Flux core welding process for large-thickness ultrahigh-strength steel with yield strength of 690 MPa

The invention discloses a flux core welding process for large-thickness ultrahigh-strength steel with the yield strength of 690 MPa. The end face of the ultrahigh-strength steel is provided with a groove angle of 25-50 degrees, the end face of a dissimilar high-strength steel forge piece is a straight face, and a proper root gap and a proper truncated edge are adopted; the pre-welding preheating temperature, namely, the cold crack resistance sensitive temperature, of the ultrahigh-strength steel is determined, and after the preheating temperature of the ultrahigh-strength steel is determined, a welded plate is integrally preheated; the two pieces of ultrahigh-strength steel are in butt joint, consumable electrode mixed gas shielded welding is adopted, and forward and reverse welding is conducted through a flux core wire; and after welding is completed, an electric heating sheet is immediately used for carrying out 200DEG C heat preservation. According to the process, the large-thickness ultrahigh-strength ste