Study On Mechanical Properties And Welding Properties Of New Easy-welded Hull Steel

After nearly half a century of development,high-strength hull structural steel in our country has basically met the needs of various domestic sources.However,there is still a long way to go in terms of high strength,lightweight and efficient welding.In this paper,a new type of easily weldable hull steel was designed and its mechanical properties were investigated.The effects of different heat treatment processes on the microstructure and mechanical properties of the test steel were investigated by experimental methods such as optical microscopy?OM?,scanning electron microscopy?SEM?,backscattered electron diffraction?EBSD?,and X-ray diffraction?XRD?.Through the welding thermal simulation test,the continuous cooling transition curve?SH-CCT curve?and phase transition kinetic curve of the test steel heat affected zone were drawn,the effects of different t_8/5?800⁵00°C cooling time?on the microstructure of test steel were analyzed,the phase transition kinetics of test steel under continuous cooling conditions was studied.The following main results were obtained:1.The test steel is tempered at different temperatures after water quenching or oil quenching.With the increase of tempering temperature,the yield strength and hardness are gradually reduced.The strength and hardness of the test steel after water quenching are higher than oil quenching;The toughness increases with the increase of tempering temperature;and its large angle grain boundary increases with the increase of tempering temperature.2.The microstructure of the steel tempered at different temperatures after water quenching or oil quenching is tempered sorbite.With the increase of tempering temperature,martensite lath bundles gradually merge and widen,and the direction distribution between lath bundles tends to be disordered,accompanied by the precipitation and aggregation of cementite and the formation of polygonal ferrite.3.Through the welding thermal simulation test,the SH-CCT curve of the test steel was drawn.The microstructure of the weld heat affected zone is martensite and bainite.When t_8/5?10 s,all are martensite structure;When 10 s<t_8/5<50 s,martensite and bainite structure exist simultaneously.When t_8/5?50 s,the microstructure is bainite;the hardness of the test steel decreases continuously with the increase of t_8/5,and the hardness is less than 350 HV1 when t_8/5>30 s.4.The phase transition kinetics curves are calculated by the thermal expansion curves under different cooling conditions.It is found that the phase transition temperature and the end temperature of the test steel gradually increases with the decrease of the cooling rate,the phase transition temperature range is basically unchanged,and the phase transition rate gradually decreases.