A welding wire for δ-TRIP steel and a laser welding method

Abstract

This invention discloses a welding wire for δ-TRIP steel and a laser welding method, belonging to the field of laser welding technology. The welding wire, by mass percentage, contains: C 0.08–0.2%, Si 0.4–1.2%, Mn 0.8–1.8%, modified titanium powder 3–10%, S ≤0.005%, P ≤0.02%, with the balance being Fe and unavoidable impurities. Titanium is added in the form of modified titanium powder, which is prepared by high-energy ball milling of titanium powder mixed with rare earth oxides and boron powder. The laser welding method employs annular spot welding, combined with a composite induction preheating-post-heating process, using compressed air as the shielding gas. This method can significantly increase the volume fraction of Ti(C,N) strengthening phase in the weld to 5.5%–7.5%, effectively refine the weld microstructure, suppress the formation of coarse columnar δ-ferrite, and give the welded joint excellent strength, plasticity, and formability.

Description

Technical Field

[0001] This invention belongs to the field of laser welding technology, specifically relating to a welding wire for δ-TRIP steel and a laser welding method. Background Technology

[0002] In recent years, with the increasing demands for lightweighting and safety in the automotive industry, the development of advanced steels that combine high strength and high plasticity has become a research hotspot in materials science. Currently, these steels mainly include duplex (DP) steel, transformation-induced plasticity (TRIP) steel, and hot-formed boron steel. δ-Transformation-Induced Plasticity (TRIP) steel is a novel, low-density steel with high strength-plasticity product, improved based on the TRIP steel system. Its key characteristic is the addition of 3-5% Al. The addition of Al reduces the metal density and improves ferrite stability, transforming its microstructure into a multiphase structure of δ-ferrite + retained austenite + a very small amount of bainite. During tensile deformation, the retained austenite in the microstructure undergoes a TRIP effect, transforming into martensite, which brings high plasticity to the material.

[0003] Laser welding technology enables precise joining of steel plates of varying thicknesses and materials, providing an efficient solution for manufacturing complex automotive body structural components. This technology offers advantages such as narrow weld seams, high welding speed, and minimal thermal deformation, making it particularly suitable for automotive body structural components with high precision requirements. However, unlike welding ordinary steel, the addition of Al (Al) to δ-TRIP steel brings high strength and ductility, but also presents new challenges for welding this material. During laser welding, the increased Al content in the molten pool leads to the formation of Fe-Al intermetallic compounds (IMC) or δ-ferrite in the weld seam. For example, in the laser welding of aluminum-silicon coated steel, Al in the weld joint causes the formation of δ-ferrite. The poor bonding between δ-ferrite and the martensitic grain boundaries in the weld seam makes it prone to cracking. Furthermore, the formation of coarse columnar grain structures in the δ-TRIP steel weld joint severely weakens the strength and ductility of the weld joint.

[0004] At present, the industry mainly adopts the following two solutions to address the technical challenges of laser welding of δ-TRIP steel, but both have certain limitations: (1) Laser filler wire welding technology: During the welding process, carbon steel welding wire is filled to dilute the aluminum content in the molten pool and inhibit the formation of coarse ferrite in the weld. The weld will produce a dual-phase structure of ferrite + martensite, which will improve the strength. However, the mismatch between the weld and the base material properties leads to a reduction in the forming performance of the welded joint, which cannot meet the needs of industrial production. (2) Oscillating laser beam welding technology: The molten pool is stirred by an "∞"-shaped oscillating laser beam to refine the weld grains. In addition, oscillating welding requires a high dynamic response galvanometer, and the equipment investment cost is about 60% higher than that of conventional laser welding machines, and the process window is narrow. (3) Controlling the grain size of the weld structure: Nucleating agents are used to prevent the original ferrite in the weld from changing during the welding process, inhibit the formation of coarse ferrite, and refine the weld grains. Currently, Nb, V, Ti, or rare earth elements are often used as inoculants in ferritic steels. However, considering the cost and the characteristics of the compounds precipitated in the weld, further research is needed.

[0005] Therefore, for the optimization of laser welding process of δ-TRIP steel, a ferro-based welding wire and laser welding method for δ-TRIP steel were designed based on the concept of weld metallization. This method can improve the strength, plasticity and formability of the welded joint, meet the needs of industrial production, and the welding method is simple, fast and low cost. Summary of the Invention

[0006] The purpose of this invention is to provide a welding wire for δ-TRIP steel. By utilizing the titanium element weld metallization concept, a steel-based welding wire is designed to control the elemental composition in the weld, thereby improving the phenomenon of excessively large ferrite grain size in the weld caused by excessive aluminum content during laser welding of δ-TRIP steel, and refining the ferrite structure in the weld.

[0007] The purpose of this invention is to provide a laser welding method for δ-TRIP steel welding wire. This method uses compressed air as a shielding gas and utilizes the ultra-high energy density of the laser to ionize the air into C, N, and O ions, which are then introduced into the molten pool. Titanium reacts with these ions to form nanoscale precipitates, thereby inhibiting grain growth within the weld. The coupling of fine-grain strengthening and precipitation strengthening mechanisms improves the strength, plasticity, and formability of the welded joint. Furthermore, this laser filler wire welding process uses an annular laser spot, which significantly reduces spatter. The energy distribution of the annular spot is characterized by lower energy in the central region and higher energy in the outer annular region. The high-energy outer ring rapidly melts the welding wire and base material, forming a molten pool. The low-energy central region allows the molten metal vapor to escape smoothly upwards without being re-ionized or violently disturbed by the high-intensity central laser beam. This greatly reduces the possibility of violent fluctuations in the molten pool and the violent ejection of molten droplets / metal (i.e., spatter) caused by the back pressure of the metal vapor. Moreover, the central region is the main escape channel for the metal vapor, which also helps to reduce the formation of porosity.

[0008] To achieve the above objectives, this invention provides a welding wire for δ-TRIP steel and a laser welding method. By using the filler wire described in this invention to perform filler wire welding on δ-TRIP steel plates, a welded plate component is obtained. The welded plate of this invention exhibits excellent mechanical properties at the joint, the welding process is simple and easier to implement, and it has greater adaptability to industrial production and low cost.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] The first aspect of the present invention provides a welding wire for δ-TRIP steel welded plates (i.e., a welding wire for δ-TRIP steel), wherein the welding wire comprises Ti, Fe and unavoidable impurities by mass fraction.

[0011] The chemical composition of the welding wire for δ-TRIP steel welded plates is as follows (by mass fraction): C 0.08–0.2%, Si 0.4–1.2%, Mn 0.8–1.8%, modified titanium powder 3–10%, S ≤0.005%, P ≤0.02%, with the balance being Fe and unavoidable impurities.

[0012] The specific preparation process of the welding wire (i.e., a type of δ-TRIP steel welding wire) first requires the preparation of modified titanium powder (titanium powder + submicron-sized rare earth oxides + nano / micron-sized boron powder, after high-energy ball milling). The modified titanium powder is then smelted with other elements (C, Si, Mn, etc.) through powder metallurgy to form a uniform alloy steel ingot. Subsequently, the steel ingot is hot-rolled into wire rod, and then drawn to the target diameter (0.8-1.8 mm) using a small-taper continuous drawing technique. Finally, the welding wire undergoes copper plating surface treatment to improve conductivity, rust prevention, and wire feeding smoothness.

[0013] Preferably, the unavoidable impurity content in the welding wire for the δ-TRIP steel welded plate does not exceed 1 wt%.

[0014] Preferably, the unavoidable impurities include Al, and the Al content does not exceed 0.5 wt%.

[0015] Furthermore, the titanium element in the welding wire is added in the form of modified titanium powder, which is prepared by the following method: titanium powder with a particle size of 10-50 μm is mixed with 0.5%-2% of rare earth oxides (such as Y2O3, La2O3) of 2-4 μm by mass of titanium powder and 0.1%-0.5% of boron powder of 0.5-1 μm by mass of titanium powder. The mixture is then subjected to high-energy ball milling under argon protection for 2-6 hours, with a ball-to-material ratio of 5:1-10:1 and a rotation speed of 200-400 r / min, to obtain surface-modified titanium powder. This modified titanium powder can promote the formation of finer and more uniformly distributed Ti(C,N) precipitates during welding, and introduces rare earth and boron elements, further refining the weld grains, forming new strengthening phases, and improving the strength and toughness of the weld joint.

[0016] Among them, titanium element weld metallization solves the problem of coarse structure in direct welding. The fundamental reason why coarse ferrite is prone to intragranular fracture (transgranular fracture) is the combined effect of its crystal structure characteristics, grain size effect and impurity segregation behavior. The specific mechanism is as follows: (1) Crystal structure characteristics: The slip system is limited. Ferrite has a body-centered cubic (BCC) structure, and its slip system is much smaller than that of the face-centered cubic (FCC) structure, which leads to limited dislocation movement ability, difficulty in plastic deformation, and difficulty in releasing energy through slip when stress is concentrated, which easily leads to the initiation of microcracks inside the grain. (2) Grain size effect: Stress concentration is aggravated. Coarse grains lead to a reduction in the number of grain boundaries, and the hindering effect of grain boundaries on dislocation movement is weakened. In summary, the essence of intragranular fracture of coarse ferrite is the result of the synergistic effect of poor plasticity of BCC structure and stress concentration of coarse grains, which directly leads to a decrease in the toughness and formability of welded joints.

[0017] When using welding wire containing modified titanium powder, the titanium reacts fully with elements in the air to generate finer Ti(C,N) precipitates. Furthermore, the introduction of rare earth elements and boron can form new strengthening phases, such as rare earth oxysulfides and TiB2. These precipitates and strengthening phases can play a role in grain refinement and precipitation strengthening respectively during the initial formation of the molten pool and the solidification process, synergistically enhancing the strength, plasticity, and formability of the welded joint.

[0018] Titanium content is an important component of the welding wire alloy composition. The titanium content value indicates the ability of the composition system described in this invention to refine ferrite grains and form precipitates. Since the specific titanium content in the weld microstructure is related to the Al content and the welding wire dilution rate, the titanium content in the weld microstructure will fluctuate with changes in process parameters. When the modified titanium powder content in the weld is less than 3 wt%, the ferrite grains cannot be refined during high-speed welding, easily leading to weld brittleness; when the modified titanium powder content in the weld is greater than 10 wt%, brittle intermetallic compounds are easily formed in the weld, which also reduces the mechanical properties of the weld. Therefore, the composition system of the welding wire described in this invention is set at modified titanium powder: 3-10 wt%.

[0019] In laser welding, compressed air, as a shielding gas, plays a crucial role in regulating the microstructure of the weld. It has two main effects on the weld microstructure: (1) Grain refinement: Ti(C,N) acts as a nucleating agent, with Ti(C,N) particles serving as heterogeneous nucleation nuclei during molten pool solidification, promoting heterogeneous ferrite nucleation. Compared to inert gas shielding, grain size is reduced by 30%~50%, microstructure uniformity is significantly improved, and the formation of coarse ferrite is suppressed. (2) Precipitation strengthening: Nanoscale precipitates such as TiN / TiC pin dislocations, increasing strength. Furthermore, compared to traditional inert shielding gases, compressed air has the core advantages of low cost and significant grain refinement.

[0020] In laser wire filler welding, the use of annular spot lasers offers several significant advantages over traditional single-point Gaussian spots, particularly in improving process stability, reducing defects, and improving weld formation. The main benefits are as follows: (1) Significantly reduced spatter: The energy distribution of annular spot lasers is characterized by lower energy in the central region and higher energy in the outer annular region. The high-energy outer ring rapidly melts the welding wire and base material, forming a molten pool. The low-energy central region allows the molten metal vapor to escape smoothly upwards without being reionized or violently disturbed by the high-intensity central laser beam. This greatly reduces the likelihood of violent fluctuations in the molten pool caused by the back pressure of the metal vapor and the violent ejection of molten droplets / metal (i.e., spatter). (2) Improved welding process stability: The presence of the low-energy central region stabilizes the dynamic behavior of the molten pool, making droplet transition smoother and more controllable, reducing defects such as unevenness and undercut on the weld surface. (3) Improved gap bridging capability: The wider energy distribution of the annular spot increases the welding tolerance to assembly gaps and misalignments. (4) Avoid porosity: The molten pool is more stable and the fluctuations are reduced, which is conducive to the escape of gas in the molten pool. The central area is the main escape channel of metal vapor, which also helps to reduce the formation of porosity. (5) Wider process window: The annular spot technology usually allows for stable and low spatter welding effects within a wider range of laser power, defocusing amount, and wire feed speed, reducing the difficulty of process debugging and the stringent requirements for parameter accuracy.

[0021] The second aspect of this invention provides a laser welding method for δ-TRIP steel (i.e., a laser welding method for δ-TRIP steel welding wire), which uses the δ-TRIP steel welding wire described in the first aspect for laser welding, and includes the following steps:

[0022] S1. Provide a first substrate and a second substrate;

[0023] The welding surfaces of the first and second substrates are polished sequentially with 240#-400#-600# sandpaper until they are bright, and are placed at intervals to form gaps between the δ-TRIP steels (the butt joint bevel between the δ-TRIP steels adopts an "I" type bevel, and the width of the gap is not greater than the diameter of the welding wire used for welding the δ-TRIP steel plates).

[0024] Preferably, sandpaper is used to remove rust from the joint of the plates, acetone or ethanol is used to remove impurities from the surface of the steel plates to be welded, and the plates are then air-dried and cooled.

[0025] S2. Under the protective atmosphere of compressed air, fill the gap with δ-TRIP steel welding wire, and use a fiber laser to output an annular spot for laser welding to weld the first substrate and the second substrate together.

[0026] The welding parameters are as follows: welding power 3-6kW, outer ring power ratio 70%-90%, inner ring power ratio 10-30%, welding speed 2-4m / min, and gas flow rate 10-30L / min.

[0027] As a further improvement of the present invention, in step S2, during laser welding, a composite induction preheating-postheating device is used to perform thermal management on the weld and its adjacent area, specifically including:

[0028] a) Preheating before welding: In front of the laser welding gun, a high-frequency induction heating device (Ambrell EASYHEAT series, USA, frequency 150-400 kHz) is used to preheat the bevel of the steel plate to be welded and the area 5-10mm on both sides. The preheating temperature is 300-500℃.

[0029] b) Post-weld heat treatment: After the laser welding torch is used, a medium-frequency induction heating device (Shaanxi Haishan KGPS series medium-frequency furnace, frequency 100 Hz - 8 kHz) is used to perform post-weld heat treatment on the newly formed weld. The post-heating temperature is 600-800℃ and the holding time is 10-30s.

[0030] The present invention discloses a laser welding method for δ-TRIP steel. By introducing a multi-field coupling process of composite induction preheating and postheating, combined with an annular spot laser and modified titanium powder welding wire, the welding thermal cycle is precisely controlled, further refining the weld microstructure and significantly increasing the volume fraction of Ti(C,N) strengthening phase (up to 5.5%-7.5%), thereby achieving a better strength-plasticity match in the welded joint.

[0031] Furthermore, in S1, the thicknesses of the first substrate and the second substrate are independently selected from 0.8-2 mm.

[0032] Furthermore, in S1, the first substrate and the second substrate are made of δ-TRIP steel.

[0033] Furthermore, in S2, the diameter of the welding wire is 0.8-1.8 mm.

[0034] Furthermore, in S2, the welded joint is a type I butt joint, and the gap is no higher than the diameter of the welding wire.

[0035] Furthermore, in S2, the wire feeding speed ranges from 3.0 to 6.0 m / min.

[0036] Furthermore, in S2, the outer ring spot diameter is 1-2mm, the inner ring spot diameter is 0.3-0.6mm, and the defocusing amount is -2-0mm.

[0037] Furthermore, in S2, the protective atmosphere is compressed air.

[0038] Furthermore, the ferrite grains in the weld of the welded component are refined, suppressing the formation of coarse columnar δ-ferrite structure.

[0039] This invention is based on the idea of ​​weld metallization and uses laser filler wire welding technology to prepare δ-TRIP steel welded joints. The purpose of this invention is to obtain welded joints that reach the level of the base material in a low-cost and high-efficiency manner.

[0040] The beneficial effects of this invention are:

[0041] This invention provides a welding wire and laser welding method for δ-TRIP steel. Utilizing the titanium element's alloying principle, it controls the elemental composition of the weld seam and designs an iron-based welding wire to improve the excessively large ferrite grain size in the weld seam caused by excessive aluminum content during laser welding of δ-TRIP steel. This refines the ferrite microstructure in the weld seam. Furthermore, compressed air is used as a shielding gas, and titanium reacts with it to form nanoscale precipitates, thereby inhibiting grain growth within the weld seam. The coupling of grain refinement strengthening and precipitation strengthening mechanisms improves the strength, plasticity, and formability of the welded joint. Moreover, this laser filler wire welding process uses an annular laser spot, which significantly reduces defects such as spatter and porosity. This invention meets the performance requirements of high-strength steel in automobile manufacturing while achieving high-quality welding using low-cost welding wire, simplifying the welding process, improving production efficiency, and reducing production costs.

[0042] This invention employs a laser filler wire welding method, which has low assembly requirements, strong weldability, high efficiency, and more stable welding quality. It optimizes the laser welding process for δ-TRIP steel, especially by controlling the elemental composition of the weld, refining the ferrite grains of the weld, and improving the strength and plasticity of the welded joint.

[0043] Compared with existing technologies, this invention, through titanium powder modification and a multi-field coupled annular spot laser welding process involving composite induction preheating and post-heating, has the following outstanding advantages:

[0044] 1. Induction preheating reduces the cooling rate during welding, which is beneficial for the full diffusion of Ti elements in the molten pool and their reaction with C and N elements, thereby increasing the nucleation rate of Ti(C,N).

[0045] 2. Induction heating provides the energy conditions for the full precipitation and growth of Ti(C,N) precipitates, avoiding insufficient precipitation caused by excessively rapid cooling, and significantly increasing the volume fraction of Ti(C,N) reinforcing phase.

[0046] 3. This composite thermal management process effectively reduces welding residual stress, decreases crack sensitivity, and further improves the toughness and formability of the welded joint.

[0047] 4. The rare earth and boron elements introduced through titanium powder modification can form finer and more uniformly distributed Ti(C,N) precipitates, and generate new reinforcing phases such as rare earth oxysulfides and TiB2, which further refine the weld grains and improve the strength and toughness of the welded joint.

[0048] This invention provides a welding wire for δ-TRIP steel and a laser welding method, belonging to the field of laser welding technology. The welding wire, by mass percentage, contains: C 0.08–0.2%, Si 0.4–1.2%, Mn 0.8–1.8%, modified titanium powder 3–10%, S ≤0.005%, P ≤0.02%, with the balance being Fe and unavoidable impurities. Titanium is added in the form of modified titanium powder, which is prepared by high-energy ball milling of titanium powder mixed with rare earth oxides and boron powder. The laser welding method employs annular spot welding, combined with a composite induction preheating-post-heating process, using compressed air as the shielding gas. This method can significantly increase the volume fraction of Ti(C,N) strengthening phase in the weld to 5.5%–7.5%, effectively refine the weld microstructure, suppress the formation of coarse columnar δ-ferrite, and give the welded joint excellent strength, plasticity, and formability. Attached Figure Description

[0049] Figure 1 The microstructure of the weld seam of the welded plate in Comparative Example 1 of the present invention;

[0050] Figure 2 The microstructure of the weld seam of the welded plate in Comparative Example 2 of the present invention;

[0051] Figure 3 The microstructure of the weld seam of the welded plate in Embodiment 1 of the present invention;

[0052] Figure 4 The microstructure of the weld seam of the welded plate in Embodiment 2 of the present invention. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0054] This embodiment provides a welding wire and laser welding method for δ-TRIP steel. A special welding wire is filled at the joint of the δ-TRIP steel, and compressed air is used as a shielding gas during laser welding. The laser generates an annular spot that acts on the joint of the plates to form a molten pool. At the same time, a composite induction preheating-postheating device is used to manage the heat of the welding process.

[0055] This embodiment provides a δ-TRIP steel welding wire. This special welding wire can refine the ferrite grains in the weld and, along with the formation of precipitates, effectively suppress the brittle fracture problem of the weld joint caused by coarse ferrite in the weld. Due to the ultra-high energy density of the laser, titanium can react with C and N ions ionized from compressed air in the molten pool to form precipitates, such as Ti(C,N). Larger precipitates mostly act as nucleating agents, serving as heterogeneous nucleation nuclei during the solidification of the molten pool, promoting non-uniform nucleation of ferrite, and inhibiting grain growth during cooling and solidification, thereby refining the weld grains. Nanoscale phases such as TiN / TiC will precipitate within the ferrite grains, acting as dislocation pinning agents and improving the strength of the weld. The synergistic effect of grain refinement strengthening and precipitation strengthening improves the strength, plasticity, and formability of the weld joint. Furthermore, this invention uses compressed air as a protective gas, which not only further refines the weld grains but is also less expensive than inert gases. Using an annular spot can significantly reduce the generation of defects such as spatter and porosity. The composite induction preheating-postheating process further optimizes the welding thermal cycle and promotes the precipitation and growth of Ti(C,N) strengthening phases.

[0056] This embodiment utilizes a laser-assisted wire-filled welding method to suppress the formation of coarse grains within the weld and refine the ferrite grains. The tensile strength, toughness, and other mechanical properties of the welded joint are significantly improved, meeting the performance requirements of third-generation automotive steel welded joints. Weld metallization effectively addresses the embrittlement problem of δ-TRIP steel during welding, reducing the impact of excessive aluminum content on weld quality. This invention enables high-quality welding, simplifies welding procedures, improves production efficiency, and reduces production costs.

[0057] This embodiment also provides a laser welding method for δ-TRIP steel using the aforementioned special welding wire, comprising the following steps:

[0058] S1. Provide a first substrate and a second substrate;

[0059] The welding surfaces of the first and second substrates are sequentially polished with 240#-400#-600# sandpaper to achieve a bright surface, and are placed at intervals to create gaps between the δ-TRIP steels. The thickness of the first and second substrates is independently selected from 0.8-2mm. The chemical composition of the welding wire used for the δ-TRIP steel welding plate is: C 0.08~0.2%, Si 0.4~1.2%, Mn 0.8~1.8%, Ti 3~10%, S≤0.005%, P≤0.02%, with the balance being Fe and unavoidable impurities.

[0060] S2. Under the protective atmosphere of compressed air, δ-TRIP steel welding wire is filled into the gap, and a fiber laser outputs a ring spot for laser welding to weld the first substrate and the second substrate together. At the same time, a composite induction preheating-postheating device is used to perform thermal management on the weld and its adjacent area.

[0061] The welding parameters are as follows: welding power 3-6kW, outer ring power ratio 70%-90%, inner ring power ratio 10-30%. Outer ring spot diameter 1-2mm, inner ring spot diameter 0.3-0.6mm, defocusing amount -2-0mm. Welding speed 2-4m / min, gas flow rate 10-30L / min.

[0062] As a specific example, in S2, the welded joint is a type I butt joint, with a gap not exceeding the diameter of the welding wire. The diameter of the welding wire is 0.8-1.8 mm, and the wire feeding speed ranges from 3.0 to 6.0 m / min. The annular laser spot acts on the substrate joint along the preset path of the laser welding system, completely melting the welding wire into the molten pool, and then blowing compressed air into the molten pool.

[0063] As a specific example, in S1, the steel plate is made of δ-TRIP steel.

[0064] Furthermore, the ferrite grains in the weld of the welded component are refined, suppressing the formation of coarse columnar δ-ferrite structure. The strength of the welded joint can reach over 592 MPa, the elongation can reach over 40%, and the volume fraction of Ti(C,N) strengthening phase can reach 5.5%-7.5%.

[0065] This embodiment uses laser wire filler welding technology, which has low assembly requirements, strong weldability, high efficiency, and more stable welding quality.

[0066] The following are specific examples. Comparative Example 1

[0067] The difference between this comparative example and the embodiment is that the protective gas compressed air is replaced with pure argon.

[0068] Chemical composition of welding wire for δ-TRIP steel welded plates: C 0.1%, Si 0.8%, Mn 1.2%, S≤0.005%, P≤0.02%, balance being Fe and unavoidable impurities.

[0069] This comparative example relates to a direct laser welding method for δ-TRIP steel. The welding plates are δ-TRIP steel, and the thickness of the two welding plates is 1.5mm. To facilitate welding and installation, the length and width of the welding plates are uniformly cut to 100mm*50mm. The welding is carried out by butt welding, I-type joint, with a preset gap of 0.1mm. Before welding, the surface of the welding plates is cleaned with acetone and alcohol.

[0070] The specific welding process is as follows: laser defocusing amount is 0mm, outer ring spot diameter is 1.5mm, inner ring spot diameter is 0.5mm, shielding gas is pure argon, and flow rate is 20L / min. Specific welding process parameters are: welding power 3kW, outer ring power ratio 70%, inner ring power ratio 30%, welding speed 3m / min.

[0071] The welding process was stable with no spatter and no obvious welding defects. The resulting weld contained significant coarse δ-ferrite. Specifically, the microstructure of the weld seam of the buttress plate was as follows: Figure 1 As shown. Figure 1 The microstructure is coarse and contains neither modified titanium powder nor titanium powder, exhibiting poor mechanical properties. Phase analysis of the weld revealed that the volume fraction of the Ti(C,N) reinforcing phase was approximately 0%. Comparative Example 2

[0072] This comparative example relates to a welding wire and laser welding method for δ-TRIP steel. The method involves filling the joint of the plates with welding wire (6wt% Ti content) that has not undergone titanium powder modification, using compressed air as the shielding gas, and employing an induction preheating-post-heating process. The welded plates are δ-TRIP steel, with two plates each 1.5mm thick. To facilitate welding and installation, the plates are uniformly cut to 100mm x 50mm. Welding is performed using a butt joint, type I joint, with a pre-set gap of 0.1mm. Before welding, the surfaces of the welded plates are cleaned with acetone and alcohol.

[0073] The specific welding process is as follows: laser defocusing is 0mm, outer ring spot diameter is 1.5mm, inner ring spot diameter is 0.5mm, and the shielding gas is compressed air with a flow rate of 20L / min. The specific welding process parameters are: welding power 3kW, outer ring power ratio 70%, inner ring power ratio 30%, welding speed 3m / min, welding wire diameter 1.2mm, and wire feed speed 3.0m / min.

[0074] Preheating before welding: A high-frequency induction heating device is used in front of the laser welding gun to preheat the bevel of the steel plate to be welded and 6mm on both sides of it. The preheating temperature is 400℃.

[0075] Post-weld heat treatment: A medium-frequency induction heating device is used behind the laser welding gun to perform post-weld heat treatment on the newly formed weld. The post-heating temperature is 700℃ and the holding time is 20s.

[0076] The welding process was stable with no spatter and no obvious welding defects. Phase analysis of the weld revealed a volume fraction of approximately 2.8% for the Ti(C,N) strengthening phase. The microstructure of the weld seam of the assembled plate is as follows: Figure 2 As shown. Figure 2 Comparison Figure 1With the increase of titanium content, the microstructure is refined, and the mechanical properties are improved accordingly. Example 1

[0077] A δ-TRIP steel welding wire, wherein the chemical composition of the welding wire, by mass percentage, comprises: C 0.1%, Si 0.8%, Mn 1.5%, modified titanium powder 6%, S 0.001%, P 0.01%, with the balance being Fe and unavoidable impurities.

[0078] Modified titanium powder was prepared by the following method: titanium powder with a particle size of about 30 μm was mixed with 1% rare earth oxide Y2O3 and 0.3% boron powder by mass of titanium powder, and then ball-milled under argon protection for 4 hours with a ball-to-material ratio of 8:1 and a rotation speed of 300 r / min.

[0079] This embodiment relates to a welding wire and laser welding method for δ-TRIP steel. A special welding wire prepared with modified titanium powder (6wt% modified titanium powder content) is used to fill the joint of the plates, and compressed air is used as the shielding gas. A composite induction preheating-post-heating device is also employed. The welding plates are δ-TRIP steel, with two plates each 1.5mm thick. To facilitate welding and installation, the plates are uniformly cut to 100mm*50mm in length and width. Welding is performed using a butt joint, type I joint, with a pre-set gap of 0.1mm. Before welding, the surface of the welding plates is cleaned with acetone and alcohol.

[0080] The specific welding process is as follows: laser defocusing amount is 0mm, outer ring spot diameter is 1.5mm, inner ring spot diameter is 0.5mm, and the shielding gas is compressed air with a flow rate of 20L / min. The specific welding process parameters are: welding power 3kW, outer ring power ratio 70%, inner ring power ratio 30%, welding speed 3m / min. The welding wire diameter is 1.2mm, and the wire feed speed is 3.0m / min. Induction preheating temperature: 400℃; induction postheating temperature: 700℃, holding time: 20s.

[0081] The welding process was stable with no spatter and no obvious welding defects. The ferrite structure in the resulting weld was significantly refined. Specifically, the microstructure of the weld seam of the buttress plate was as follows: Figure 3 As shown. Figure 3 and Figure 2 In comparison, replacing titanium powder with modified titanium powder resulted in a finer microstructure and improved mechanical properties. Phase analysis of the weld revealed that the volume fraction of the Ti(C,N) reinforcing phase was approximately 6.2%. Example 2

[0082] The difference between this embodiment and Embodiment 1 is that the titanium content of the modified titanium powder in the welding wire is 10wt%, the induction preheating temperature is 450℃, the induction heating temperature is 750℃, and the holding time is 25s. Other conditions are the same as in Embodiment 1. Phase analysis of the weld revealed that the volume fraction of the Ti(C,N) strengthening phase was approximately 7.1%. The microstructure of the weld seam of the welded plate is as follows: Figure 4 As shown. Figure 4 and Figure 3 In contrast, when the titanium content in the weld increases, the grain size of the microstructure decreases, and fine-grain strengthening can improve the mechanical properties of the welded joint.

[0083] Specifically, the chemical composition of the welding wire includes: C 0.2%, Si 1.2%, Mn 1.8%, modified titanium powder 10%, S 0.005%, P 0.02%, with the balance being Fe and unavoidable impurities. Example 3

[0084] A δ-TRIP steel welding wire, wherein the chemical composition of the welding wire, by mass percentage, comprises: C 0.08%, Si 0.4%, Mn 0.8%, modified titanium powder 3%, S 0.002%, P 0.005%, with the balance being Fe and unavoidable impurities.

[0085] Modified titanium powder was prepared by the following method: titanium powder with a particle size of about 10 μm was mixed with 0.5% rare earth oxide La2O3 and 0.1% boron powder by mass of titanium powder, and the mixture was ball-milled under argon protection for 2 hours, with a ball-to-material ratio of 5:1 and a rotation speed of 200 r / min.

[0086] This embodiment relates to a laser welding method for δ-TRIP steel using welding wire. A special welding wire prepared with modified titanium powder (3wt% modified titanium powder content) is used to fill the joint of the plates, and compressed air is used as the shielding gas. A composite induction preheating-post-heating device is also employed. The welding plates are δ-TRIP steel, with two plates each 0.8mm thick. To facilitate welding and installation, the plates are uniformly cut to 100mm*50mm in length and width. Welding is performed using a butt joint, type I joint, with a pre-set gap of 0.08mm. Before welding, the surface of the welding plates is cleaned with acetone and alcohol.

[0087] The specific welding process is as follows: laser defocusing amount is -2mm, outer ring spot diameter is 1mm, inner ring spot diameter is 0.3mm, and the shielding gas is compressed air with a flow rate of 10L / min. The specific welding process parameters are: welding power 6kW, outer ring power ratio 90%, inner ring power ratio 10%, welding speed 2m / min. The welding wire diameter is 0.8mm, and the wire feed speed is 3.0m / min. Induction preheating temperature: 300℃; induction postheating temperature: 600℃, holding time: 10s.

[0088] The welding process was stable with no spatter and no obvious welding defects. The ferrite structure in the resulting weld was significantly refined. Phase analysis of the weld revealed that the volume fraction of the Ti(C,N) strengthening phase was approximately 5.5%. Example 4

[0089] A δ-TRIP steel welding wire, wherein the chemical composition of the welding wire, by mass percentage, comprises: C 0.2%, Si 0.4%, Mn 1.2%, modified titanium powder 10%, S 0.003%, P 0.02%, with the balance being Fe and unavoidable impurities.

[0090] Modified titanium powder was prepared by the following method: titanium powder with a particle size of about 50 μm was mixed with 2% rare earth oxide La2O3 and 0.5% boron powder by mass of titanium powder, and the mixture was ball-milled under argon protection for 6 hours with a ball-to-material ratio of 10:1 and a rotation speed of 400 r / min.

[0091] This embodiment relates to a laser welding method for δ-TRIP steel using welding wire. A special welding wire prepared with modified titanium powder (10wt%) is used to fill the joint of the plates, and compressed air is used as the shielding gas. A composite induction preheating-post-heating device is also employed. The welding plates are δ-TRIP steel, with two plates each 2.0mm thick. To facilitate welding and installation, the plates are uniformly cut to 100mm*50mm in length and width. Welding is performed using a butt joint, type I joint, with a pre-set gap of 0.2mm. Before welding, the surface of the welding plates is cleaned with acetone and alcohol.

[0092] The specific welding process is as follows: laser defocusing amount is -1mm, outer ring spot diameter is 2mm, inner ring spot diameter is 0.6mm, and the shielding gas is compressed air with a flow rate of 30L / min. The specific welding process parameters are: welding power 5kW, outer ring power ratio 80%, inner ring power ratio 20%, welding speed 4m / min. The welding wire diameter is 1.8mm, and the wire feed speed is 6.0m / min. Induction preheating temperature: 500℃; induction postheating temperature: 800℃, holding time: 30s.

[0093] The welding process was stable with no spatter and no obvious welding defects. The ferrite structure in the resulting weld was significantly refined. Phase analysis of the weld revealed that the volume fraction of the Ti(C,N) strengthening phase was approximately 7.5%.

[0094] Test method for Ti(C,N) strengthening phase volume fraction:

[0095] The determination of the volume fraction of the Ti(C,N) strengthening phase was performed in accordance with GB / T 3488.3-2021 "Metallographic Determination of Microstructures of Cemented Carbide Part 3: Metallographic Determination of Microstructures of Ti(C,N) and WC Cubic Carbide-Based Cemented Carbide" and the standard principle of determining the volume fraction of phases using microscopic imaging methods related to fine ceramics. The cut-off method was used. The specific steps are as follows:

[0096] 1. Prepare metallographic specimens on the welded joint samples.

[0097] 2. Take microscopic images of the weld area using a scanning electron microscope (SEM) or an optical microscope at at least 5 random fields of view.

[0098] 3. Image analysis software was used to identify and threshold-segment the Ti(C,N) precipitates in the microscopic images.

[0099] 4. Calculate the volume fraction (i.e., area fraction) of the Ti(C,N) phase using the intercept method. The formula is: Vv = AA = LL = PP, where the volume fraction Vv is estimated by measuring the area fraction AA of the phase.

[0100] 5. The reported results are the average of at least 5 fields of view.

[0101] The standard for cupping value testing is GB / T 4156-2020: the maximum indentation depth to which the punch pushes the specimen to breakage.

[0102] The performance of the welded joints in the δ-TRIP steel of Comparative Example 1, Comparative Example 2, and Examples 1-4 was tested, and the results are shown in Table 1.

[0103] Table 1 Performance Parameters

[0104]

[0105] As shown in Table 1, the welded joints of Examples 1-4 of the present invention have superior mechanical properties, with tensile strength and elongation after fracture being significantly higher than those of the comparative examples, and good forming performance (cupping value).

[0106] By comparing Comparative Example 2 and Example 1, it can be found that, under the same conditions of using titanium-containing welding wire and annular spot, Example 1, which uses modified titanium powder welding wire, has a significantly higher volume fraction of Ti(C,N) reinforcing phase in the weld (6.2%) than Comparative Example 2 (2.8%). Correspondingly, the tensile strength, elongation, and cupping value are also significantly improved. This demonstrates the effectiveness of the composite process used in this invention in increasing the Ti(C,N) reinforcing phase content and improving the overall performance of the joint.

[0107] As can be seen from the comparison between Example 1 and Comparative Example 1, after replacing the protective compressed air with pure argon, the coarse δ-ferrite in the direct laser welding weld seam causes the weld joint to become brittle, thus deteriorating the strength and plasticity of the weld seam.

[0108] As can be seen from the comparison of Examples 1-2, the titanium content in the weld is within the critical value. The higher the titanium content, the stronger the grain refinement effect, and the better the joint strength, plasticity, and formability.

[0109] In the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, this disclosure should not be construed as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, the inventive aspect lies in fewer than all features of the foregoingly disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0110] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0111] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A welding wire for δ-TRIP steel, characterized in that, It contains, by mass percentage: C 0.08-0.2%, Si 0.4-1.2%, Mn 0.8-1.8%, modified titanium powder 3-10%, S≤0.005%, P≤0.02%, with the balance being Fe and unavoidable impurities; Modified titanium powder is prepared by the following method: titanium powder with a particle size of 10-50μm is mixed with 0.5%-2% of rare earth oxides and 0.1%-0.5% of boron powder by mass of titanium powder, and then subjected to high-energy ball milling under argon protection for 2-6 hours, with a ball-to-material ratio of 5:1-10:1 and a rotation speed of 200-400r / min.

2. The δ-TRIP steel welding wire according to claim 1, characterized in that, Rare earth oxides include Y2O3 or La2O3.

3. A laser welding method for δ-TRIP steel welding wire according to claim 1 or 2, characterized in that, Includes the following steps: S1. After cleaning the δ-TRIP steel, place it at intervals to create gaps between the steel plates; S2. Under the protective atmosphere of compressed air, fill the gap with δ-TRIP steel welding wire with a diameter of 0.8 to 1.8 mm, and use a fiber laser to output an annular spot for laser welding. At the same time, use a composite induction preheating-post-heating process to perform thermal management on the weld and its adjacent area.

4. The laser welding method according to claim 3, characterized in that, In S1, the purification process is as follows: the welding surfaces of δ-TRIP steel are polished sequentially with 240#-400#-600# sandpaper until the surface is bright, and impurities on the surface of the steel plate to be welded are removed with acetone or ethanol and then air-dried and cooled.

5. The laser welding method according to claim 3, characterized in that, In S1, the method of spacing is as follows: the butt joint between δ-TRIP steels adopts an I-type bevel, and the width of the gap is not greater than the diameter of the welding wire used for this type of δ-TRIP steel.

6. The laser welding method according to claim 3, characterized in that, In S2, the laser welding parameters are as follows: outer ring spot diameter 1-2mm, inner ring spot diameter 0.3-0.6mm, defocusing amount -2-0mm, welding laser power 3-6kW, outer ring power ratio of the annular spot 70%-90%, inner ring power ratio 10-30%, welding speed 2.0-4.0m / min, compressed air flow rate 10-30L / min; wire feed speed 3.0-6.0m / min.

7. The laser welding method according to claim 3, characterized in that, In S2, the composite induction preheating-postheating process includes the following steps: a) Preheating before welding: In front of the laser welding gun, a high-frequency induction heating device is used to preheat the bevel of the steel plate to be welded and the area on both sides for 5-10mm. The preheating temperature is 300-500℃. b) Post-weld heat treatment: A medium-frequency induction heating device is used behind the laser welding gun to perform post-weld heat treatment on the newly formed weld. The post-heating temperature is 600-800℃ and the holding time is 10-30s.

8. The δ-TRIP steel welded plate prepared by the laser welding method according to claim 3, characterized in that, The welded joints of δ-TRIP steel welded plates have a tensile strength of over 592 MPa, an elongation of over 40%, and a volume fraction of Ti(C,N) reinforcing phase of 5.5%-7.5%.

9. The application of the δ-TRIP steel welded plate according to claim 8 in automotive body structural components.

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