Microstructure and Mechanical Properties of Welded Joint of BS700MC Low Carbon Micro-alloy Steel for Automobile Beam

Microstructure and mechanical properties of welded joint of BS700MC low carbon micro-alloy steel for automobile beam were investigated by testing cold crack sensitivity, micro-hardness, bending performance, impact property, tensile property and microstructure observation. The results show that the BS700MC steel had good crack resistance, and didn′t need heat treatment before welding with WH80-G welding wires. The microstructure of the weld was acicular ferrite, very little bainite and proeutectoid ferrite. The welded joint had good comprehensive mechanical property. The hardness of weld was 380 HV, which was close to the base metal. But the hardness had obvious fluctuation at the bottom of welded joint, and softened phenomenon appeared in the HAZ. The welded joint had good impact toughness in the temperature range from -20 ℃ to 20 ℃. The tensile strength of BS700MC welded joint reached 815 MPa, which was 97.1% of the base metal, and fracture position located in the HAZ. Tensile fracture showed mixed type of dimples and cleavage steps.