A double-sided friction stir welding method; a cold-rolled steel strip and plated steel strip manufacturing method; a double-sided friction stir welding device; and cold-rolled steel strip and plated steel strip manufacturing equipment are provided. In this double-sided friction stir welding method, while rotating in mutually opposite directions a pair of rotation tools arranged on one side and the other side of an abutting portion or overlapping portion of steel strips, said pair of rotation tools are pressed against the abutting portion or overlapping portion of the steel strips and moved in a joining direction, and, while softening the still-unjoined portion of the steel strips by friction heat between the rotation tools and the unjoined portion of the steel strips, plastic flow is generated by stirring the softened area by means of the aforementioned rotation tools, thereby bonding steel strips. The pair of rotation tools has a front end formed in a round and flat shape, and the front end is of a material harder than that of the steel strips.
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