Friction stir spot welded joint and production method therefor, and friction stir spot welding method

A friction stir spot welded joint comprising:

• two or more overlapping steel sheets;
• a welded portion of the steel sheets;
• and
• an annular groove on an upper surface of the welded portion,
• wherein the welded portion includes a recess, a first flow portion adjacent to the recess, a second flow portion adjacent to the first flow portion, and a welding interface that is a boundary between the first flow portion and the second flow portion,
• a deepest point of the recess is located at a distance satisfying at least one of 0.5 mm or more below the lower mating surface of the steel sheets and TL×0.50 or more below the lower mating surface of the steel sheets,
• a highest point of the welding interface is located at a distance of TU×0.50 or more above an upper mating surface of the steel sheets,
• in the welded portion, a microstructure of a region extending vertically from a distance of TU×0.20 to a distance of TU×0.50 above the upper mating surface of the steel sheets, and extending in a horizontal range from a first point located 400 μm inward from the welding interface to a second point located 100 μm outward from the welding interface, with a center position of the recess as a reference position, has a volume fraction of polygonal ferrite of 70% or more and 99% or less, a volume fraction of hard phase of 1% or more and 30% or less, wherein the hard phase is composed of one or more of martensite and cementite, and an average aspect ratio of the polygonal ferrite of 3.0 or less,
• an average hardness of the welded portion at a same vertical position as the upper mating surface of the steel sheets is 400 HV or less,
• the two or more overlapping steel sheets include an upper sheet and a lower sheet that are respectively a steel sheet located at a top and a steel sheet located at a bottom,
• TU is a thickness in mm of the upper sheet,
• TL is a thickness in mm of the lower sheet,
• the upper mating surface is a mating surface between the upper sheet and a steel sheet adjacent to the upper sheet, and
• the lower mating surface is a mating surface between the lower sheet and a steel sheet adjacent to the lower sheet.