A laser welding method for cold rolling of high carbon steel and a welded joint

Abstract

This invention discloses a laser welding method and welded joint for cold continuous rolling of high-carbon steel, applicable to high-carbon steel strip with a carbon mass percentage of C: 0.60~1.0%. The laser welding method includes a continuous welding stage at the welding station and a post-weld online heat treatment stage. A complete heat treatment is performed on the weld seam immediately after welding, transforming the hardened microstructure of the weld seam and heat-affected zone into a well-toughened balanced microstructure online. This significantly reduces the weld hardness and improves its ductility and toughness, ultimately reducing the weld breakage rate of high-carbon steel to below 0.3%.

Description

Technical Field

[0001] This invention belongs to the field of metal material manufacturing technology, specifically relating to a laser welding method for cold continuous rolling of high carbon steel and a welded joint. Background Technology

[0002] Cold-rolled high-carbon steel (usually referring to steel grades with a carbon mass fraction C ≥ 0.60%) is widely used in high-end manufacturing fields such as tools and springs due to its excellent strength and wear resistance. In the cold continuous rolling production process, in order to achieve continuous and efficient production, the heads and tails of the front and rear coils of steel must be reliably connected to form "endless rolling". This connection process mainly relies on high-power laser welding technology, such as the German Mibach laser welding machine, and is usually supplemented by a filler wire process.

[0003] However, the extremely poor weldability of high-carbon steel is a recognized technical challenge in the industry. During laser welding, the extremely high heating and cooling rates cause the weld and heat-affected zone to easily form hardened structures such as high-carbon martensite and bainite. These structures are highly hard and brittle, especially on the operational side (weld edge). During the post-weld edge trimming (or "crescent trimming"), the brittle weld edge often cracks directly, making the weld highly susceptible to overall fracture under rolling tension, leading to "strip breakage" accidents on the production line, causing production interruptions and equipment damage. Existing technologies mainly attempt to improve this by optimizing filler wire materials (such as using alloy welding wire to match strength) and adjusting parameters such as welding laser power and speed, but the effects are limited. For steel grades with a carbon content of 0.60% to 1.0%, the weld strip breakage rate is still as high as 10% or more (reaching 20% ​​in winter), becoming a bottleneck restricting the continuous and efficient production of high-carbon steel.

[0004] Post-weld heat treatment is an effective way to improve the microstructure and reduce the hardness of welds. While traditional laser welding machines integrate front and rear induction heating devices, their main function is to provide preheating and post-weld heat preservation during the welding process. The entire weld is heated for a short time (usually about 25 seconds), insufficient to achieve effective microstructure transformation. To extend the heat treatment time, the industry's traditional improvement solution is to add a separate weld induction heating device outside the welding machine exit. However, this method requires physical modifications to the production line, increasing costs significantly; simultaneously, the secondary heating time is significantly extended (average holding time exceeds 60 seconds), resulting in a tight buffer time at the inlet looper, severely violating the continuous and efficient production rhythm of cold rolling and affecting overall output. Therefore, how to complete welding online and in real-time on existing laser welding machine platforms without significantly extending the welding cycle or affecting production continuity, and simultaneously optimize the weld microstructure to fundamentally solve the brittle fracture problem of high-carbon steel welds, is a technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a laser welding method for cold continuous rolling of high carbon steel and a welded joint of high carbon steel, which can improve the microstructure of the weld seam online and in real time, significantly reduce its hardness and improve its toughness, thereby completely solving the problem of weld band breakage in high carbon steel.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for laser welding of high-carbon steel cold rolling, wherein the composition of the high-carbon steel by mass percentage includes: C: 0.60~1.0%, Mn: 0.5~1.5%, Si≤0.40%, Cr≤0.30%, P≤0.30%, S≤0.30%; the strip thickness of the high-carbon steel is 1.5mm~6.0mm, and the laser source of the laser welding machine is a fast axial flow CO2 laser with a rated laser power of 12kW.

[0007] The welding method includes a welding stage and a post-weld online heat treatment stage performed sequentially, with the two stages seamlessly connected and completed at the same station of the welding machine.

[0008] The welding stage includes: Welding wire selection: Use low-carbon welding wire with a carbon mass content of ≤0.10% as filler wire to dilute the carbon equivalent of the weld and reduce the tendency to form ultra-high hardness structure from the source. Induction heating device start position setting: Before setting, the start position of the induction heating device is 20mm~30mm away from the edge of the strip. After setting, the start position of the induction heating device is 40mm~60mm away from the edge of the strip. This position setting is intended to guide the heat more effectively to the center area of ​​the strip thickness direction, avoiding overheating or underheating at the edge.

[0009] Preheating before welding: Start the induction heating device to preheat the welding joint area. The preheating power is 20% to 35% of the rated power of the induction heating device. Laser welding: Start the laser welding machine to perform welding. During the welding process, the welding speed is 16% to 30% of the rated welding speed of the laser welding machine to ensure sufficient heat input; the welding laser power is maintained at 90% to 95% of the rated laser power of the laser welding machine. Preliminary heat preservation after welding: During the welding process, the induction heating device is started simultaneously to provide preliminary heat preservation for the newly formed weld. The heat preservation power is 30% to 50% of the rated power of the induction heating device. This step constitutes the first thermal cycle. At this time, the C-frame of the welding machine is located on the transmission side.

[0010] Post-weld online heat treatment stage: After welding is completed, the weld seam stops between the inlet and outlet clamps, the welding wire is stopped, and the laser welding machine is turned off; Adjust the starting position of the induction heating device: Adjust the starting position of the front induction heating device to 40mm~60mm from the edge of the strip, and adjust the starting position of the rear induction heating device to 20mm~30mm from the edge of the strip. This position adjustment complements the position setting of the welding stage: the welding stage focuses on heating the middle of the weld, while the heat treatment stage deliberately places the rear induction heating device closer to the edge of the strip to enhance the heat treatment effect on the edge area of ​​the weld, which is the most brittle.

[0011] Secondary heating annealing: While keeping the clamps firmly holding the weld, control the C-frame of the welding machine to move towards the operating side. Simultaneously, in the adjusted new position, restart the front and rear induction heating devices to perform a second heating annealing on the weld. The process parameters for the second heating are: the heating power of the front induction heating device is 20%~35% of its rated power, and the heating power of the rear induction heating device is 30%~50% of its rated power. This step constitutes the second thermal cycle, which is closely coupled with the first thermal cycle in time and space to jointly achieve online "tempering" treatment of the weld microstructure.

[0012] The setup and adjustment of the induction heating devices before and after the welding stage, as well as before and after the heat treatment stage, ensured that the weld as a whole (especially the vulnerable edges) received uniform and sufficient heat treatment. The heating layout in the welding stage lays the foundation for forming a high-quality weld, while the targeted adjustment of the heating position in the heat treatment stage is used to solve the problem of brittleness in the weld edge structure. The two work together.

[0013] The high-carbon steel welded joints obtained by the above-described method have a microstructure dominated by pearlite in the weld and heat-affected zone, and a Vickers hardness below 400 HV. 0.3 .

[0014] The specific technical principles and effects of this invention are as follows: 1. The method of the present invention seamlessly completes the welding stage (first thermal cycle) and the post-weld online heat treatment stage (second thermal cycle) in the same welding station and within a continuous time. The starting positions of the induction heating devices before and after the welding and heat treatment stages are set and adjusted purposefully and differently. In particular, the heating of the fragile weld edge (operation side) is strengthened in the heat treatment stage.

[0015] The key to this invention lies in achieving "real-time online heat treatment" of the weld seam without interrupting the production line or occupying additional production cycles. Traditional solutions require additional independent heating equipment outside the welding machine for post-weld heat treatment, significantly extending processing time (average >60 seconds) and severely disrupting the high-speed, continuous production cycle. This invention cleverly utilizes and reconfigures the welding machine's existing integrated front and rear induction heating devices. After welding, the laser and wire feeding stop, but the weld seam is clamped and centered at the workstation. Then, by adjusting the induction heating position and restarting, a second heating annealing is immediately completed at the same workstation. The entire process is continuous, with almost no additional time delay, perfectly meeting the high-efficiency, continuous production requirements of cold rolling mills.

[0016] This invention achieves online microstructure optimization of welds without significantly affecting production efficiency, which makes it possible to perform effective post-weld heat treatment on high-carbon steel welds without large-scale physical modifications to existing production lines or sacrificing output. It provides a practical and feasible process path for solving the problem of brittle fracture in high-carbon steel welds.

[0017] 2. This invention achieves precise orientation of heat input to different areas of the weld by strategically adjusting the activation positions of the front and rear induction heating devices during both the welding and heat treatment stages. In the welding stage, the activation positions are set (front induction heating device activation position 20mm~30mm / rear induction heating device activation position 40mm~60mm) to direct heat towards the central area of ​​the strip thickness, forming a high-quality weld. In the heat treatment stage, the activation positions of the front and rear induction heating devices are interchanged (adjusted to 40mm~60mm front / 20mm~30mm rear), bringing the rear induction heating device closer to the operating side of the strip (i.e., the weld edge) during heat treatment. During welding, the weld edge, due to its fastest cooling rate, is the area most prone to forming brittle and hard microstructure (high-carbon martensite) and exhibits the most severe brittleness issues. The heat treatment stage specifically intensifies heating of this edge area, effectively increasing its temperature and promoting the transformation of the brittle and hard microstructure in this area into a more resilient pearlite microstructure. This fundamentally solves the core problem of weld edges being extremely prone to cracking during subsequent "crescent-shaped" finishing and rolling processes. By applying more targeted heat treatment to the weld edge area, the microstructure and toughness of the area are significantly improved, resulting in a more uniform and reliable overall weld performance and greatly reducing the risk of subsequent "stripping" accidents caused by brittle cracking at the weld edge.

[0018] 3. This invention achieves a complete online "tempering" process by tightly coupling the welding stage (first thermal cycle) and the post-weld online heat treatment stage (second thermal cycle). The welding stage forms the weld and is accompanied by initial heat preservation, while the heat treatment stage involves secondary heating and annealing. The effect of this dual thermal cycle is to transform the ultra-high hardness, high-brittleness hardened microstructure (hardness up to 580~620 HV0.3), mainly composed of high-carbon martensite and bainite, formed during the extremely high cooling rate in the first welding process, into a balanced microstructure mainly composed of pearlite with good toughness (hardness significantly reduced to 270~400 HV0.3, below 400 HV0.3) immediately after welding, through the temperature and time conditions provided by the post-weld heat treatment. This achieves fundamental optimization of the microstructure of the weld and heat-affected zone and a qualitative improvement in mechanical properties. The microstructure transforms from brittle martensite / bainite to tough pearlite, while the hardness decreases significantly, resulting in a significant improvement in the overall mechanical properties of the welded joint, especially its plasticity and toughness. The welded joint thus possesses sufficient strength and toughness reserves to withstand the complex rolling tension and deformation stress during subsequent cold rolling, thereby preventing the weld from breaking during rolling at the source.

[0019] 4. After adopting the process of this invention, the weld exhibits excellent crack resistance in edge bending and simulated rolling tests, with improved microstructure and significantly reduced hardness. Most importantly, the high-carbon steel (C: 0.60%~1.0%) welded joints obtained by this method have demonstrated extremely high reliability in long-term, continuous production. The weld breakage rate of high-carbon steel (C: 0.60%~1.0%) has been reduced from 10%~20% (over 20% in winter) in traditional processes to below 0.3%. This order-of-magnitude reduction means that the risks of production line downtime, equipment damage, and material waste caused by weld fracture are greatly controlled, thereby significantly improving the continuity, stability, and overall economic benefits of high-carbon steel cold rolling production.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention seamlessly connects the welding process (first thermal cycle) and the post-weld online heat treatment (second thermal cycle) at the same station on the production line within a continuous time. This is equivalent to instantly completing a complete "normalizing" or "high-temperature tempering" treatment for the weld at the high speed of the production line, without requiring large-scale modifications to the production line or taking up almost no additional production cycle time.

[0021] 2. By purposefully differentiating and adjusting the start-up positions of the induction heating devices before and after the welding stage and the post-weld online heat treatment stage, especially by strengthening the heating of the strip edge during the heat treatment stage, the temperature of the weld edge can be significantly increased, effectively improving the brittle structure formed in this area under rapid cooling. This fundamentally solves the core problem of easy cracking of the weld edge during edge cutting and rolling.

[0022] 3. Through dual thermal cycling control, the material formed in the first welding process is mainly composed of high-carbon martensite and bainite, with a hardness of up to 580~620HV. 0.3 The hardened structure was transformed online into a structure dominated by pearlite with good toughness, and the hardness was significantly reduced to 400 HV. 0.3 The following equilibrium structure. This structural transformation significantly improves the plasticity and toughness of the welded joint, making it capable of withstanding the complex stresses of subsequent cold rolling. This reduces the weld breakage rate of high-carbon steel (C: 0.60%~1.0%) from 10%~20% in traditional processes to below 0.3%, significantly improving the continuity, stability, and economic efficiency of production. Attached Figure Description

[0023] Figure 1 This is a comparative photograph of cracking after the weld edge has been cut away; Figure 2 This is a schematic diagram illustrating the rolling cracking of a weld seam as a comparison. Figure 3 The microstructure of the weld is shown in the metallographic image (mainly martensite / bainite). Figure 4 This is a schematic diagram illustrating the microhardness of a comparative weld. Figure 5 This is a photograph showing no cracking after the weld edge was cut away, as an example. Figure 6 This is a schematic diagram illustrating the crack-free rolling process of the weld seam in an example. Figure 7 The image shows the metallographic structure of the weld seam in the example (mainly pearlite). Figure 8 This is a schematic diagram of the microhardness of the weld seam in an example. Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0025] Example The welding object is high-carbon steel 65Mn, with a chemical composition of C: 0.62%~0.70% and Mn: 0.90%~1.20%, and a strip thickness of 3.2mm. The laser source of the laser welding machine is a fast axial-flow CO2 laser with a rated power of 12kW.

[0026] Welding stage: Use low-carbon welding wire with a carbon content of ≤0.10%.

[0027] The starting position of the front induction heating device is set to 40mm from the edge of the strip, and the starting position of the rear induction heating device is set to 60mm from the edge of the strip.

[0028] The preheating power of the front induction heating device is set to 30% of the rated power of the front induction heating device to preheat the welded joint.

[0029] Start laser welding at 20% of the rated welding speed of the laser welding machine, and maintain the laser power at 93% of the rated laser power of the laser welding machine.

[0030] Simultaneously with welding, the rear induction heating device is activated to provide initial heat preservation for the newly formed weld. The initial heat preservation power is 45% of the rated power of the rear induction heating device. At this point, the first thermal cycle is completed, and the C-frame is located on the drive side.

[0031] Post-weld online heat treatment stage: After welding is completed, the laser and wire feeding stop, and the weld is fixed in the center of the workstation by clamps.

[0032] The starting position of the front induction heating device was adjusted to 60mm from the edge of the strip, and the starting position of the rear induction heating device was adjusted to 40mm from the edge of the strip. This adjustment aims to direct the heat of the heat treatment more concentratedly to the weld edge area.

[0033] The C-frame is moved towards the operating side, and the front and rear induction heating devices are restarted at the newly set position for secondary heating and annealing. The heating power of the front induction heating device is 30% of its rated power, and the heating power of the rear induction heating device is 50% of its rated power. This process constitutes the second thermal cycle.

[0034] Comparison of effects: Comparative Example: A traditional welding process was used (only one thermal cycle, with the induction heating device fixed at 0mm from the edge). The initial induction heating power was 30%, and the subsequent induction heating power was 45%. After welding, an edge-cutting and bending test was performed on the weld. A crescent-shaped crack appeared along the weld, and cracking occurred on the operating side when passing through the rolling mill. Figure 1 , Figure 2 As shown; tests were conducted on the microstructure and hardness of the weld and heat-affected zone: the microstructure mainly consisted of high-carbon martensite and bainite, with a hardness of 580~620 HV. 0.3 ,like Figure 3 , Figure 4 As shown, in the simulated rolling tension test, 3 out of 10 samples broke at the weld.

[0035] In this invention embodiment: After adopting the above-described online dual thermal cycling method, the weld quality is significantly improved. After welding, an edge-cutting and bending test is performed on the weld; no cracking occurs during the crescent bend, and the weld remains normal and crack-free when passing through the rolling mill. Figure 5 , Figure 6 As shown. Tests were conducted on the microstructure and hardness of the weld and heat-affected zone: the microstructure mainly consisted of fine lamellar pearlite and a small amount of bainite, with martensite essentially eliminated. The hardness decreased significantly to 270-400 HV. 0.3 ,like Figure 7 , Figure 8 As shown; in the same simulated rolling tension test, all 10 samples passed without any breakage.

[0036] Production statistics: In 450 consecutive welding operations, only one weld seam broke. The breakage rate using the complete process of this invention is less than 0.3%, significantly reducing the risk of breakage.

[0037] The above description is only a specific example of the present invention. It should be noted that the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the technical concept and technical solution of the present invention, or the direct application of the technical concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A laser welding method for cold continuous rolling of high-carbon steel, applied to high-carbon steel strip with a carbon mass percentage of C: 0.60~1.0%; characterized in that, The laser welding method includes a welding stage that is performed continuously at the welding station and a post-weld online heat treatment stage: The welding stage includes: Low-carbon welding wire with a carbon content of ≤0.10% was used for filler wire welding; Set the starting position of the front induction heating device to be 20mm~30mm from the edge of the strip, and the starting position of the rear induction heating device to be 40mm~60mm from the edge of the strip; Before starting, the induction heating device preheats the welding joint. After preheating, laser welding is performed. During the welding process, the induction heating device initially keeps the weld warm after starting. The post-weld online heat treatment stage is performed after the welding stage is completed, and includes: After welding is completed, stop feeding the welding wire and turn off the laser; the weld seam stops at the welding station. Adjust the starting position of the front induction heating device to 40mm~60mm from the edge of the strip, and adjust the starting position of the rear induction heating device to 20mm~30mm from the edge of the strip; the front and rear induction heating devices perform secondary heating and annealing on the weld.

2. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: The preheating power of the front induction heating device for preheating the weld joint is 20% to 35% of the rated power of the front induction heating device.

3. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: The welding speed of the laser welding is 16% to 30% of the rated welding speed of the laser welding machine, and the laser power is maintained at 90% to 95% of the rated laser power of the laser welding machine.

4. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: The initial insulation power is 30% to 50% of the rated power of the subsequent induction heating device.

5. The laser welding method for cold continuous rolling of high carbon steel according to claim 1, characterized in that: The heating process parameters in the second heating annealing are as follows: the heating power of the front induction heating device is 20% to 35% of the rated power of the front induction heating device, and the heating power of the rear induction heating device is 30% to 50% of the rated power of the rear induction heating device.

6. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: During the secondary heating and annealing, the welding machine C-frame moves toward the operating side.

7. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: The thickness of the high-carbon steel strip is 1.5mm to 6.0mm.

8. The laser welding method for cold rolling of high carbon steel according to claim 1, characterized in that: The laser source of the laser welding machine is a fast axial-flow CO2 laser.

9. A high-carbon steel welded joint, characterized in that, The welded joint is obtained by welding using any one of claims 1 to 8. The microstructure of the weld and heat-affected zone is mainly pearlite, and the Vickers hardness is less than 400 HV. 0.3 .

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