Manufacturing method of high strength and plastic product super high strength steel laser welding joint

Abstract

A method for manufacturing a laser-welded joint of high-strength, high-ductility, ultra-high-strength steel includes: butt welding high-strength, high-ductility, ultra-high-strength steel; heating a local area of ​​the welded joint, and obtaining the welded joint after cooling; wherein the high-strength, high-ductility, ultra-high-strength steel matrix has a carbon content greater than 0.3% by mass and its microstructure contains martensite and retained austenite; the heating treatment employs a laser beam to scan a local area of ​​the welded joint, including: detecting the width k (mm) of the local area of ​​the welded joint containing the weld seam and the heat-affected coarse-grained and fine-grained regions; setting the scanning range A of the laser beam to satisfy: k≤A≤2k; scanning the local area at least once with the laser beam; and after the above scanning is completed, scanning the local area at least once with a lower laser beam energy. The laser processing method of this invention can significantly improve the strength and elongation of laser-welded joints of high-carbon quenched-partitioned-tempered steel.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and more specifically, to a method for manufacturing a laser-welded joint of high-strength, high-density, high-performance steel. Background Technology

[0002] Automotive lightweighting technology has developed rapidly against the backdrop of global energy conservation and emission reduction. Lightweighting not only improves fuel economy but also effectively reduces vehicle emissions, thereby lessening the burden on the environment. In this context, high-strength steel, due to its excellent strength-to-weight ratio, has become an important material choice in automobile manufacturing, significantly reducing overall vehicle weight while ensuring vehicle safety. Based on the strength-ductility product, advanced high-strength steel is divided into three generations. Currently, the research and development trend of high-strength steel mainly focuses on the third generation of advanced high-strength steel, which offers both low cost and excellent performance. The strength-ductility product of third-generation advanced high-strength steel is generally greater than 30 GPa%, with representative examples including medium-manganese steel, quenched-distribution steel, and quenched-distribution-tempered steel. These materials have good adaptability in structural design and can meet the high standards of lightweighting required in modern automobile manufacturing. In particular, quenched-distribution-tempered steel has achieved a dual improvement in strength and ductility in recent years by introducing inexpensive carbon elements, demonstrating excellent strength-ductility product and cost-effectiveness. However, the high carbon content also leads to a significant decrease in weldability, mainly manifested in problems such as easy cracking, reduced joint strength, and poor plasticity during the welding process, which has become the main obstacle restricting its industrial application.

[0003] Therefore, how to achieve high-quality, high-performance welding of high-carbon quenched-partitioned-tempered steel through appropriate welding processes has become a pressing research challenge. Currently, research on manufacturing methods for high-carbon quenched-partitioned-tempered steel mainly focuses on the base material itself, while research on its welding and post-weld treatment processes is scarce. Furthermore, post-weld treatment processes for other types of third-generation advanced high-strength steels are often difficult to effectively transfer to high-carbon quenched-partitioned-tempered steel, resulting in a lack of targeted solutions in practical operation. In addition, while post-weld heat treatment can improve the performance of welded joints to some extent, it increases operational complexity and hinders efficiency improvements in industrial production.

[0004] Patent document 1 (CN108356417B) discloses a heat treatment method for improving the plasticity of laser-welded joints of high-strength and high-ductility manganese steel. After heat treatment in the two-phase region, the martensite in the weld zone is transformed into austenite and ferrite. This method is not applicable to high-carbon quenched-partitioned-tempered steel with a martensitic matrix, and the heat treatment increases the complexity of the process.

[0005] Patent document 2 (CN112222667B) discloses a post-weld treatment method to improve the performance of spot welded joints of medium manganese steel. The better pull performance is achieved by heat preservation in the two-phase region. However, this method is not suitable for high carbon quenched-distributed-tempered steel, and the heat treatment increases production time and reduces process efficiency.

[0006] Reference 1, "Liu H, Wang Y, Zhu Z, et al. Improving laser welded joint performance of medium-Mn steel through C and Mn partition by intercritical annealing process[J]. Materials Today Communications, 2024, 39: 109077," describes how two-phase annealing improves the strength and ductility of medium-Mn steel joints, but significantly reduces the joint strength. This indicates that while two-phase annealing can improve ductility in medium-Mn steel, it negatively impacts strength. This problem may be even more pronounced in high-carbon quenched-partitioned-tempered steels. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for manufacturing laser-welded joints of high-strength, high-ductility, ultra-high-strength steel. This method replaces traditional heat treatment with online laser processing, butt-welding high-strength, high-ductility, ultra-high-strength steel using a laser beam, and subjecting specific heat treatment to localized areas of the welded joint. This significantly improves the toughness of the joint, solves the problem of brittleness and hardness, and yields high-strength, high-ductility, high-carbon quenched-partitioned-tempered steel laser-welded joints. This method is particularly suitable for ultra-high-strength steels with a matrix carbon content higher than 0.3% and a microstructure containing martensite and retained austenite.

[0008] The technical solution of the present invention is as follows:

[0009] A method for manufacturing a laser-welded joint of high-strength, high-density, high-density steel, characterized by the following steps:

[0010] S1. Butt welding of high-strength, high-ductility, ultra-high-strength steel with a matrix carbon content greater than 0.3% and a microstructure containing martensite and retained austenite is carried out using a laser beam;

[0011] S2. A local area of ​​the welded joint is heated and then cooled to obtain the welded joint. The local area includes the weld seam and the coarse-grained and fine-grained regions of the heat-affected zone. The brittle martensite in the local area of ​​the cooled welded joint is transformed into tempered martensite, transition carbides and reversed austenite.

[0012] Preferably, the high-strength-ductility-product ultra-high-strength steel matrix has a martensite volume fraction of not less than 60% and a matrix strength-ductility-product of not less than 15 GPa.

[0013] Preferably, the steel plate matrix manufacturing process includes a carbon quenching-partitioning-tempering (QPT) process. The matrix microstructure, by volume fraction, comprises 65–92% martensite, 5–40% retained austenite, 1–3% transition carbides, and no more than 1% stable nano-precipitated carbides. The strength-ductility product of the steel plate matrix ranges from 15–60 GPa%, the strength ranges from 1000–1900 MPa, and the elongation at break ranges from 15%–35%.

[0014] Preferably, the steel plate matrix elements, by mass percentage, include 0.3–0.75% C, 0.5–3.0% Mn, 0.5–3.0% Si, 0.02–0.5% Nb+V, as well as Fe and unavoidable impurities.

[0015] Preferably, the steel plate matrix elements, by mass percentage, also include 0-2.0% Cr, 0-3.0% Ni, 0-3.0% Mo, and 0-5.0% Al.

[0016] Preferably, the laser beam is Gaussian or uniformly distributed, and the laser beam can be composed of one or more light spots, with the welded joint cross-sectional shape being X-shaped or Y-shaped.

[0017] Preferably, the heat treatment employs a laser beam to scan a local area of ​​the welded joint, including the following steps:

[0018] Step 1: Inspect the width k (mm) of the weld joint, including the weld seam and the local area of ​​the heat-affected coarse-grained zone and fine-grained zone;

[0019] Step 2: Set the scanning range A of the laser beam to satisfy: k≤A≤2k;

[0020] Step 3: The laser beam scans the local area at least once, with the laser beam input energy acting on the local area ranging from 0.2 to 20 J / mm². 2 ;

[0021] Step 4: After the above scanning is completed, scan the local area at least once using a lower laser beam energy, wherein the laser beam energy range is 0.05–15 J / mm. 2 .

[0022] Preferably, in step 3, the laser spot diameter D (mm) is 0.1 to 10 mm, the laser oscillation amplitude f (mm) is 0.01 to 10 mm, and the oscillation trajectory can be a sawtooth path, a circular path, a linear path, an "8" shaped path, an "∞" shaped path, or a mixed path.

[0023] Preferably, in step 3, the laser power g1 (W) is 50-5000W, the scanning speed is 0.5-10m / min, and the laser beam defocusing amount is -10 to 90mm.

[0024] Preferably, in step 3, the temperature is measured by an infrared thermal imager. During the laser scanning process, the overall average temperature of the scanned joint area is between 100 and 500°C, and the local instantaneous temperature near the laser spot is between 300 and 1000°C.

[0025] Preferably, the interior of the weld, the coarse-grained zone of the heat-affected zone, and the fine-grained zone of the heat-affected zone transform from predominantly martensite to tempered martensite, transition carbides, and reversed austenite. The surface portion of the weld will re-austenite, forming martensite upon cooling.

[0026] Preferably, the laser power g2 (W) satisfies g2≤g1, the scanning speed is 0.5-10m / min, and the laser beam defocusing amount is -10~90mm.

[0027] Preferably, by measuring temperature with an infrared thermal imager, the overall average temperature of the scanned joint area during laser scanning is between 100 and 300°C, and the local instantaneous temperature near the laser spot during laser scanning is between 200 and 700°C.

[0028] Preferably, the weld undergoes further distribution aging, and the martensite formed on the weld surface in step 4 is distributed and aged, transforming into tempered martensite, transition carbides and reversed austenite.

[0029] Preferably, after the above heat treatment, the high-carbon quenched-distributed-tempered steel laser welded joint has the following mechanical properties: tensile strength of 1000-1900 MPa and elongation of 15-35%.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] After laser welding of high-carbon quenched, partitioned, and tempered steel, the weld seam, heat-affected zone (HAZ), coarse-grained region, and fine-grained region are all composed of brittle and hard high-carbon martensite. Traditional post-weld heat treatment increases process steps and production time, reducing production efficiency. This invention employs online laser treatment. By controlling the online laser treatment process parameters, the joint undergoes partitioned aging. After treatment, the high-carbon martensite in the weld seam and the HAZ, coarse-grained and fine-grained regions transforms into tempered martensite, transition carbides, and reversed austenite. The hardness is significantly reduced. Due to the precipitation of transition carbides, the high carbon content of the martensite is depleted, improving toughness. Furthermore, the transition carbides can play a precipitation strengthening role. Simultaneously, the reversed austenite can generate dislocations across the martensite / austenite interface and a phase transformation-induced plasticity effect, thereby further improving the strength and toughness of the joint. The joint strength is comparable to that of the base metal, and tensile fracture occurs in the base metal. Furthermore, this method only treats the weld and adjacent areas, without affecting the microstructure of the base material, thus maintaining the high strength and plasticity of the base material. Compared with traditional heat treatment methods, it can significantly reduce the number of process steps and processing time, thereby greatly improving production efficiency. Attached Figure Description

[0032] Figure 1 This is a schematic diagram showing the different locations of high-strength plastic volume laser-welded joints without online laser processing.

[0033] Figure 2 This is a schematic diagram of the online laser processing of a high-strength plastic volume laser-welded joint.

[0034] Figure 3 This is a schematic diagram showing the different locations of the high-strength plastic volume laser-welded joint after online laser processing.

[0035] Figure 4 This is a scanning electron microscope image of the high-carbon quenched-distributed-tempered steel base material from Example 1.

[0036] Figure 5 This is a scanning electron microscope image of the weld seam after laser welding of high-carbon quenched-distributed-tempered steel in Example 1.

[0037] Figure 6 This is a scanning electron microscope image of the weld seam of the laser-welded joint of high-carbon quenched-distributed-tempered steel in Example 1 after heat treatment. Detailed Implementation

[0038] The present invention will now be described in detail with reference to specific embodiments, and preferred embodiments of the present application are shown in the accompanying drawings. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0039] This invention provides a method for manufacturing a laser-welded joint of high-strength, high-ductility, high-carbon quenched-distributed-tempered steel, specifically comprising:

[0040] First, a steel billet is provided, which can be a flat plate or a stamped part with a certain shape, with a plate thickness ≤10mm and a composition (mass fraction, %): C: 0.30~0.75, Mn: 0.5~3.0, Si: 0.5~3.0, Cr: 0~2.0, Ni: 0~3.0, Mo: 0~3.0, Al: 0~5.0, Nb+V: 0.02~0.5, with the remainder being iron. The steel manufacturing process is quenching-partitioning-tempering (QPT) heat treatment. The microstructure of the steel is as follows: martensite matrix content 65–92%, retained austenite content 5–40%, transition carbides 1–3%, and stable nano-precipitated carbides less than 1%. In the high-carbon quenched-partitioned-tempered steel, the martensite matrix consists of primary lath martensite and fine twinned secondary martensite; the retained austenite is fine lamellar or granular austenite, coherent with the martensite matrix using the KS or NW method; the transition carbides are η or ε carbides; and the stable nano-precipitated carbides are niobium carbide and / or vanadium carbide and / or molybdenum carbide. This high-carbon quenched-partitioned-tempered steel has a strength of 1000–1900 MPa and an elongation of 15–35%. The high-carbon quenched-partitioned-tempered steel is butt-welded and then laser-welded. During laser welding, the laser beam can consist of one or more beams, and the beam energy distribution can be Gaussian, average, or point-ring distribution, among other forms. During the welding process, the laser beam can travel along the welding direction in a fixed or synchronous high-speed motion, including oscillations. The oscillation shape can include various forms such as circles, broken lines, figure-eights, and infinity symbols. The oscillation frequency is generally 50-500Hz, and the oscillation amplitude is between 0.2-1.5mm. The workpiece butt gap b should not exceed 20% of the plate thickness B. The laser welding power is 1-15kW, the welding speed is 1-12m / min, the laser beam defocusing distance is -10-90mm, argon gas is used to protect the front side, and the gas flow rate is 5-30L / min. The resulting laser-welded joint cross-sectional morphology is X-shaped or Y-shaped.

[0041] The width k (mm) of the weld joint, including the weld seam and the local area of ​​the heat-affected coarse-grained and fine-grained regions, is detected. The scanning range A of the laser beam is set to satisfy: k≤A≤2k. The laser spot diameter D (mm), laser oscillation amplitude f (mm), and oscillation mode of the laser online processing are determined. The laser spot diameter D (mm) is 0.1~10mm, the laser oscillation amplitude f (mm) is 0.01~10mm, and the oscillation trajectory can be a sawtooth path, a circular path, a linear path, an "8" shaped path, an "∞" shaped path, or a mixed path, and D and f satisfy: k≤D+2f≤2k.

[0042] Next, set the laser power g1 (W) to 50-5000W, the scanning speed to 0.5-10m / min, and the laser beam defocus to -10 to 90mm.

[0043] Then, the front and back welds and the coarse-grained and fine-grained zones of the heat-affected zone of the high-carbon quenched-partitioned-tempered steel were subjected to n1 laser scanning processes, where 1 ≤ n1. Infrared thermal imager measurements showed that the overall average temperature of the scanned joint area during laser scanning ranged from 100 to 500℃, while the local instantaneous temperature near the laser spot ranged from 300 to 1000℃. During the scanning process, the weld interior, the coarse-grained zone, and the fine-grained zone of the heat-affected zone transformed from predominantly martensite to tempered martensite, transition carbides, and reversed austenite. The weld surface portion underwent re-austenitization, forming martensite upon cooling.

[0044] Set the laser power g2 (W) to satisfy g2≤g1, the scanning speed to 0.5-10m / min, and the laser beam defocusing amount to -10~90mm.

[0045] The front and back welds and the coarse-grained and fine-grained zones of the heat-affected zone of high-carbon quenched-partitioned-tempered steel were subjected to n² laser scanning processes, where 1 ≤ n². Temperature measurements were taken using an infrared thermal imager. During the laser scanning process, the overall average temperature of the scanned joint area ranged from 100 to 300°C, while the local instantaneous temperature near the laser spot ranged from 200 to 700°C. During the scanning process, the weld underwent further partitioning aging, transforming the martensite formed on the weld surface into tempered martensite, transition carbides, and reversed austenite.

[0046] After undergoing the above-mentioned online laser treatment, the mechanical properties of the laser-welded joints of high-carbon quenched-distributed-tempered steel reach: tensile strength of 1000-1900MPa and elongation of 15-35%.

[0047] Example

[0048] Table 1 shows the specific composition, mechanical properties, and laser welding process parameters of the high-carbon quenched-distributed-tempered steel base material in the embodiments and comparative examples of the present invention. The plate thickness is 1.25 mm, and the tensile test is carried out in accordance with GB / T 228-2002 "Metallic Materials - Tensile Testing at Room Temperature".

[0049]

[0050]

[0051]

[0052] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for manufacturing a laser-welded joint of high-strength, high-density, high-density steel, characterized in that, Includes the following steps: S1. Butt welding of high-strength, high-ductility, ultra-high-strength steel with a matrix carbon content greater than 0.3% and a microstructure containing martensite and retained austenite is carried out using a laser beam; S2. A local area of ​​the welded joint is heated and then cooled to obtain the welded joint. The local area includes the weld seam and the coarse-grained and fine-grained regions of the heat-affected zone. The brittle martensite in the local area of ​​the cooled welded joint is transformed into tempered martensite, transition carbides and reversed austenite.

2. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 1, characterized in that, The high-strength, high-ductility ultra-high-strength steel matrix has a martensite volume fraction of not less than 60% and a matrix strength-ductility product of not less than 15 GPa.

3. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 2, characterized in that, The steel plate matrix manufacturing process includes a carbon quenching-partitioning-tempering (QPT) process. The matrix structure, by volume fraction, contains 65-92% martensite, 5-40% retained austenite, 1-3% transition carbides, and no more than 1% stable nano-precipitated carbides.

4. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 3, characterized in that, The strength-ductility volume range of the steel plate substrate is 15–60 GPa.

5. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 2, characterized in that, The matrix elements of the steel plate, by mass percentage, include 0.3–0.75% C, 0.5–3.0% Mn, 0.5–3.0% Si, 0.02–0.5% Nb+V, as well as Fe and unavoidable impurities.

6. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 5, characterized in that, The steel plate matrix elements, by mass percentage, also include 0–2.0% Cr, 0–3.0% Ni, 0–3.0% Mo, and 0–5.0% Al.

7. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 1, characterized in that, The laser beam is Gaussian or uniformly distributed, and the laser beam can be composed of one or more light spots. The cross-sectional shape of the welded joint is X-shaped or Y-shaped.

8. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 1, characterized in that, The heat treatment employs a laser beam to scan a local area of ​​the welded joint, and includes the following steps: Step 1: Inspect the width k (mm) of the weld joint, including the weld seam and the local area of ​​the heat-affected coarse-grained zone and fine-grained zone; Step 2: Set the scanning range A of the laser beam to satisfy: k≤A≤2k; Step 3: The laser beam scans the local area at least once, with the laser beam input energy acting on the local area ranging from 0.2 to 20 J / mm². 2 The laser spot diameter D (mm) is 0.1-10mm, the laser oscillation amplitude f (mm) is 0.01-10mm, and the oscillation trajectory is a sawtooth path, a circular path, a linear path, an "8" shaped path, an "∞" shaped path, or a mixed path. The laser power g1 (W) is 50-5000W, the scanning speed is 0.5-10m / min, the laser beam defocus is -10-90mm, and the temperature is measured by an infrared thermal imager. During the laser scanning process, the overall average temperature of the scanned joint area is 100-500℃, and the local instantaneous temperature near the laser spot is 300-1000℃. Step 4: After the above scanning is completed, scan the local area at least once using a lower laser beam energy, wherein the laser beam energy range is 0.05–15 J / mm. 2 The laser power g2 (W) satisfies g2≤g1, the scanning speed is 0.5-10m / min, the laser beam defocus is -10~90mm, and the temperature is measured by an infrared thermal imager. During the laser scanning process, the overall average temperature of the scanned joint area is 100~300℃, and the local instantaneous temperature near the laser spot is 200~700℃.

9. The method for manufacturing high-strength, high-density, ultra-high-strength steel laser-welded joints as described in claim 8, characterized in that, In step 3, the interior of the weld, the coarse-grained zone of the heat-affected zone, and the fine-grained zone of the heat-affected zone transform from mainly martensite to tempered martensite, transition carbides, and reversed austenite. The surface of the weld will be re-austenitized and form martensite after cooling.

10. The method for manufacturing a high-strength, high-density, ultra-high-strength steel laser-welded joint as described in claim 8, characterized in that, In step 4, the weld undergoes further distribution aging. The martensite formed on the weld surface in step 4 is distributed and aged, transforming into tempered martensite, transition carbides, and reversed austenite.

Images