High-grade non-oriented silicon steel welded joint and friction stir welding method thereof

Abstract

The application discloses a high-grade non-oriented silicon steel welded joint and a friction stir welding method thereof, and the application places a low-grade non-oriented silicon steel strip between two high-grade non-oriented silicon steel strips along a running direction, and connects the three strips at the head and tail to form two parallel butt seams; two stirring heads are used to synchronously stir the two butt seams, the rotating directions of the two stirring heads are opposite, the high-grade strip is located at the retreat side of the weld seam, and the low-grade strip is located at the advancing side of the weld seam. The gradient transition formed by the composition difference of the two can effectively relieve the brittleness problem of the weld seam area, and the specific double-head reverse synchronous welding process can solve the problems of high strip breakage rate and poor weld appearance quality of the high-grade non-oriented silicon steel weld seam in the continuous production process.

Description

Technical Field

[0001] This invention relates to the field of metal material welding technology, specifically to a high-grade non-oriented silicon steel welded joint and its friction stir welding method. Background Technology

[0002] Non-oriented silicon steel, as a key soft magnetic alloy material, exhibits superior magnetic properties in its high-grade products due to significantly increased silicon and aluminum content. However, this also leads to decreased plasticity and increased brittleness, posing unique challenges for welding connections in subsequent continuous processing. On continuous production lines such as normalizing pickling and continuous rolling, hot-rolled coils need to be joined end-to-end by welding to ensure continuous operation. Traditionally, fusion welding methods such as laser welding are commonly used. However, these methods have inherent limitations when welding high-grade non-oriented silicon steel: the molten pool and heat-affected zone formed during welding are subjected to high temperatures, making grain coarsening and growth highly likely; furthermore, due to the high silicon-aluminum alloy content and low carbon content of high-grade non-oriented silicon steel, it remains in the ferrite phase region throughout the welding thermal cycle, unable to refine the grains through phase transformation, resulting in significant deterioration of the toughness of the joint area, especially the heat-affected zone. Although existing technologies have attempted to improve weld composition and refine solidification structure by adding welding wire and introducing low-grade materials as lead-in strips, it is still difficult to overcome the fundamental problem of grain coarsening in the heat-affected zone. This results in a high risk of strip breakage under the tensioning, correction, and repeated bending of tension rollers, correction rollers, and loopers on the production line, which seriously restricts production stability and efficiency.

[0003] Friction stir welding (FSW), a solid-state joining technology, has been successfully applied in joining materials such as aluminum alloys and magnesium alloys, and in recent years has begun to be explored for welding steel materials. However, when FSW is directly applied to non-oriented silicon steel with high silicon and aluminum content, the high silicon and aluminum content significantly increases the material's resistance to deformation at welding temperatures and deteriorates its plastic flowability. If the process control is not proper, internal quality problems such as incomplete penetration and tunnel-type defects can easily occur in the weld. These internal defects are prone to propagate under the bending stress of subsequent continuous production, ultimately leading to early fracture in the weld area and causing production breakage. In addition, improper process control can also cause appearance defects such as grooves, peeling, and flash on the weld surface, affecting the weld formation quality and failing to meet the requirements of weld appearance consistency for continuous production. Summary of the Invention

[0004] This invention provides a high-grade non-oriented silicon steel welded joint and its friction stir welding method to solve the problems of high breakage rate and poor weld appearance quality caused by insufficient welding performance of high-grade non-oriented silicon steel welds during continuous production.

[0005] In a first aspect, the present invention provides a friction stir welding method for high-grade non-oriented silicon steel welded joints, comprising the following steps: A low-grade non-oriented silicon steel strip is placed between two high-grade non-oriented silicon steel strips along the strip running direction. The two end faces of the low-grade non-oriented silicon steel strip are then joined with the welding end faces of the two high-grade non-oriented silicon steel strips to form two butt joints. Here, joining means that the two end faces of the low-grade non-oriented silicon steel strip are in close contact with and aligned with the welding end faces of the two high-grade non-oriented silicon steel strips, so that the three strips are connected end to end along the strip running direction and the two butt joints are formed at the contact points. Two stirring heads were used to perform synchronous friction stir welding on two butt joints. The two stirring heads rotated in opposite directions, with the high-grade non-oriented silicon steel strip positioned on the retreating side of the weld and the low-grade non-oriented silicon steel strip positioned on the advancing side. The downward speed of the stirring heads was 0.20 mm / s to 0.30 mm / s, and the rotational speed W satisfied the following: W = 20 × [100 × (Si% + Al% + 0.7Mn%)]. 3 The welding speed V satisfies the following formula: V = 600 - 50 × [100 × (Si% + Al% + 0.7Mn%)] mm / min, where Si%, Al%, and Mn% are the mass percentages of Si%, Al%, and Mn% in high-grade non-oriented silicon steel strip, respectively. The side where the tangential direction of the stirring head rotation is the same as the welding direction is called the advancing side, and the other side is called the retreating side. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, includes: C≤0.0030%, Si: 3.05%~3.43%, Al: 0.55%~0.95%, Mn: 0.20%~0.65%, P≤0.025%, Si+Al content is 3.65%~4.30%, and the remainder is Fe and unavoidable inclusions; The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, includes: C: 0.0040%~0.0060%, Si: 0.15%~0.25%, Al: 0.15%~0.25%, Mn: 0.50%~0.72%, P: 0.020%~0.040%, Ni: 0.10%~0.15%, Cu: 0.01%~0.03%, with the remainder being Fe and unavoidable inclusions; The sum of the Si content of the high-grade non-oriented silicon steel strip and the Si content of the low-grade non-oriented silicon steel strip is 3.25%~3.55%.

[0006] Based on the compositional design of the high-grade and low-grade non-oriented silicon steel strips, the higher silicon and aluminum content in the high-grade non-oriented silicon steel strip ensures its magnetic properties. The carbon content in the low-grade non-oriented silicon steel strip is controlled at 0.0040%~0.0060%, and when mixed with the high-grade strip, it allows the weld pool to undergo austenitization, followed by grain refinement through phase transformation during cooling. Simultaneously, the lower silicon and aluminum content in the low-grade strip helps reduce the brittleness of the weld region, while appropriate amounts of manganese, phosphorus, and added nickel and copper improve the fluidity of the weld pool, effectively alleviating the difficulties in plastic flow during friction stir welding of the high-grade non-oriented silicon steel strip. This synergistic compositional design, through the effective dilution of the high-grade material by the low-grade material and the optimization of the phase transformation behavior and rheological properties of the weld, provides a compositional basis for obtaining welded joints with high plasticity and high toughness.

[0007] In one optional embodiment, the length of the low-grade non-oriented silicon steel strip along the running direction is 400 mm to 550 mm. And / or, the high-grade non-oriented silicon steel strip and the low-grade non-oriented silicon steel strip have the same thickness; optionally, the thickness of both the high-grade non-oriented silicon steel strip and the low-grade non-oriented silicon steel strip is 2.2 mm to 3.2 mm. And / or, the high-grade non-oriented silicon steel strip has the same width as the low-grade non-oriented silicon steel strip.

[0008] In one optional embodiment, the process parameters of the friction stir welding are: holding time 4 s to 10 s, and welding angle 1.5° to 2.2°.

[0009] In one optional embodiment, the shoulder radius R of the stirring head is 7.5~10.5; preferably, the shoulder radius R of the stirring head is determined by the following formula: R=200×(Si%+Al%); where Si% and Al% are the mass percentages of Si% and Al% in high-grade non-oriented silicon steel strip, respectively, and the unit of R is mm. And / or, the shoulder pressing amount D of the stirring head is 0.03 mm to 0.06 mm.

[0010] In one optional embodiment, the stirring pin of the stirring head has a frustum-shaped structure with a spherical crown at the end, an end diameter of 3.0 mm to 4.0 mm, and a root diameter to shoulder diameter ratio of 0.5 to 0.6. And / or, the length L of the stirring pin of the stirring head is 1.8~3.0; preferably, the length L of the stirring pin of the stirring head is determined by the following formula: L=HD-0.14; where L is in mm; H is the thickness of the high-grade non-oriented silicon steel strip in mm; and D is the shoulder pressing amount in mm.

[0011] In one optional embodiment, the friction stir welding is performed under a protective atmosphere; preferably, the protective atmosphere is an inert gas, more preferably, the inert gas is argon. And / or, for the two stirring heads performing synchronous friction stir welding, the relative deviation of their rotational speed is controlled within 15 rpm, and the relative deviation of their welding speed is controlled within 10 mm / min.

[0012] In one optional embodiment, the high-grade non-oriented silicon steel strip and the low-grade non-oriented silicon steel strip are both pre-treated by shearing and grinding on their weldable ends before being spliced ​​together.

[0013] In an optional embodiment, after the butt joint is formed and before friction stir welding is performed, the method further includes a step of flattening and fixing the non-oriented silicon steel strips on both sides of the butt joint using a clamp, and a step of preheating the area of ​​the butt joint; optionally, the preheating temperature is 180°C to 250°C; optionally, the preheating step is achieved by heating a pad set below the butt joint with a flame; wherein, the flame nozzle is arranged below the pad and close to the side of the high-grade non-oriented silicon steel strip, 8 mm to 12 mm away from the butt joint, and the flame nozzle is located 30 mm to 60 mm in front of the stirring head in the direction of travel.

[0014] In one optional embodiment, after the synchronous friction stir welding is completed, the welded joint is cooled at a cooling rate of 30℃ / s to 80℃ / s, and the keyhole area formed at both ends of the weld due to the lifting of the stirring pin is subjected to crescent shearing with a radius of 300 mm to 500 mm and a shearing depth of 60 mm to 100 mm. At the same time, the surface of the weld is ground.

[0015] Secondly, the present invention also provides a high-grade non-oriented silicon steel welded joint, which is prepared by the above-mentioned friction stir welding method for high-grade non-oriented silicon steel welded joints. The welded joint consists of two high-grade non-oriented silicon steel strips and a low-grade non-oriented silicon steel strip located between the two. The three strips are connected end to end along the running direction of the strips. The two ends of the low-grade non-oriented silicon steel strip are respectively connected to the two high-grade non-oriented silicon steel strips by friction stir welds. The weld microstructure is ferrite with a grain size of 10μm~50μm, preferably 10μm~30μm.

[0016] The technical solution of this invention has the following advantages: 1. This invention employs friction stir welding to weld high-grade non-oriented silicon steel strips. By controlling process parameters such as the downward pressure speed, rotation speed, and welding speed of the stirring head, low heat input is achieved, preventing abnormal grain growth in the heat-affected zone and avoiding weld breakage during operation. Specifically, a low-grade non-oriented silicon steel strip is used as an intermediate transition section, joined end-to-end with two high-grade non-oriented silicon steel strips along the operating direction to form a "high-low-high" structural layout. The sum of the Si content of the high-grade and low-grade strips is controlled at 3.25%~3.55%, reducing the content of major elements such as silicon and aluminum in the stirred molten pool region and improving the plasticity of the weld zone. Simultaneously, the two stirring heads are set to rotate in opposite directions, placing the high-grade strip on the weld retraction side and the low-grade strip on the weld advancement side. This reduces the shear strain and thermal load on the high-grade non-oriented silicon steel during welding, enhances plastic flow in the weld zone, avoids weld defects, and yields a high-quality weld. Furthermore, this invention designs the alloy composition of low-grade non-oriented silicon steel with a C content of 0.0040%~0.0060% and a Si content of 0.15%~0.25%, thereby increasing the C content and decreasing the Si content in the stirred molten pool region. This allows for the formation of an austenitic phase region during the welding heating process, followed by a phase transformation during cooling to refine the grains and improve the strength and toughness of the weld joint. Simultaneously, by adding appropriate amounts of Mn and small amounts of P, Ni, and Cu, high-temperature rheological stress is reduced, high-temperature ductility is improved, and grain boundary slip resistance is reduced, thereby enhancing the fluidity of the stirred molten pool metal and preventing surface defects such as grooves, peeling, and flash from appearing on the weld surface.

[0017] 2. This invention further defines the relationship between the silicon and aluminum content in high-grade silicon steel strip and the shoulder radius of the stirring head, namely, the shoulder radius R = 200 × (Si% + Al%), where Si% and Al% are the mass percentages of Si% and Al% in the high-grade non-oriented silicon steel strip, respectively. Higher silicon and aluminum content results in greater material brittleness, higher strength, and a greater required heat input; therefore, a larger shoulder radius ensures more stable stirring of the molten pool.

[0018] 3. This invention further defines the length L of the stirring pin as L = HD - 0.14, where H is the thickness of the high-grade strip steel and D is the shoulder pressing amount, both in millimeters. This formula directly links the length of the stirring pin to the actual thickness of the strip steel and the pressing amount, ensuring that the stirring pin welds through without puncturing the backing plate. The welding process and tool dimensions are adjusted according to the brittleness and hardness of the material and the variation in plate thickness, making the welding process more stable and resulting in a more uniform joint structure. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the friction stir welding process in Embodiment 1 of the present invention; Explanation of reference numerals in the attached figures: 1-1. High-grade non-oriented silicon steel strip 1; 1-2. High-grade non-oriented silicon steel strip 2; 1-3. Low-grade non-oriented silicon steel strip; 1-4. Stirring head 1; 1-5. Stirring head 2. Detailed Implementation

[0021] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0022] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0023] Example 1 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The friction stir welding process is as follows: Figure 1 As shown, the specific steps are as follows: (1) Prepare two high-grade non-oriented silicon steel strips and one low-grade non-oriented silicon steel strip. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0025%, Si: 3.09%, Al: 0.78%, Mn: 0.42%, P: 0.024%, with Si+Al content of 3.87%, and the remainder being Fe and unavoidable inclusions. The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0048%, Si: 0.19%, Al: 0.22%, Mn: 0.62%, P: 0.030%, Ni: 0.12%, Cu: 0.013%, with the remainder being Fe and unavoidable inclusions. Both the high-grade and low-grade strips have a thickness of 2.2 mm and the same width. The sum of the Si content of the high-grade and low-grade strips is 3.28%. The length of the low-grade strip steel along the running direction is 450 mm. The weldable end faces of the three strip steel pieces are sheared and ground.

[0024] (2) Place the low-grade strip steel between two high-grade strip steels along the strip steel running direction, and then assemble and weld the two end faces of the low-grade strip steel to the end faces of the two high-grade strip steels to be welded, forming two parallel butt joints.

[0025] (3) Place a pad directly below the two butt joints and use a clamp to flatten and fix the strips on both sides of the butt joints. Preheat the butt joint area with flame heating. The flame nozzle is arranged below the pad and close to the side of the high grade strip, 10 mm away from the butt joint, and 50 mm in front of the direction of travel of the stirring head. The preheating temperature is 220℃.

[0026] (4) Two stirring heads were used to perform synchronous friction stir welding on the two butt joints. The two stirring heads rotated in opposite directions, with the high-grade strip on the retreating side of the weld and the low-grade strip on the advancing side. The shoulder radius R of the stirring head was 8 mm, and the shoulder pressure D was 0.04 mm. The stirring pin of the stirring head had a frustum conical structure with a spherical crown at the end, a diameter of 3.5 mm at the end, and a root diameter to shoulder diameter ratio of 0.5. The length of the stirring pin was L = HD - 0.14 = 2.2 - 0.04 - 0.14 = 2.02 mm. The welding process parameters were: pressure drop speed 0.25 mm / s, holding time 5 s, rotation speed 1444 r / min, welding speed 392 mm / min, and welding tilt angle 1.7°. The welding process was carried out under an argon protective atmosphere. The relative deviation of the actual rotation speed of the two stirring heads was controlled within ±12 rpm, and the relative deviation of the welding speed was controlled within ±8 mm / min.

[0027] (5) After welding, the flame heating was turned off, and the welded joint was cooled at a cooling rate of 45℃ / s. Then, the keyhole areas formed at both ends of the weld due to the lifting of the stirring pin were sheared in a crescent shape with a radius of 400 mm and a shearing depth of 70 mm. The weld surface was then ground to obtain a high-grade non-oriented silicon steel welded joint. The weld microstructure of the welded joint was found to be ferrite with a grain size of 21 μm.

[0028] Example 2 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints, the specific steps of which are as follows: (1) Prepare two high-grade non-oriented silicon steel strips and one low-grade non-oriented silicon steel strip. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0027%, Si: 3.28%, Al: 0.67%, Mn: 0.37%, P: 0.020%, with Si+Al content of 3.95%, and the remainder being Fe and unavoidable inclusions. The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0056%, Si: 0.22%, Al: 0.18%, Mn: 0.57%, P: 0.035%, Ni: 0.10%, Cu: 0.027%, and the remainder being Fe and unavoidable inclusions. Both the high-grade and low-grade strips have a thickness of 2.5 mm and the same width. The sum of the Si content of the high-grade and low-grade strips is 3.50%. The length of the low-grade strip steel along the running direction is 450 mm. The weldable end faces of the three strip steel pieces are sheared and ground.

[0029] (2) Place the low-grade strip steel between two high-grade strip steels along the strip steel running direction, and then assemble and weld the two end faces of the low-grade strip steel to the end faces of the two high-grade strip steels to be welded, forming two parallel butt joints.

[0030] (3) Place a pad directly below the two butt joints and use a clamp to flatten and fix the strips on both sides of the butt joints. Preheat the butt joint area with flame heating. The flame nozzle is arranged below the pad and close to the side of the high grade strip, 10 mm away from the butt joint, and 36 mm in front of the direction of travel of the stirring head. The preheating temperature is 199℃.

[0031] (4) Two stirring heads were used to perform synchronous friction stir welding on the two butt joints. The two stirring heads rotated in opposite directions, with the high-grade strip on the retreating side of the weld and the low-grade strip on the advancing side. The shoulder radius R of the stirring head was 8 mm, and the shoulder pressure D was 0.05 mm. The stirring pin of the stirring head had a frustum conical structure with a spherical crown at the end, a diameter of 3.5 mm at the end, and a root diameter to shoulder diameter ratio of 0.5. The length of the stirring pin was L = HD - 0.14 = 2.5 - 0.05 - 0.14 = 2.31 mm. The welding process parameters were: pressure drop speed 0.27 mm / s, holding time 4 s, rotation speed 1491 r / min, welding speed 390 mm / min, and welding tilt angle 1.5°. The welding process was carried out under an argon protective atmosphere, and the relative deviation of the actual rotation speed of the two stirring heads was controlled within ±8 rpm, and the relative deviation of the welding speed was controlled within ±6 mm / min.

[0032] (5) After welding, the flame heating was turned off, and the welded joint was cooled at a cooling rate of 37℃ / s. Then, the keyhole areas formed at both ends of the weld due to the lifting of the stirring pin were sheared in a crescent shape with a radius of 350 mm and a shearing depth of 60 mm. The weld surface was then ground to obtain a high-grade non-oriented silicon steel welded joint. The weld microstructure of the welded joint was found to be ferrite with a grain size of 14 μm.

[0033] Example 3 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints, the specific steps of which are as follows: (1) Prepare two high-grade non-oriented silicon steel strips and one low-grade non-oriented silicon steel strip. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0018%, Si: 3.17%, Al: 0.59%, Mn: 0.25%, P: 0.023%, with Si+Al content of 3.76%, and the remainder being Fe and unavoidable inclusions. The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0051%, Si: 0.17%, Al: 0.23%, Mn: 0.67%, P: 0.038%, Ni: 0.14%, Cu: 0.023%, with the remainder being Fe and unavoidable inclusions. Both the high-grade and low-grade strips have a thickness of 2.7 mm and the same width. The sum of the Si content of the high-grade and low-grade strips is 3.34%. The length of the low-grade strip steel along the running direction is 480 mm. The weldable end faces of the three strip steel pieces are sheared and ground.

[0034] (2) Place the low-grade strip steel between two high-grade strip steels along the strip steel running direction, and then assemble and weld the two end faces of the low-grade strip steel to the end faces of the two high-grade strip steels to be welded, forming two parallel butt joints.

[0035] (3) Place a pad directly below the two butt joints and use a clamp to flatten and fix the strips on both sides of the butt joints. Preheat the butt joint area with flame heating. The flame nozzle is arranged below the pad and close to the side of the high grade strip, 10 mm away from the butt joint, and 42 mm in front of the direction of travel of the stirring head. The preheating temperature is 183℃.

[0036] (4) Two stirring heads were used to perform synchronous friction stir welding on the two butt joints. The two stirring heads rotated in opposite directions, with the high-grade strip on the retreating side of the weld and the low-grade strip on the advancing side. The shoulder radius R of the stirring head was 8 mm, and the shoulder pressure D was 0.06 mm. The stirring pin of the stirring head had a frustum conical structure with a spherical crown at the end, a diameter of 3.5 mm at the end, and a root diameter to shoulder diameter ratio of 0.5. The length of the stirring pin was L = HD - 0.14 = 2.7 - 0.06 - 0.14 = 2.50 mm. The welding process parameters were: pressure drop speed 0.23 mm / s, holding time 7 s, rotation speed 1219 r / min, welding speed 403 mm / min, and welding tilt angle 2.1°. The welding process was carried out under an argon protective atmosphere, and the relative deviation of the actual rotation speed of the two stirring heads was controlled within ±3 rpm, and the relative deviation of the welding speed was controlled within ±5 mm / min.

[0037] (5) After welding, the flame heating was turned off, and the welded joint was cooled at a cooling rate of 59℃ / s. Then, the keyhole areas formed at both ends of the weld due to the lifting of the stirring pin were sheared in a crescent shape with a radius of 450 mm and a shearing depth of 80 mm. The weld surface was then ground to obtain a high-grade non-oriented silicon steel welded joint. The weld microstructure of the welded joint was found to be ferrite with a grain size of 17 μm.

[0038] Example 4 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints, the specific steps of which are as follows: (1) Prepare two high-grade non-oriented silicon steel strips and one low-grade non-oriented silicon steel strip. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0023%, Si: 3.40%, Al: 0.87%, Mn: 0.62%, P: 0.019%, with Si+Al content of 4.27%, and the remainder being Fe and unavoidable inclusions. The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, is: C: 0.0043%, Si: 0.15%, Al: 0.25%, Mn: 0.52%, P: 0.027%, Ni: 0.11%, Cu: 0.019%, with the remainder being Fe and unavoidable inclusions. Both the high-grade and low-grade strips have a thickness of 3.1 mm and the same width. The sum of the Si content of the high-grade and low-grade strips is 3.55%. The length of the low-grade strip steel along the running direction is 480 mm. The weldable end faces of the three strip steel pieces are sheared and ground.

[0039] (2) Place the low-grade strip steel between two high-grade strip steels along the strip steel running direction, and then assemble and weld the two end faces of the low-grade strip steel to the end faces of the two high-grade strip steels to be welded, forming two parallel butt joints.

[0040] (3) Place a pad directly below the two butt joints and use a clamp to flatten and fix the strips on both sides of the butt joints. Preheat the butt joint area with flame heating. The flame nozzle is arranged below the pad and close to the side of the high grade strip, 10 mm away from the butt joint, and 58 mm in front of the direction of travel of the stirring head. The preheating temperature is 243℃.

[0041] (4) Two stirring heads were used to perform synchronous friction stir welding on the two butt joints. The two stirring heads rotated in opposite directions, with the high-grade strip on the retreating side of the weld and the low-grade strip on the advancing side. The shoulder radius R of the stirring head was 9 mm, and the shoulder pressure D was 0.03 mm. The stirring pin of the stirring head had a frustum conical structure with a spherical crown at the end, a diameter of 3.5 mm at the end, and a root diameter to shoulder diameter ratio of 0.5. The length of the stirring pin was L = HD - 0.14 = 3.1 - 0.03 - 0.14 = 2.93 mm. The welding process parameters were: pressure drop speed 0.28 mm / s, holding time 8 s, rotation speed 2082 r / min, welding speed 365 mm / min, and welding tilt angle 1.9°. The welding process was carried out under an argon protective atmosphere, and the relative deviation of the actual rotation speed of the two stirring heads was controlled within ±6 rpm, and the relative deviation of the welding speed was controlled within ±3 mm / min.

[0042] (5) After welding, the flame heating was turned off, and the welded joint was cooled at a cooling rate of 73℃ / s. Then, the keyhole areas formed at both ends of the weld due to the lifting of the stirring pin were sheared in a crescent shape with a radius of 500 mm and a shearing depth of 90 mm. The weld surface was then ground to obtain a high-grade non-oriented silicon steel welded joint. The weld microstructure of the welded joint was found to be ferrite with a grain size of 27 μm.

[0043] Example 5 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this embodiment and Embodiment 1 is that the shoulder radius of the stirring head is 10mm (theoretically calculated shoulder radius is 8mm). All other conditions are exactly the same as in Embodiment 1.

[0044] Example 6 This embodiment provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this embodiment and Embodiment 1 is that the length of the stirring pin is 2.15 mm (the theoretically calculated length of the stirring pin is 2.02 mm). All other conditions are exactly the same as in Embodiment 1.

[0045] Comparative Example 1 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this method and Example 1 is that, instead of adding low-grade non-oriented silicon steel strip as an intermediate transition layer, the end faces of two high-grade non-oriented silicon steel strips to be welded are directly spliced ​​together to form a butt joint, and a single stirring head is used for friction stir welding. Other conditions are the same as in Example 1.

[0046] Comparative Example 2 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the two stirring heads rotate in the same direction, while the other conditions are the same as in Example 1.

[0047] Comparative Example 3 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the rotation direction of the two stirring heads is such that the high-grade strip is located on the weld advancing side and the low-grade strip is located on the weld retreating side. Other conditions are the same as in Example 1.

[0048] Comparative Example 4 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the downward speed of the stirring head is 0.40 mm / s, while other conditions are the same as in Example 1.

[0049] Comparative Example 5 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the stirring head rotation speed is 2000 rpm, while other conditions are the same as in Example 1.

[0050] Comparative Example 6 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the Si content of the high-grade non-oriented silicon steel strip is 4.20%, while other conditions are the same as in Example 1.

[0051] Comparative Example 7 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this comparative example and Example 1 is that the Si content of the low-grade non-oriented silicon steel strip is 0.50%, while other conditions are the same as in Example 1.

[0052] Comparative Example 8 A laser welding method for high-grade non-oriented silicon steel welded joints is provided. The only difference between this method and Example 1 is that laser welding (laser power of 11kW and welding speed of 4.2m / min) is used instead of friction stir welding. That is, the two butt joints formed by splicing are welded separately using a laser welding machine without using a stirring head. Other conditions are the same as in Example 1.

[0053] Comparative Example 9 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this method and Example 1 is that step-by-step welding is used instead of synchronous welding: first, one stirring head is used to perform friction stir welding on one butt joint, and after the weld has cooled to room temperature, another stirring head is used to perform friction stir welding on the other butt joint. Other conditions are the same as in Example 1.

[0054] Comparative Example 10 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this method and Example 1 is that in the low-grade non-oriented silicon steel strip, Ni ≤ 0.01% and Cu ≤ 0.005%, while other conditions are the same as in Example 1.

[0055] Comparative Example 11 This comparative example provides a friction stir welding method for high-grade non-oriented silicon steel welded joints. The only difference between this method and Example 1 is that the welding speed is 200 mm / min (theoretically calculated welding speed is 392 mm / min), while other conditions are the same as in Example 1.

[0056] Test Example 1 The mechanical properties of the welded joints obtained in Examples 1 to 6 and Comparative Examples 1 to 11 were tested. The specific test conditions are as follows: 1. After welding, the edges of the weld are cut 100mm on both sides. The remaining weld area is subjected to cupping tests on the upper and lower plates every 100mm. That is, each weld is subjected to 5 cupping tests on the upper and lower surfaces. The cupping value is greater than 5 and the crack direction is not parallel to the weld as the acceptance standard. The number of cupping tests is ≥9 (that is, at least 9 out of 10 tests are qualified for each weld). The cupping test of the weld is considered to be qualified. 2. The location of the fracture shall be determined in accordance with the requirements of GB / T228 and GB / T 2651 standards. The weld strength shall be deemed to be superior to the base metal strength if the fracture point is not in the weld or the affected zone. 3. In the reverse bending test, the bending angle is recorded according to the requirements of GB / T 2653-2008 standard. The weld is considered to be qualified when the bending angle is greater than 120°.

[0057] Only when the above three tests are passed can the weld be considered to be of qualified quality. At the same time, observe the appearance quality and the breakage of the weld. The characterization results are shown in Table 1.

[0058] Table 1. Performance test results of welded joints in each embodiment and comparative example.

[0059] Based on the above test results, it can be seen that the weld cupping success rate of Examples 1 to 6 of the present invention is ≥9 times, the bending angle is >120°, the fracture location is the base material fracture, and the breakage rate is ≤0.03%, indicating that the technical solutions of the present invention can all obtain welded joints that meet the requirements of continuous production. Among them, Examples 1 to 4 have the best overall performance, while the performance of Examples 5 and 6 is slightly reduced but still meets the requirements.

[0060] In contrast, the performance of Comparative Examples 1 through 11 all showed varying degrees of degradation. Comparative Examples 1 through 3 exhibited severe weld defects due to the lack of a low-grade intermediate layer, unidirectional rotation of both ends, or the high-grade material being located on the advancing side; the bending angle was ≤30° and the breakage rate was ≥18%. Comparative Example 4's excessive pressing speed caused tearing around the keyhole, resulting in a repeat welding rate of 40%. Comparative Example 5's excessive rotation speed led to grain coarsening, reducing the number of cupping successes to 7. Comparative Examples 6 and 7 had poor weld formation due to excessive high-grade or low-grade Si content, resulting in only 3 cupping successes for each. Comparative Example 8's laser welding resulted in coarse grains due to high heat input, with a bending angle of only 40°. Comparative Example 9's step-by-step welding, while achieving acceptable performance, suffered from low production efficiency and a deviation rate of 15%. Comparative Example 10's lack of Ni and Cu resulted in insufficient molten pool fluidity, reducing the number of cupping successes to 8. Comparative Example 11's low welding speed led to excessive heat input, reducing the number of cupping successes to 7.

[0061] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A friction stir welding method for high-grade non-oriented silicon steel welded joints, characterized in that, Includes the following steps: A low-grade non-oriented silicon steel strip is placed between two high-grade non-oriented silicon steel strips along the strip running direction, and the two end faces of the low-grade non-oriented silicon steel strip are respectively joined with the end faces to be welded of the two high-grade non-oriented silicon steel strips to form two butt joints. Two stirring heads were used to perform synchronous friction stir welding on two butt joints. The two stirring heads rotated in opposite directions, with the high-grade non-oriented silicon steel strip positioned on the retreating side of the weld and the low-grade non-oriented silicon steel strip positioned on the advancing side. The downward speed of the stirring heads was 0.20 mm / s to 0.30 mm / s, and the rotational speed W satisfied the following: W = 20 × [100 × (Si% + Al% + 0.7Mn%)]. 3 The welding speed V satisfies the following formula: V = 600 - 50 × [100 × (Si% + Al% + 0.7Mn%)] mm / min, where Si%, Al%, and Mn% are the mass percentages of Si%, Al%, and Mn% in the high-grade non-oriented silicon steel strip, respectively. The chemical composition of the high-grade non-oriented silicon steel strip, by mass percentage, includes: C≤0.0030%, Si: 3.05%~3.43%, Al: 0.55%~0.95%, Mn: 0.20%~0.65%, P≤0.025%, Si+Al content is 3.65%~4.30%, and the remainder is Fe and unavoidable inclusions; The chemical composition of the low-grade non-oriented silicon steel strip, by mass percentage, includes: C: 0.0040%~0.0060%, Si: 0.15%~0.25%, Al: 0.15%~0.25%, Mn: 0.50%~0.72%, P: 0.020%~0.040%, Ni: 0.10%~0.15%, Cu: 0.01%~0.03%, with the remainder being Fe and unavoidable inclusions; The sum of the Si content of the high-grade non-oriented silicon steel strip and the Si content of the low-grade non-oriented silicon steel strip is 3.25%~3.55%; The shoulder radius R of the stirring head is 7.5~10.5; the shoulder radius R of the stirring head is determined by the following formula: R=200×(Si%+Al%); where Si% and Al% are the mass percentages of Si% and Al% in high-grade non-oriented silicon steel strip, respectively, and the unit of R is mm. The shoulder pressing amount D of the stirring head is 0.03 mm to 0.06 mm.

2. The friction stir welding method according to claim 1, characterized in that, The length of the low-grade non-oriented silicon steel strip along the running direction is 400 mm to 550 mm. And / or, the high-grade non-oriented silicon steel strip has the same thickness as the low-grade non-oriented silicon steel strip; the thickness of both the high-grade and low-grade non-oriented silicon steel strips is 2.2 mm to 3.2 mm. And / or, the high-grade non-oriented silicon steel strip has the same width as the low-grade non-oriented silicon steel strip.

3. The friction stir welding method according to claim 1 or 2, characterized in that, The process parameters for friction stir welding are: holding time 4 s to 10 s, and welding angle 1.5° to 2.2°.

4. The friction stir welding method according to claim 1 or 2, characterized in that, The stirring needle of the stirring head has a frustum-shaped structure with a spherical crown at the end. The diameter of the end is 3.0mm to 4.0mm, and the ratio of the root diameter to the shoulder diameter is 0.5 to 0.

6. And / or, the stirring pin length L of the stirring head is 1.8~3.0; the stirring pin length L of the stirring head is determined by the following formula: L=HD-0.14; where L is in mm; H is the thickness of high grade non-oriented silicon steel strip in mm; and D is the shoulder pressing amount in mm.

5. The friction stir welding method according to claim 1 or 2, characterized in that, The friction stir welding is performed under a protective atmosphere; the protective atmosphere is an inert gas, specifically argon. And / or, for the two stirring heads performing synchronous friction stir welding, the relative deviation of their rotational speed is controlled within 15 rpm, and the relative deviation of their welding speed is controlled within 10 mm / min.

6. The friction stir welding method according to claim 1 or 2, characterized in that, Before being joined together, the high-grade non-oriented silicon steel strip and the low-grade non-oriented silicon steel strip both undergo pretreatment of shearing and grinding on their weldable ends.

7. The friction stir welding method according to claim 1 or 2, characterized in that, After the butt joint is formed and before friction stir welding is performed, the process includes a step of flattening and fixing the non-oriented silicon steel strips on both sides of the butt joint using a clamp, and a step of preheating the area of ​​the butt joint. The preheating temperature is 180℃~250℃. The preheating step is achieved by heating a pad set below the butt joint with a flame. The flame nozzle is arranged below the pad and close to the side of the high-grade non-oriented silicon steel strip, 8 mm~12 mm away from the butt joint, and the flame nozzle is located 30 mm~60 mm in front of the stirring head in the direction of travel.

8. The friction stir welding method according to claim 1 or 2, characterized in that, After the synchronous friction stir welding is completed, the welded joint is cooled at a cooling rate of 30℃ / s to 80℃ / s, and the keyhole area formed at both ends of the weld due to the lifting of the stirring pin is subjected to crescent shearing with a radius of 300 mm to 500 mm and a shearing depth of 60 mm to 100 mm. At the same time, the surface of the weld is ground.

9. A high-grade non-oriented silicon steel welded joint, characterized in that, It is prepared by friction stir welding of high-grade non-oriented silicon steel welded joints according to any one of claims 1 to 8.

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