A laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel

Abstract

This invention provides a laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel, comprising adding welding wire to the weld pool and performing laser welding to form a weld; the welding wire is a hardenable welding wire, the composition of which includes C 0.04-0.20, Si 0.30-0.90, Mn 1.00-1.50, Mo 0.40-1.00, Ni 0.20-1.0, W 0.1-1.0%, P ≤0.020, S ≤0.015, and one or more of V, Ti, B, and Nb, with the balance being Fe and unavoidable impurity elements. The weld microstructure of this invention is martensitic, and the tensile strength after heat treatment is not less than 1500MPa.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a welding method for 1500MPa grade uncoated hot-formed steel, and more particularly to a laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel. Background Technology

[0002] The use of hot-formed steel in automobile bodies solves the problem of weight reduction while increasing body strength and safety. Currently, widely used hot-formed components, such as door rings, are formed by welding multiple steel plates together using laser welding. After welding, the workpiece undergoes a hot-forming process, quenching it in a mold to create a hot-formed component with a predominantly martensitic microstructure. During this process, the weld also undergoes hot forming, and the weld joint transforms from austenite to martensite within the mold. Therefore, the entire workpiece has a high-strength microstructure dominated by martensite. Since laser welding is a deep-penetration welding process, the laser melts the butt joint of the workpiece, forming a molten pool, which then forms the weld during cooling. Due to factors such as low workpiece butt joint precision and laser ablation, the actual weld fullness is lower than that of the steel plate surface. The weld becomes a stress concentration point for the entire workpiece, easily leading to stress concentration and crack initiation. During component use, fracture can easily occur in the weld area, causing component failure. Meanwhile, due to factors such as incomplete weld filling, the weld and mold cannot make full contact during hot forming, resulting in insufficient cooling and inadequate quenching. This leads to low weld strength, failing to meet the requirements for component research. Traditional MIG or MAG welding wires, when used for laser filler welding, cannot achieve fully martensitic welds after hot forming heating and quenching, resulting in low weld strength and failing to meet the requirements for hot forming welding. For example, existing patent CN 111390425 B describes a welding wire and welding method for laser welding of hot-stamped Al-Si coated plates. This patented welding wire design is mainly for laser welding of hot-stamped Al-Si coated plates, containing over 6% Ni, resulting in extremely high production costs. Existing patent CN 103331529B discloses a mixed gas-shielded welding wire with a tensile strength ≥1100MPa and its application method. The welding wire described in this patent requires a reverse DC power supply for welding, using an 80% Ar + 20% CO2 mixed gas shielding gas to achieve a weld tensile strength ≥1100MPa. However, laser welding does not use a DC power supply and employs 100% Ar shielding, making it unsuitable for laser filler wire welding. Furthermore, the welding wire has a complex composition and high alloy content, placing high demands on its smelting and manufacturing processes.

[0003] To address these issues, a novel welding wire for uncoated hot-formed steel is needed. This wire possesses strong quenching properties, fully meeting the requirements for laser welding of hot-formed steel. Furthermore, a new welding method is required to achieve a joint strength exceeding 800 MPa after welding uncoated hot-formed cold-rolled sheets. Following the hot-forming process, the weld seam can be quenched to obtain a fully martensitic structure, resulting in a joint strength exceeding 1500 MPa. Summary of the Invention

[0004] To address the aforementioned technical issues, a laser filler wire welding method is provided for uncoated hot-formed steel of grade 1500MPa, achieving a weld joint strength ≥800MPa after welding and a weld strength ≥1500MPa after heating and quenching.

[0005] The technical means employed in this invention are as follows:

[0006] A laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel includes:

[0007] Two test plates are butt-jointed on their welding sides and fixed with clamps. The gap between the two test plates is 0-0.2 mm. Welding wire is added to the molten welding pool for laser welding to form a weld. The molten welding pool is protected by an inert gas. After the welding wire is melted by the laser beam, it enters the molten welding pool in liquid form. The molten metal of the welding wire and the molten metal of the test plate are fused together to form the weld. The metal structure in the weld is martensitic. In the laser welding described in this patent, the weld metal is not lower than the surface of the test plate, ensuring that the weld and the mold are in full contact during the thermoforming process.

[0008] The welding wire is a quenchable welding wire, and its composition and mass percentage are as follows: C: 0.04-0.20%; Si: 0.30-0.90%; Mn: 1.00-1.50%; Mo: 0.40-1.00%; Ni: 0.20-1.0%; W: 0.1-1.0%; P: ≤0.020%; S: ≤0.015%, and may also include one or more of V: ​​0-0.15%, Ti: 0-0.15%, B: 0.002-0.008%, and Nb: 0-0.15%, with the balance being Fe and unavoidable impurity elements; the weld microstructure is martensitic.

[0009] Preferably, the weld strength of the welded steel plate is ≥800MPa, and the weld strength of the welded steel plate after hot forming is ≥1500MPa. In the hot forming process, the welding wire is heated to the austenitizing temperature to fully austenitize, and then cooled at a cooling rate of not less than 30℃ / s.

[0010] Preferably, the diameter of the welding wire is 0.8 mm to 1.2 mm.

[0011] Preferably, the surface of the welding wire is plated with copper, and the thickness of the copper plating layer is 0.1μm to 0.4μm.

[0012] The method of the present invention can be used to weld between composite materials of different materials, grades, and thicknesses.

[0013] The functions and purposes of each component in the welding wire are as follows:

[0014] Carbon (C) is an important element for improving weld strength and increasing weld metal hardenability. Higher C content results in stronger steel, but requires multiple annealing processes for welding wire production, leading to higher production costs. Conversely, low C content results in insufficient weld metal strength and poor weld hardenability. Considering all factors, the designed C content is 0.04–0.10% by mass.

[0015] Si and Mn can both improve weld strength and deoxidize the weld metal. Sufficient deoxidation of the weld metal effectively reduces the precipitation of two-phase particles, lowering the probability of wire breakage during drawing. It also reduces the generation of weld defects. Considering all factors, the Si content of the welding wire in this invention is controlled between 0.30% and 0.90%, and the Mn content is controlled between 1.00% and 1.50%.

[0016] P and S are harmful elements, and the lower their content in welding wire, the better. However, deep P and S removal smelting costs are high. Considering all factors, the welding wire of this invention has P ≤ 0.020% and S ≤ 0.015%.

[0017] Mo (Mo) is an element that increases the hardenability of steel and improves weld strength. Mo exhibits high strength and creep resistance at high temperatures and reduces temper brittleness. However, excessive addition leads to a high hardenability of the weld metal, making it prone to forming twinned substructures. These microstructures easily form microcracks, resulting in weld quality defects. Therefore, a Mo content of 0.40%–1.00% is recommended.

[0018] Ni is an element that forms and stabilizes austenite, and can expand the austenite phase region. When used in combination with elements such as chromium and molybdenum, it improves the plasticity and toughness of the weld, and increases the weld's corrosion resistance. Considering all factors, the Ni content in the welding wire of this invention is 0.2%–1.0%.

[0019] Nb, V, and Ti in welding wire all contribute to grain refinement, increasing strength without reducing weld toughness. In steel, they can form precipitates with C and N, inhibiting austenite grain growth, reducing failure susceptibility and cold brittleness, and improving weldability. However, a large amount of precipitates directly affects the wire's drawability, causing breakage during pull-out. Therefore, appropriate addition is necessary.

[0020] Boron (B) is an element that strongly improves the hardenability of steel; even trace amounts of B can significantly enhance the hardenability of steel plates. B has a strong affinity for O and N, easily leading to non-metallic inclusions. When the B content is too high, the weld is prone to numerous inclusions and martensitic twin defects, reducing weld strength and plasticity. Therefore, the B content used in this invention is 0.002–0.008%.

[0021] W (tungsten) contributes to a high thermal conductivity and low coefficient of thermal expansion in the weld, effectively reducing stress concentration caused by weld microstructure transformation and improving weld strength. Simultaneously, tungsten effectively inhibits grain growth during heating, minimizing the performance degradation caused by coarse grains. When used in combination with C, Ni, and Mo, tungsten enables the weld to form a martensitic structure during hot forming, maintaining high strength and low brittleness, resulting in fracture at the steel plate during tensile testing after hot forming. However, tungsten is a precious metal, and excessively high content increases welding wire costs. Therefore, a tungsten content of 0.1% to 1.0% is recommended.

[0022] This invention also discloses a method for preparing a quenchable welding wire, comprising: smelting molten steel containing the above-mentioned components, followed by casting, wire rod rolling, wire drawing, and surface copper plating to obtain the welding wire. Surface copper plating increases the welding wire's corrosion resistance in air.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] This invention enables the production of full welds during laser welding of hot-formed steel, reducing stress concentration at the weld joint. By adding elements with high hardenability to the weld wire, this invention improves the weld's hardening ability, resulting in a fully martensitic structure and increased weld strength. After hot forming, tensile testing of the welded joint shows fracture at the base material, with a strength of not less than 1500 MPa. This invention uses welding wire with low alloy element content, resulting in low production costs. This invention can be implemented on existing laser welding equipment without requiring equipment modifications.

[0025] Based on the above reasons, this invention can be widely applied in fields such as welding of 1500MPa grade uncoated hot-formed steel. Detailed Implementation

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of this invention can be combined with each other. The technical solutions in the embodiments of this invention are clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0027] A laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel includes:

[0028] The welding sides of two test plates are butt-jointed and fixed with clamps. The gap between the two test plates is 0-0.2 mm, and at least one surface of the two test plates is on the same plane. Laser welding is performed by adding welding wire into the weld pool to form a weld, and the weld pool is protected by an inert gas. After the welding wire is melted by the laser beam, it enters the weld pool in liquid form. The molten metal of the welding wire and the molten metal of the test plate are fused together to form the weld. The metal structure in the weld is martensitic. In the laser welding described in this patent, the weld metal is not lower than the surface of the test plate to ensure that the weld is in full contact with the mold during the thermoforming process.

[0029] The welding wire is a quenchable welding wire, and its composition and mass percentage are as follows: C: 0.04-0.10%; Si: 0.30-0.90%; Mn: 1.00-1.50%; Mo: 0.40-1.00%; Ni: 0.20-1.0%; W: 0.1-1.0%; P: ≤0.020%; S: ≤0.015%, and one or more of V: ​​0-0.15%, Ti: 0-0.15%, B: 0.002-0.008%, and Nb: 0-0.15%, with the balance being Fe and unavoidable impurity elements; the welding wire forms a martensitic structure after laser melting.

[0030] The weld strength of the welded steel plate is ≥800MPa, and the weld strength of the welded steel plate after hot forming is ≥1500MPa.

[0031] In the hot forming process, the welding wire is heated (930-960℃) to the austenitizing temperature and fully austenitized, and then cooled at a cooling rate of not less than 30℃ / s.

[0032] The diameter of the welding wire is 0.8mm to 1.2mm.

[0033] The surface of the welding wire is plated with copper, and the thickness of the copper plating layer is 0.1μm to 0.4μm.

[0034] The method of the present invention can be used to weld between composite materials of different materials, grades, and thicknesses.

[0035] This invention also discloses a method for preparing a quenchable welding wire, comprising: smelting molten steel containing the above-mentioned components, followed by casting, wire rod rolling, wire drawing, and surface copper plating to obtain the welding wire. Surface copper plating increases the welding wire's corrosion resistance in air.

[0036] The following describes in detail a laser filler wire welding method for 1500MPa grade uncoated hot-formed steel proposed in this specific embodiment, and the properties of the material obtained by using this method, with four embodiments and three comparative examples.

[0037] Example 1:

[0038] Uncoated 22MnB5 hot-formed steel with a thickness of 1.5mm was selected as the material for the welding plate. A high-precision shearing machine was used to process the welding test plate. The two steel plates were butt-jointed and fixed with clamps, with a 0mm gap between the plates, ensuring the lower surfaces of the steel plates were on the same plane. Welding was performed using the welding wire disclosed in Example 1 of Table 1, with a wire diameter of 1.0mm. A fiber laser was used, with a defocus of 0mm, laser power adjusted to 3.5kW, welding speed adjusted to 0.06m / s, and wire feed speed adjusted to 0.06m / s. Ar gas was used as the shielding gas for laser wire feed welding. The weld reinforcement on the upper surface was approximately 0.3mm, and on the lower surface, approximately 0.2mm. The welded plate was placed in a 950℃ heating furnace and held for 5 minutes. After the time was up, it was removed and placed in a hot-forming mold for quenching. After cooling, the welded plate was removed, and the weld strength is shown in Table 2.

[0039] Example 2:

[0040] Uncoated 22MnB5 hot-formed steel with thicknesses of 1.5 mm and 1.2 mm was selected as the material for the welding plates. A high-precision shearing machine was used to process the welding test plates. The two steel plates were butt-jointed and fixed with clamps, with a gap of 0.1 mm between the plates, ensuring the lower surfaces of the steel plates were on the same plane. Welding was performed using the welding wire disclosed in Example 2 of Table 1, with a wire diameter of 1.2 mm. A fiber laser was used, with a defocusing amount of -1 mm, laser power adjusted to 3.5 kW, welding speed adjusted to 0.06 m / s, and wire feed speed adjusted to 0.07 m / s. Ar gas was used as the shielding gas for laser wire feed welding. The welded plates were placed in a 950°C heating furnace and held for 5 minutes. After the time was up, they were removed and placed in a hot-forming mold for quenching. After cooling, the welded plates were removed, and the weld strength is shown in Table 2.

[0041] Example 3:

[0042] Two uncoated 22MnB5 hot-formed steel plates with thicknesses of 1.0 mm and 1.0 mm were selected as the materials for the welding plates. A high-precision shearing machine was used to process the welding test plates. The two steel plates were butt-jointed and fixed with clamps, with a gap of 0.15 mm between the plates, ensuring the lower surfaces of the steel plates were on the same plane. The welding wire disclosed in Example 3 of Table 1 was used for welding, with a wire diameter of 1 mm. A fiber laser was used, with a defocusing amount of 0 mm, laser power adjusted to 3 kW, welding speed adjusted to 0.05 m / s, and wire feed speed adjusted to 0.05 m / s. Ar gas was used as the shielding gas for laser wire feed welding. The welded plates were placed in a heating furnace at 960°C for 5 minutes. After the time was up, they were removed and placed in a hot-forming mold for quenching. After cooling, the welded plates were removed, and the weld strength is shown in Table 2.

[0043] Example 4:

[0044] Uncoated 22MnB5 hot-formed steel with thicknesses of 1.0 mm and 1.2 mm was selected as the material for the welding plates. High-precision shearing machines were used to process the welding test plates. The two steel plates were butt-jointed and fixed with clamps, with a 0 mm gap between the plates, ensuring the lower surfaces of the steel plates were on the same plane. Welding was performed using the welding wire disclosed in Example 4 of Table 1, with a wire diameter of 1 mm. A fiber laser was used, with a defocusing amount of 0 mm, laser power adjusted to 3 kW, welding speed adjusted to 0.05 m / s, and wire feed speed adjusted to 0.04 m / s. Ar gas was used as the shielding gas for laser wire feed welding. The welded plates were placed in a 950°C heating furnace and held for 5 minutes. After the time was up, they were removed and placed in a hot-forming mold for quenching. After cooling, the welded plates were removed, and the weld strength is shown in Table 2.

[0045] Table 2 shows that the weld microstructure formed by this method and welding wire is entirely martensitic, while the weld microstructure formed by welding wires provided in Comparative Examples 1-3 is martensitic + bainitic. During the tensile strength test, the steel welded by this method fractured at a pressure above 1500 MPa, with the fracture location being the base metal. The steel welded by welding wires provided in Comparative Examples 1-3 fractured at a pressure of 975-1020 MPa, with the fracture location being the weld seam. Based on the above tests and compositional observations, it is clear that welding using the method and welding wire provided by this invention produces a fully martensitic weld microstructure with a tensile strength of not less than 1500 MPa.

[0046] Table 1. Chemical composition of steel ingots and comparative examples (wt%)

[0047] Example 1 0.20 0.35 1.14 0.014 0.005 0.45 0.95 0.11 0.14 0.003 Example 2 0.06 0.67 1.47 0.015 0.006 1.0 0.70 0.61 0.12 0.12 Example 3 0.08 0.90 1.02 0.009 0.007 0.86 0.63 0.36 0.05 0.008 Example 4 0.09 0.46 1.36 0.015 0.010 0.25 0.40 0.87 0.10 0.002 0.04 Comparative Example 1 0.05 0.52 1.20 0.010 0.005 0.30 Comparative Example 2 0.06 0.47 1.67 0.011 0.006 0.27 Comparative Example 3 0.06 0.65 1.58 0.012 0.009 0.21

[0048] Table 2 Mechanical properties of laser-welded panels

[0049] Example 1 1514 Martensite parent material Example 2 1520 Martensite parent material Example 3 1545 Martensite parent material Example 4 1535 Martensite parent material Comparative Example 1 1020 Martensite + Bainite weld Comparative Example 2 980 Martensite + Bainite weld Comparative Example 3 975 Martensite + Bainite weld

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A laser filler wire welding method suitable for 1500MPa grade uncoated hot-formed steel, characterized in that, include: The welding sides of the two test plates are butt-jointed and fixed, with the gap between the two test plates controlled to be 0~0.2mm; welding wire is added to the weld pool during laser welding, and inert gas protection is used; the welding wire is a hardenable welding wire, and its composition and mass percentage are: C: 0.04~0.10%; Si: 0.30~0.90%; Mn: 1.00~1.50%; Mo: 0.40~1.00%; Ni: 0.20~1.0%; W：0.1~1.0%； P：≤0.020%； S: ≤0.015%, balance being Fe and unavoidable impurity elements; During laser welding, the weld metal is controlled to be no lower than the surface of the test plate to ensure that the weld and the mold are in full contact during the subsequent thermoforming process; After welding, the steel plate is heated to the austenitizing temperature to fully austenitize it, and then cooled at a cooling rate of not less than 30℃ / s to make the weld structure martensitic. After the weld is completed and the steel plate is hot-formed, the weld strength is ≥1500MPa.

2. The laser filler wire welding method for 1500MPa grade uncoated hot-formed steel according to claim 1, characterized in that, The welding wire also includes one or more of the following: V: 0~0.15%, Ti: 0~0.15%, B: 0.002~0.008%, and Nb: 0~0.15%.

3. The laser filler wire welding method for 1500MPa grade uncoated hot-formed steel according to claim 1, characterized in that, The weld strength of the completed steel plate is ≥800MPa.