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EP-4382238-B1 - ELECTRICAL STEEL STRIP, FRICTION STIR WELDING METHOD AND METHOD OF PRODUCING ELECTRICAL STEEL STRIP

EP4382238B1EP 4382238 B1EP4382238 B1EP 4382238B1EP-4382238-B1

Inventors

  • MATSUSHITA, MUNEO
  • IWATA, Shohei
  • KITANI, Yasushi

Dates

Publication Date
20260513
Application Date
20220727

Claims (15)

  1. An electrical steel strip comprising a first electrical steel strip (1), a second electrical steel strip (2) and a welded joint joining the first electrical steel strip (1) and the second electrical steel strip (2), the welded joint comprising a joined portion (4) and a thermo-mechanically affected zone (4-1, 4-2) adjacent to the joined portion (4), characterized in that the steel microstructures of the joined portion (4) and the thermo-mechanically affected zone (4-1, 4-2) are mainly ferrite phase, and the following Expressions (1) to (4) are satisfied, Dsz ≤ 200 μm Dhaz 1 ≤ Dbm 1 Dhaz 2 ≤ Dbm 2 0.9 × Hbm 1 + Hbm 2 / 2 ≤ Hsz ≤ 1.2 × Hbm 1 + Hbm 2 / 2 wherein Dsz is an average value (µm) of ferrite grain size of the joined portion (4), Dhaz1 is an average value (µm) of ferrite grain size of the thermo-mechanically affected zone (4-1) on a first electrical steel strip side, Dhaz2 is an average value (µm) of ferrite grain size of the thermo-mechanically affected zone (4-2) on a second electrical steel strip side, Dbm1 is an average value (µm) of ferrite grain size of the base metal portion of the first electrical steel strip (1), Dbm2 is an average value (µm) of ferrite grain size of the base metal portion of the second electrical steel strip (2), Hsz is an average value of hardness of the joined portion (4), Hbm1 is an average value of hardness of the base metal portion of the first electrical steel strip (1), and Hbm2 is an average value of hardness of the base metal portion of the second electrical steel strip (2).
  2. The electrical steel strip according to claim 1, satisfying the relationships of the following Expressions (5) and (6), 0.8 × TbmL ≤ TszL TszH ≤ 1.3 × TbmH wherein TszL is the minimum value (mm) of the thickness of the joined portion (4), TszH is the maximum value (mm) of the thickness of the joined portion (4), TbmL is the thickness (mm) of the thinner of the first electrical steel strip (1) and the second electrical steel strip (2), TbmH is the thickness (mm) of the thicker of the first electrical steel strip (1) and the second electrical steel strip (2), and when the thicknesses of the first electrical steel strip (1) and the second electrical steel strip (2) are the same, TbmL = TbmH.
  3. The electrical steel strip according to claim 1 or 2, wherein the thickness of the first electrical steel strip (1) is different from the thickness of the second electrical steel strip (2).
  4. An electrical steel strip friction stir welding method for joining a first electrical steel strip (1) and a second electrical steel strip (2) following the first electrical steel strip (1), comprising: pressing a rotating tool (3) into an unjoined portion that is a butted portion or an overlapped portion of an end of the first electrical steel strip (1) and an end of the second electrical steel strip (2) while rotating the rotating tool (3); and joining the first electrical steel strip (1) and the second electrical steel strip (2) by moving the rotating tool (3) in a joining direction, wherein the joining is performed under conditions that the steel microstructures of the joined portion (4) and the thermo-mechanically affected zone (4-1, 4-2) formed by the joining of the first electrical steel strip (1) and the second electrical steel strip (2) become mainly ferrite phase and the relationships of the following Expressions (1) to (4) are satisfied, Dsz ≤ 200 μm Dhaz 1 ≤ Dbm 1 Dhaz 2 ≤ Dbm 2 0.9 × Hbm 1 + Hbm 2 / 2 ≤ Hsz ≤ 1.2 × Hbm 1 + Hbm 2 / 2 wherein Dsz is an average value (µm) of ferrite grain size of the joined portion (4), Dhaz1 is an average value (µm) of ferrite grain size of the thermo-mechanically affected zone (4-1) on a first electrical steel strip side, Dhaz2 is an average value (µm) of ferrite grain size of the thermo-mechanically affected zone (4-2) on a second electrical steel strip side, Dbm1 is an average value (µm) of ferrite grain size of the base metal portion of the first electrical steel strip (1), Dbm2 is an average value (µm) of ferrite grain size of the base metal portion of the second electrical steel strip (2), Hsz is an average value of hardness of the joined portion (4), Hbm1 is an average value of hardness of the base metal portion of the first electrical steel strip (1), and Hbm2 is an average value of hardness of the base metal portion of the second electrical steel strip (2).
  5. The electrical steel strip friction stir welding method according to claim 4, wherein the joining is performed under conditions satisfying the relationships of the following Expressions (5) and (6), 0.8 × TbmL ≤ TszL TszH ≤ 1.3 × TbmH wherein TszL is the minimum value (mm) of the thickness of the joined portion (4), TszH is the maximum value (mm) of the thickness of the joined portion (4), TbmL is the thickness (mm) of the thinner of the first electrical steel strip (1) and the second electrical steel strip (2), TbmH is the thickness (mm) of the thicker of the first electrical steel strip (1) and the second electrical steel strip (2), and when the thicknesses of the first electrical steel strip (1) and the second electrical steel strip (2) are the same, TbmL = TbmH.
  6. The electrical steel strip friction stir welding method according to claim 4 or 5, wherein the rotating tool (3) is a pair of rotating tools (3-1, 3-2) facing each other, and the rotating tools (3-1, 3-2) are pressed from both sides of the unjoined portion while rotating in opposite directions to each other, the thickness of the first electrical steel strip (1) is different from the thickness of the second electrical steel strip (2), the leading ends (5-3, 5-4) of the rotating tools each include a center portion (5-5, 5-6) and an outer circumferential portion (5-7, 5-8) disposed adjacent to a periphery of the center portion (5-5, 5-6), and the outer circumferential portion (5-7, 5-8) has a tapered shape.
  7. The electrical steel strip friction stir welding method according to claim 6, wherein a taper angle α of the outer circumferential portion (5-7, 5-8) is from 2° to 20°.
  8. The electrical steel strip friction stir welding method according to claim 6 or 7, wherein a radial position of the boundary between the center portion (5-5, 5-6) and the outer circumferential portion (5-7, 5-8) is in a range from 0.15 × D to 0.35 × D, wherein the radial position is the distance from the axis of rotation along the radial direction of the leading end (5-3, 5-4) of the rotating tool (3), and D is the diameter of the leading end (5-3, 5-4) of the rotating tool (3).
  9. The electrical steel strip friction stir welding method according to any one of claims 6 to 8, wherein the surface of the outer circumferential portion (5-7, 5-8) has a spiral-shaped stepped portion spiraling in the rotation direction of the rotating tool (3).
  10. The electrical steel strip friction stir welding method according to any one of claims 6 to 9, wherein the surface of the outer circumferential portion (5-7, 5-8) has a spiral-shaped stepped portion spiraling opposite the rotation direction of the rotating tool (3).
  11. The electrical steel strip friction stir welding method according to any one of claims 6 to 10, wherein the center portion (5-5, 5-6) has a flat, convex curved, or concave curved surface.
  12. The electrical steel strip friction stir welding method according to any one of claims 6 to 11, wherein the surface of the center portion (5-5, 5-6) has a spiral-shaped stepped portion spiraling opposite the rotation direction of the rotating tool (3).
  13. The electrical steel strip friction stir welding method according to any one of claims 6 to 12, wherein the diameter D 1 (mm) of the center portion (5-5, 5-6) of the rotating tool (3) satisfies the relationship of the following Expression (10), and a rotation speed RS (r/min) of the rotating tool (3), the diameter D 1 (mm) of the center portion (5-5, 5-6) of the rotating tool (3), and a joining speed JS (mm/min), expressed as RS × D 1 3 /JS, satisfy the relationship of the following Expression (11), 4 × TJ ≤ D 1 ≤ 10 × TJ 200 × TJ ≤ RS × D 1 3 / JS ≤ 2000 × TJ wherein TJ is defined such that, when the unjoined portion is the butted portion, TJ is an average value (mm) of the thickness of the first electrical steel strip (1) and the thickness of the second electrical steel strip (2), and when the unjoined portion is the overlapped portion, TJ is the thickness (mm) of the overlapped portion.
  14. The electrical steel strip friction stir welding method according to any one of claims 6 to 13, wherein at the butted portion of the end of the first electrical steel strip (1) and the end of the second electrical steel strip (2), there is a step on only one side, the advancing side of the rotating tool (3) disposed on the side with the step is on the side of the electrical steel strip that has the smaller thickness of the first electrical steel strip (1) and the second electrical steel strip (2), and the retreating side of the rotating tool (3) disposed on the side with the step is on the side of the electrical steel strip that has the greater thickness of the first electrical steel strip (1) and the second electrical steel strip (2).
  15. A method of producing an electrical steel strip, the method comprising: joining a first electrical steel strip (1) and a second electrical steel strip (2) by the electrical steel strip friction stir welding method according to any one of claims 4 to 14 to obtain a joined steel strip; and cold rolling the joined steel strip to obtain a cold-rolled steel strip.

Description

TECHNICAL FIELD The present disclosure relates to an electrical steel strip and friction stir welding method, and a method of producing an electrical steel strip. BACKGROUND In steel sheet production lines, such as pickling, cold rolling, annealing, and coating or plating production lines, in order to improve productivity and increase yield, performing so-called coil joining then passing a steel strip through the production line is common. Here, coil joining refers to the joining of an end (trailing end) of a preceding steel strip and an end (leading end) of the steel strip following the preceding steel strip (hereinafter also referred to as trailing steel strip) in a production line. Hereinafter, a joined portion formed by coil joining is also referred to as a coil joint. The leading end is the end of the steel strip in the direction of travel on the production line. Further, the trailing end is the end of the steel strip in the direction opposite the direction of travel on the production line. Coil joining enables rolling and the like with tension applied to the entire length of the steel strip. Further, coil joining enables highly precise control of strip thickness and shape even at the leading and trailing ends of the steel strip. In coil joining, conventionally, flash butt welding and the like are commonly applied. However, with advances in laser welders, laser welding is becoming a mainstream application for coil joining in, for example, production lines for electrical steel sheets, stainless steel sheets, and high tensile strength steel sheets. As an example of such technology, Patent Literature (PTL) 1 describes: "A high-Si steel laser welding method comprising, when welding high-Si steel, welding using a filler wire containing Ni as the main component or supplying powder filler containing Ni as the main component, so that the chemical composition of weld metal satisfies the following Expression (1). X=%Ni−%Si×2.5−%Cr+%Mo×0.4≥0 Here, [%Ni], [%Si], [%Cr], and [%Mo] represent content (wt%) of Ni, Si, Cr, and Mo in the weld metal, respectively." PTL 2 describes: "In a laser welding method for butt welding a leading sheet and a trailing sheet using a filler wire, a ratio (Gap/DEPO) of the butt gap (Gap) between the leading sheet and the trailing sheet to an average width of the weld metal (DEPO) at an initial stage of welding is 0.3 to 0.8." PTL 3 describes: "In a welded portion formed by laser welding a leading sheet and a trailing sheet made of special steel that are conveyed on a continuous cold rolling line,when L1 is the minimum thickness of base metal existing below an upper extension portion consisting of weld metal that extends to the upper side of the base metal due to cold rolling, and L2 is the minimum thickness of the base metal between the upper extension portion and a lower extension portion consisting of weld metal that extends to the lower side of the base metal due to cold rolling, then at least one of L1 or L2 is greater than zero." CITATION LIST Patent Literature PTL 1: JP H5-305466 APTL 2: JP 2004-25284 APTL 3: JP 2011-140026 APTL 4: JP H07-505090 A (publication in Japan of WO 9310935 A1)PTL 5: JP 3261433 B2PTL 6: JP 4838385 B2 (see also EP2474382)PTL 7: JP 4838388 B2 Non-Patent Literature NPL 1: Cui, L.; Fujii, H.; Tsuji, N.; Nogi, K. Scripta Mater. 2007, 56, p.637-640. SUMMARY (Technical Problem) Laser welding is fusion welding, and therefore causes embrittlement due to impurity segregation during fusion and solidification, and due to hydrogen entry. As a result, deterioration of mechanical properties of the joined portion (welded portion) may occur. In particular, electrical steel sheet chemical composition contains a large amount of Si, and therefore mechanical properties of coil joints tend to deteriorate significantly. Therefore, when laser welding as in PTL 1 to 3 is applied as coil joining of electrical steel strips, there is a problem in that a fracture may occur at a coil joint, resulting in a drop in productivity due to line stoppage and the like on a production line such as a continuous cold rolling line. It would be helpful to solve the above problem and to provide an electrical steel strip welded joint that is able to inhibit the occurrence of coil joint fracture on a production line caused by deterioration of mechanical properties and shape of the coil joint. Further, it would be helpful to provide an electrical steel strip friction stir welding method to obtain the electrical steel strip welded joint. Further, it would be helpful to provide a method of producing an electrical steel strip using the electrical steel strip friction stir welding method. (Solution to Problem) The inventors conducted intensive studies to solve the technical problem outlined above. First, the inventors investigated and examined the reasons for the deterioration of mechanical properties and shape of coil joints when laser welding is applied as coil joining of electrical steel strips, and made