Effect Of Heat Treatment Conditions On Microstructure And Mechanical Properties Of EH550 Shipbuilding Steel

With the booming development of maritime trade and the development of marine resources,Higher performance and outstanding weldability of steel for ships and offshore platforms are required.Therefore,due to its excellent mechanical properties,high-quality shipbuilding steel plays an irreplaceable role in the manufacture of large ships and marine engineering with extremely harsh service environment.Currently,lenient plate in ocean engineering is more widely used,but main problems of lenient plate at present are as follows:In the process of high heat input welding,the mechanical properties of the heat-affected zone decreases sharply due to a long time in the high temperature.The purpose of this project is to study the evolution behavior of inclusions in steel during heat treatment,resulting in optimizing the microstructure in heataffected zone of steel,and finally providing a reliable theoretical basis for the production of steel plates with high heat input welding.The aim of present study is to control the behavior of non-metallic inclusions to induce the nucleation of acicular ferrite under different heat treatment conditions,leading to optimum microstructure and mechanical properties of steel,This paper takes EH550 shipbuilding steel as the research object and in the study,EH550 shipbuilding steel is regarded as target steel and OM,SEM,EDS,EBSD,DSC,CSLM and other analytical methods are applied to study the influence of cooling methods(air cooling and water cooling),holding time(5 min,20 min and 60 min),and holding temperature(1100℃,1200℃ and 1300℃)on microstructure and mechanical properties of EH550 shipbuilding steel.Under different cooling methods on EH550 shipbuilding steel,the amount of BN and SMn-Ca inclusions sharply reduced after heat treatment,and the microstructure changed from ferrite side plate,polygonal ferrite and bainite before heat treatment to polygonal ferrite of air cooling and plenty of martensite of water cooling.In addition,the hardness value sort from high to low is water cooling>before heat treatment>air cooling.Comparing different holding time,with the increase in heat treatment time,the number density of inclusions reduced,mainly due to the S-Mn-Ca type inclusions decreased.At the same time,with the extension of heat treatment time,the austenitic grain size grew up obviously.In a larger austenite grain size of ferrite transformation temperature decreases,and not easily get bainite.But it is conducive to the formation of acicular ferrite around inclusions,and the inclusions in pinning effect is more obvious.The average hardness value of samples decreases with the extension of heat treatment time.Comparing different isothermal temperature,Al2O3 complex inclusions was at most in 1300℃,but less in 1100℃ and 1200℃.The phenomenon of acicular ferrite nucleation on Al2O3 complex inclusions was evident in 1100℃ and 1200℃,but not in 1300℃ because bainite volume increased significantly.In addition,isothermal temperature does not have a significant impact on the hardness.In summary,softening phenomenon appeared in all conditions except water cooling.Moreover,under the heat treatment temperature of 1100℃ and 1200℃,longer heat treatment time and faster cooling rate are conducive to the nucleation of acicular ferrite.