A welding and rolling plant of metallic strips comprising: - at least two first reels sized to receive coils having a predetermined coil weight limit or a predetermined coil diameter limit; - a welding line arranged downstream of said at least two first reels, and provided with at least one first welder for making a rollable weld seam between each coil and the next coil coming from said at least two first reels; - a cold rolling mill, arranged downstream of said welding line and having at least two rolling stands arranged in tandem; - at least one second reel (11, 11'), arranged downstream of said at least two rolling stands, sized to wind rolled strip portions up to said predetermined coil weight limit or said predetermined coil diameter limit; - cutting means, arranged between said cold rolling mill and said at least one second reel (11, 11'), the cutting means being adapted to cut the rolled strip each time a portion of rolled strip wound on the at least one second reel (11, 11') reaches said predetermined coil weight limit or said predetermined coil diameter limit; - sensors apt to send a control signal to said cutting means each time a portion of rolled strip wound on the at least one second reel reaches said predetermined coil weight limit or said predetermined coil diameter limit; wherein the third reel and the fourth reel are sized to wind a coil weighing from 80 to 200 metric tons and/or with a diameter of up to 6 meters, defined mega coil, made by said welding line by joining a plurality of said coils coming from said at least two first reels, characterized in that there is provided a rotating platform, arranged between the welding line and the cold rolling mill, adapted to rotate about a vertical axis, and provided with a third reel and a fourth reel, positioned on said rotating platform so that alternatively the third reel is used as winding reel of a strip coming from the welding line and the fourth reel is used as strip unwinding reel for feeding the cold rolling mill, wherein the rotating platform is provided with blocking means (6, 6', 16, 16') to block the tail of the mega coil just wound on each of the third reel and fourth reel. - at least two first reels sized to receive coils having a predetermined coil weight limit or a predetermined coil diameter limit; - a welding line arranged downstream of said at least two first reels, and provided with at least one first welder for making a rollable weld seam between each coil and the next coil coming from said at least two first reels; - a cold rolling mill, arranged downstream of said welding line and having at least two rolling stands arranged in tandem; - at least one second reel (11, 11'), arranged downstream of said at least two rolling stands, sized to wind rolled strip portions up to said predetermined coil weight limit or said predetermined coil diameter limit; - cutting means, arranged between said cold rolling mill and said at least one second reel (11, 11'), the cutting means being adapted to cut the rolled strip each time a portion of rolled strip wound on the at least one second reel (11, 11') reaches said predetermined coil weight limit or said predetermined coil diameter limit; - sensors apt to send a control signal to said cutting means each time a portion of rolled strip wound on the at least one second reel reaches said predetermined coil weight limit or said predetermined coil diameter limit; wherein the third reel and the fourth reel are sized to wind a coil weighing from 80 to 200 metric tons and/or with a diameter of up to 6 meters, defined mega coil, made by said welding line by joining a plurality of said coils coming from said at least two first reels, characterized in that there is provided a rotating platform, arranged between the welding line and the cold rolling mill, adapted to rotate about a vertical axis, and provided with a third reel and a fourth reel, positioned on said rotating platform so that alternatively the third reel is used as winding reel of a strip coming from the welding line and the fourth reel is used as strip unwinding reel for feeding the cold rolling mill, wherein the rotating platform is provided with blocking means (6, 6', 16, 16') to block the tail of the mega coil just wound on each of the third reel and fourth reel.