CO2-shielded welding experiments of newly developed,780 MPa super-high strength heavy-duty truck crossbeam steel were conducted,and the microstructure,microhardness,mechanical properties,and impact toughness of the welded joint were studied.The evolution of the microstructure of the welded joint occurred as follows:welding seam(acicular ferrite+proeutectoid ferrite)→fusion zone(granular bainite-long strip M/A island)→coarse grain zone(granular bainite-long strip or short bar M/A island)→fine grain zone(ferrite+pearlite+blocky M/A island)→mixed grained zone(ferrite+granular bainite+blocky M/A island)→base metal(proeutectoid ferrite+granular bainite-blocky or granular M/A island).Increasing the density of the grain boundaries can effectively improve the impact toughness,and the blocky M/A island hindered crack propagation more effectively than the long strip M/A island.The new hot-rolled 780 MPa super-high strength steel had excellent weldability.The welding technology was applied under the following conditions:welding voltage was 20 to 21 V,welding current was 200 to 210 A,and the gas flow rate was 25 L/min.
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