlite before forming, with a carbon content of not less than 0.1%, and contains about 0.002 - 0.006 of boron element to meet the hardenability during the subsequent quenching process, enabling the formation of a fully martensitic structure after quenching, having relatively high strength. The typical tensile strength of boron steel is not less than 500 Mpa. The typical steel plate base material is grade 22MnB5, and the mass percentage of the elements it contains is mainly: 0.15% ≤ C ≤ 0.45%; 0.5% ≤ Mn ≤ 2.5%; 0.08% ≤ Si ≤ 0.4%; Al ≤ 0.45%; 0.01% ≤ Cr ≤ 0.5%; Ti ≤ 0.1%; Nb ≤ 0.1%; V ≤ 0.1%; S ≤ 0.05%; P ≤ 0.05%; 0.002% ≤ B ≤ 0.006%; the rest are Fe and inevitable impurities. As materials of a higher strength level, they generally contain more alloying elements such as C, for example, typical grades such as 34MnB5, 28MnB5, 37MnB4, etc., and their tensile strength can exceed 2000 Mpa. As a representative of higher strength, with a mass fraction of carbon content higher than 0.25%, in the present invention, the steel workpiece is the raw material with a ferrite and pearlite structure before stamping, with a carbon content of not less than 0.25%, and the tensile strength after hot stamping is not less than 1500 Mpa. Additionally, as a typical representative during the application of this material, its surface generally has aluminum-silicon coatings 111 and 112, and the surface coating generally mainly consists of aluminum and silicon, with an aluminum content of not less than 80%, preferably more than 85%, and the total thickness b of the coating is generally 5 - 50 μm.

[0053] According to Figure 1 As shown in the figure, where the two workpieces 11 and 12 can be two completely identical materials, or two materials with any difference in thickness, coating type, or strength; the thickness of the two workpieces is generally 0.8 - 3.0 mm, and the two workpieces are butt-jointed to form an assembly, where the butt-joint gap is B1, which is 0 - 0.5 mm.

[0054] When the second energy source is also a laser beam energy, it includes the spot 31 formed by the first main beam and the spot 32 formed by the second beam. The two spots generate heat and melt simultaneously in the irradiation area to form a molten pool. The molten pool has a front wall 511 and a rear wall 512. The base material is cooled and solidified to form a weld seam. At this time, the output energy ratio of the second laser heat source to the main laser heat source does not exceed 0.8, and the second energy beam does not form a keyhole. It acts on the molten metal 52 at the rear side of the keyhole and only maintains the shape of the molten pool by its heat conduction; while the first main spot 31 mainly melts the steel workpiece and forms a keyhole 71 with a depth of h1, where h1 ≤ T (plate thickness); the keyhole has a front wall 711 and a rear wall; the molten metal 51 surrounds the keyhole 71. The relative positions and energy distributions of the two spots can have various types. Figure 4 As shown, the two spots are arranged front and back along the traveling direction and have a spacing d. The projected shapes of the two irradiation spots on the top surface can have various types. Figure 4 As shown in the schematic diagram when it is circular, the spot diameters are d1 and d2 respectively; generally, d1, d2 are both ≥ 0.5 mm, preferably d2 ≥ d1, and the center spacing d of the two spots satisfies: d > (d1 + d2) / 2; the geometric shapes of the two spots can be shapes formed by the mutual composition of circular, polygonal and other broken lines and curves, which can be the same or different, such as Figure 5 As shown in the schematic diagram when the two spots are circular and rectangular respectively, at this time the irradiation area of the second spot is a rectangular shape with a length L1 and a width B1, and satisfies L1 ≥ 3 mm, B1 ≤ 1 mm, L1 ≥ L. When the irradiation spot has other non-circular shapes, the diameter of the equivalent circle of its surface area is not less than 0.5 mm. In addition, the front and back relative positions of the two spots can be converted, that is, when observed along the welding forward direction, the main spot can be located on the front side or the rear side. The so-called main spot is only a name used to distinguish multiple spots. In addition, in addition to the main laser beam 31, more than one laser beam can be included as the second laser beam energy source. Figure 6 As shown in the schematic diagram of the configuration on the top surface when three laser spots irradiate, at this time the second laser beams 32 and 33 are arranged symmetrically with respect to the docking line and behind the first main laser beam 31. Under the action of the energies of the three laser beams, a molten pool is formed, and a weld seam is formed after cooling and solidification.

[0055] Generally, the laser beams 31 and 32 are laser beams with the same or different wavelengths, and their wavelengths are 0.3 - 10 μm, especially emitted by infrared lasers with wavelengths of 0.5 - 3 μm, which is common in laser processing. The laser beam is generally emitted by a laser, and the laser can include various types, such as solid laser beams or gas laser beams. Specifically, it can include fiber lasers, disk lasers, semiconductor diode lasers, and solid lasers of the Nd:YAG type, or CO 2A gas laser. Of course, other types can also be included as long as they can generate a laser beam and form a keyhole and a molten weld pool. The first main laser beam and the second laser beam can be emitted by the same laser or by two independent lasers. The two laser beams can be generated by the same laser and decomposed into two laser beams 31 and 32 through a beam splitter in an integral laser processing head. The beam splitter achieves this purpose through any internal optical element or its combination, such as a prism, a mirror, etc.; the laser system controller can be connected to the beam splitter to control the energy distribution and irradiation area size of the two beams of light and the coordinated output during processing, such as the power level, the mutual position distance d, and the defocus amount, etc. The generation of multiple laser beams is generally composed of optical elements or their combination inside the laser processing head. It can be composed of completely fixed optical elements to form laser beams with fixed beam characteristics, or it can include optical elements with movable and deflectable parts inside, so that the irradiation areas 31 and 32 formed by the laser beams on the surface of the raw material are variable, and the laser beams can be scanned at high speed in space position through their movement, deflection, etc. during the processing. The energy distribution of the laser beam can be in various forms such as Gaussian distribution, uniform distribution, or point-ring distribution, etc. During the welding process, the laser beam can also travel in a fixed or synchronous high-speed movement in various forms such as swinging along the welding direction. At this time, the swinging shape can include various shapes such as circular, broken line, figure-eight, ∞-shaped, etc. The swinging frequency is generally 50 - 500 HZ, and the swinging amplitude is between 0.2 - 1.5 mm. During the welding process, various single or mixed shielding gases can also be added, such as Ar gas, He gas, etc., or welding can be carried out under the condition of no shielding gas. The power of the laser beams 31 and 32 is generally 3 - 15 KW, preferably 4 - 10 KW; and the traveling speed of the laser beams relative to the workpiece assembly is generally 2 - 10 m / min, preferably 2 - 8 m / min.

[0056] After cooling and solidification, a weld seam 6 is formed. Now refer to Figure 7 , Figure 7 The figure shows a cross-sectional schematic diagram of the formed welded joint, where the joint includes a base metal zone 61, a heat-affected zone 62, and a fusion zone 63. The base metal zone 61 is composed of the original ferrite and pearlite; the weld zone 63 is mainly composed of martensite formed after melting and solidification; and in the heat-affected zone 62, it is mainly composed of tempered martensite formed by austenitizing the base metal structure and then quenching and self-tempering. Among them, the Vickers hardness of the base metal zone satisfies H bm ≤300 HV, the hardness of the fusion zone is H fz ≤H / 3 - 20 HV, and the highest hardness of the heat-affected zone is H haz ≤H / 3 - 20 HV, and preferably H haz≤630 HV. For materials with a strength level of 2000 Mpa, it is generally 550 - 600 HV; and the width of its heat - affected zone is not less than 0.2 mm.

[0057] In addition, in the present invention, the second heat source can also be other heat sources besides the laser heat source, such as an arc - welding heat source. Figure 8 The figure shows a schematic diagram when the second heat source is an arc - welding heat source. The main laser beam 31 and the arc - welding torch 8 together form a composite heat source. Under the action of the composite heat source, the base material is melted to form a common molten pool and after solidification and cooling, a weld seam is formed; the relative front - rear position of the arc and the laser beam can be changed, but preferably the arc - welding torch is located at the front to guide. It is worth mentioning that during the laser welding process, solder can also be added synchronously. The solder can be in the form of powder, wire, or rod. When it is wire, it is the well - known laser wire - feeding welding in the field. As Figure 9 The figure shows a schematic diagram of synchronously adding a welding wire during welding. At this time, when observed along the welding direction, the welding wire is generally placed on the front side, and its end is at the position of the first laser beam spot. The height h2 from its intersection position with the laser beam to the top surface 12a of the assembly is 0 - 1 mm. During the laser welding process, the first beam spot heats and melts the welding wire through laser energy, making it integrate with the base material of the assembly melted by the first and second laser beams to form a molten pool; where 41 is the projected area formed by the bonding position of the welding wire 4 on the top surface of the assembly. Generally, the equivalent diameter d1 of the main laser beam spot is not less than 0.6 times the diameter of the welding wire, and preferably not less than 0.8 times the diameter of the welding wire. Its wire - feeding device can be, for example, MAG (Metal Active Gas), MIG (Metal Inert Gas), or TIG (Tungsten Inert Gas) as the wire - feeding device. During the welding process, the wire - feeding speed of the welding wire is generally 1 - 5 m / min, and the ratio of it to the welding speed is between 0.3 - 1.5. The welding wire is iron - based and can contain one or more austenitizing elements such as Ni, Mn, C, etc. and ferrite - forming elements such as Cr, Mo, etc.

[0058] Generally, the welded joint will continue to perform a hot - stamping process to form the final hot - formed part structure, which can be used to manufacture various components for locomotives, such as automotive A - pillars, B - pillars, and door ring structures. The tensile strength of its joint is not less than 1500 Mpa, and even exceeds 2000 Mpa.

[0059] Example 1

[0060] Figure 10The figure shows a cross-sectional view of a weld obtained by the present invention. The raw material to be welded is a hot-formed steel substrate with a tensile strength of 2000 Mpa after hot stamping and quenching, and the substrate thickness is 1.8 mm. The double-beam welding method is adopted during the laser welding process. The total laser power of the double beams is 4000 W, and the energy ratio is 50:50. Both light spots are circular, with a diameter of 0.5 mm, and the light spot spacing of 0.79 mm satisfies being greater than (0.5 + 0.5) / 2. The welding speed is 5.4 m / min. The morphology of the molten pool formed during the welding process is as Figure 11 shown. The overall length L of the formed molten pool is 6.37 mm, and the width B is 1.82 mm, satisfying L > 3B. The minimum width of the heat-affected zone after welding is about 0.22 mm, and the highest hardness of the heat-affected zone is about 581 HV, and its hardness satisfies < 2000 / 3 - 20. It can be seen from the results that when welding is performed using two laser beams with the same energy, no cracks appear in the heat-affected zone, and the joint has high crack resistance.

[0061] Example 2

[0062] Figure 12 The figure shows another cross-sectional view of a weld obtained by the present invention. The raw material to be welded is a hot-formed steel substrate with a tensile strength of 2000 Mpa after hot stamping and quenching, and the substrate thickness is 1.8 mm. The double-beam welding method is adopted during the laser welding process. The total laser power of the double beams is 4000 W, and the energy ratio is 20:80. Both light spots are circular, with a diameter of 0.5 mm. The energy proportion of the front light beam is lower, and the light spot spacing of 1.58 mm satisfies being greater than (0.5 + 0.5) / 2. The welding speed is 5.4 m / min. The morphology of the molten pool formed during the welding process is as Figure 13 shown. The overall length L of the formed molten pool is 6.05 mm, and the width B is 1.72 mm, satisfying L > 3B. The minimum width of the heat-affected zone after welding is about 0.21 mm, and the highest hardness of the heat-affected zone is about 585 HV, and its hardness satisfies < 2000 / 3 - 20. It can be seen from the results that when welding is performed synchronously using two laser beams with a certain energy difference, no cracks appear in the heat-affected zone, and the joint has high crack resistance.

[0063] Example 3

[0064] Figure 14The figure shows a cross-sectional view of another weld obtained by the present invention; the raw material to be welded is a hot-formed steel substrate with a tensile strength of 2000 Mpa after hot stamping and quenching, and the substrate thickness is 1.8 mm. During the laser welding process, an arc is used to guide the laser beam for welding simultaneously. The laser beam spot diameter is 0.5 mm, the arc current is 53 A, the voltage is 17.5 V, the wire feeding speed is 4 m / min, the distance between the arc center and the laser beam center is 3 mm, and the welding speed is 5.4 m / min. The minimum width of the heat-affected zone after welding is about 0.58 mm, and the highest hardness of the heat-affected zone is about 601 HV. Its hardness meets <2000 / 3 - 20; from the results, it can be seen that when using an arc with a certain distance from the laser beam to assist laser welding, no cracks appear in the heat-affected zone, and the joint has high crack resistance.

[0065] Comparative Example 1

[0066] Figure 15 The figure shows a cross-sectional view of the joint of a hot-formed steel substrate with a substrate thickness of 1.8 mm and a tensile strength of 2000 Mpa after hot stamping and quenching after ordinary single-laser welding. During the laser welding process, a single-laser beam welding method is used. The laser power is 4000 W, the spot shape is circular, the diameter is 0.5 mm, and the welding speed is 5.4 m / min. It can be seen that cracks appear in the heat-affected zone after welding and cannot meet the requirements.

[0067] Comparative Example 2

[0068] Figure 16 The figure shows a cross-sectional view of the joint of a hot-formed steel substrate with a substrate thickness of 1.8 mm and a tensile strength of 2000 Mpa after hot stamping and quenching after ordinary single-laser welding. During the laser welding process, a single-laser beam welding method is used. The laser power is 7000 W, the spot shape is circular, the diameter is 1.16 mm, and the welding speed is 3.0 m / min. It can be seen that cracks also appear in the heat-affected zone after welding and cannot meet the requirements.

Claims

1. A laser welded joint of ultra-high strength steel, comprising a base metal zone, a heat-affected zone and a melting zone; characterized in that: The matrix area structure is composed of ferrite and pearlite, and the heat-affected zone near the matrix is ​​a tempered martensite structure formed by quenching and self-tempering after austenitization of the matrix structure. The Vickers hardness of the matrix area is H bm ≤ 300HV, the melting zone hardness H fz ≤H / 3-20HV, the maximum hardness of the heat affected zone is H haz ≤H / 3-20HV, the minimum width of the heat affected zone is not less than 0.2mm; H is the tensile strength obtained after the material is thermoformed.

2. The laser welded joint of ultra-high strength steel according to claim 1, characterized in that: The structure of the heat-affected zone at the highest hardness is tempered martensite, and the carbon content thereof is 0.25% to 0.4% by mass.

3. The laser welded joint of ultra-high strength steel according to claim 1, characterized in that: The maximum hardness of the heat affected zone is H haz ≤630HV.

4. A laser welding method for ultra-high strength steel, characterized in that: Includes steps: Providing a pair of ultra-high-strength steel workpieces, butted to form an assembly; Using a primary laser heat source and a secondary heat source to act together on the assembly; A deep melting hole is formed by the main laser heat source, and the morphology of the molten pool is regulated by the second heat source, and the ratio of the length L to the width B of the molten pool is controlled to satisfy L / B≥3, so as to prolong the existence time of the molten pool; After cooling and solidification, a laser welded joint as described in any one of claims 1 to 3 is formed.

5. The laser welding method for ultra-high strength steel according to claim 4, characterized in that: The length L of the molten pool formed by the laser welding is not less than 6 mm.

6. The laser welding method for ultra-high strength steel according to claim 4, characterized in that: The energy of the light spot formed by the main laser heat source irradiating on the top surface of the assembly is evenly distributed, and under the action of the laser heat source, it is sufficient to completely melt the assembly along the thickness direction.

7. The laser welding method for ultra-high strength steel according to claim 4, characterized in that: The second heat source is a laser heat source, and the ratio of its energy to the energy of the main laser heat source does not exceed 0.8; The center distance between the beam spots of the second laser heat source and the main laser heat source is d>(d1+d2) / 2, wherein d1 and d2 are the equivalent circular diameters of the irradiation areas of the main laser heat source and the second laser heat source on the surface of the assembly, respectively.

8. The laser welding method of ultra-high strength steel according to claim 8, characterized in that: The equivalent circular diameters d1 and d2 of the irradiation areas of the main laser heat source and the second laser heat source on the surface of the assembly are not less than 0.5 mm.

9. The laser welding method for ultra-high strength steel according to claim 4, characterized in that: The second heat source is a laser heat source, which irradiates a rectangular shape on the top surface of the assembly, and the energy is evenly distributed in the rectangular area. The length and width of the rectangle are L1 and B1 respectively, satisfying L1≥3mm, B1≤1mm, and L1≥L.

10. The laser welding method for ultra-high strength steel according to claim 4, characterized in that: The second heat source is an arc heat source, which is arranged front and back with the main laser heat source along the welding direction to melt the assembly together to form a molten pool.

11. The laser welding method for ultra-high strength steel according to claim 1, characterized in that: During the laser welding process, the welding wire is simultaneously filled and melted into the molten pool.

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