Study On Microstructure And Mechanical Properties Of 600MPa Cu-rich Nanophase Strengthened Steel Welded Joints

As a new type of low-alloy high-strength steel,copper-rich nanophase strengthened steel has broad application prospects in the fields of shipbuilding,marine engineering and automobiles due to its high strength,good plasticity,toughness and weldability.The matrix is strengthened by introducing nano-scale copper-rich precipitation for precipitation strengthening and fine grain strengthening.In this paper,the weldability of 600 MPa copperrich nanophase strengthened steel with tempered martensite and bainite,and the microstructure and mechanical properties of welded joints were studied through a combination of theoretical analysis,welding thermal simulation experiments and real welding experiments.The matching welding wire was designed and developed preliminarily.In this paper,the continuous cooling transition behavior of 600 MPa copper-rich nanophase strengthened steel was studied by quenching thermal expander,and the SH-CCT curve was drawn according to the evolution of microstructure,original austenite grain size and microhardness at different cooling rates.It provided the reference and basis for the selection of thermal cycle parameters for subsequent welding thermal simulation experiments.With the decrease of cooling rate,the microstructure of the test steel gradually changed from lath-like martensite and lath-like bainite to granular bainite,and the original austenite grain became coarser.The welding thermal simulation experiments of different heat input were carried out by Gleeble-3800 thermal simulation test machine.With the continuous increase of heat input,the grain size of the simulated coarse grain HAZ of the test steel increased constantly,and the microstructure changed from LM/LB to GB,the bulk M-A component was distributed in the GB matrix.The dislocation density and grain boundary density decreased,and the impact energy decreased at the same time.The welding thermal simulation experiments of different peak temperature were carried out to simulate different areas of the heat affected zone.Affected by welding thermal cycle,the copper-rich nanophase in the coarse grain HAZ,fine grain HAZ and intercritical HAZ redissolved in the matrix.The hardness of the fine grain HAZ and intercritical HAZ reduced significantly,but the tensile strength of the HAZ has hardly changed.The coarse grain HAZ is the weakest part in the heat affected zone,due to the coarsest grain and the worst impact properties.In this paper,the weldability of 600 MPa copper-rich nanophase strengthened steel was theoretically analyzed through the calculation of carbon equivalent and crack sensitivity,and the cold crack sensitivity experiments of the test steel and a tyle of common marine steel10 CrNi3MoV were carried out by the Tekken test.The results showed that copper-rich nanophase strengthened steel had good cold crack resistance,which was significantly better than 10 CrNi3MoV.Then,the welding experiments of welded joints with different heat input were carried out,and the differences in microstructure and mechanical properties of welded joints were studied.When the heat input was 15 k J/cm,the comprehensive mechanical properties were the best.The matching welding wire of 600 MPa copper-rich nanophase strengthened steel was designed,and the annealing process of wire was explored.The welding wire was subjected to a deposition metal welding test and a welded joint welding test,and the microstructure and mechanical properties were tested.The results showed that the welding wire had high strength,good toughness and good matching with the base metal.