Studies On Welding Properties Of 1000MPa Copper-rich Nanoprecipitate-strengthened Steel

Copper-rich nanoprecipitate strengthened steels,process high strength while maintaining the excellent plasticity and toughness at low temperatures.More important,the lower carbon content and carbon equivalent,lead to a excellent welding performance,providing a broad application prospect.In this paper,theoretical and experimental research are carried out on1000 MPa grade copper-rich nanoprecipitate strengthened steel.The welding performance of the steel and the actual welded joint structure and properties are systematically evaluated,which provides a reliable reference for the research and application of this kind of steel.Firstly,the SH-CCT diagrams of the steels were drawn for the first time by using thermal simulation technology.The influence of cooling rate on phase transition was investigated,which provides an important reference for the selection of welding thermal cycle parameters.The results show that when the cooling rate is between 0.3 and 5℃/s,the microstructure of the tested steel is lath martensite.When the cooling rate is over 20℃/s,the lath martensite is dominant in the test steel.When the cooling rate is between the two,the HAZ is the microstructure of lath martensite mixed with lath bainite.Then,welding thermal simulation experiments were carried out with different welding heat inputs and peak temperatures.The results show that with the increase in heat input,the original austenite grain boundary migrates,the microstructure of coarse-grained HAZ gradually changes from strip to granular,and the hardness value tends to decrease.With the increase of peak temperatures,the hardness value decreases firstly and then increases,the impact energy increases slightly first and then decreases,and the fracture mechanism changes from dimple fracture to quasi-cleavage fracture.In addition,the influence of the secondary heat cycle on the microstructure and properties of coarse-grained HAZ was studied by the secondary heat cycle experiments,and four different reheat coarse-grained regions were obtained.The results show that when the peak temperatures of the secondary thermal cycle are 900℃ and 1050℃,the grains in the simulated supercritical reheated coarse-grained HAZ are refined,and the structure is dominated by lath martensite and lath bainite,and the island M-A component is distributed in the lath.The low temperature toughness of the simulated supercritical reheated coarse-grained HAZ is significantly improved compared with that of coarse-grained HAZ.Finally,the theoretical weldability of the steel was calculated and analyzed,and the sensitivity of cold crack was evaluated by the maximum hardness test in the weld heat affected zone and the oblique Y-groove welding crack tests.The welding performance of real joints was evaluated by gas shielded welding of test steel with different welding heat input process parameters.The results show that the steel is slightly hardened at room temperature,and the crack rate is obviously reduced by increasing the preheating temperature.The change trend of microstructure and properties of heat affected zone obtained from thermal simulation test can effectively reflect the change characteristics of real welded joints.In order to obtain good welded joints,the welding heat input should not be too large,and the optimal welding line energy is about 14 k J/cm.