Effects Of Mg On The Microstructure And Mechanical Properties Of EH36 Shipbuilding Steel

Heavily influenced by global economic integration and market competition,global ocean transportation is now moving towards large-sized patterns with high strength and thickness.Plate steels used by large-sized ships demand better mechanical properties comparing with traditional generations.In the shipbuilding industry,welding is an important process and takes up 40%of working hours during shipbuilding.In order to improve the productivity and reduce production cost,high heat input welding have gained engineering applications due to enhanced construction efficiency.However,high heat input welding usually leads to significant coarsening of austenite grains and formation of brittle microstructures such as ferrite side plate and upper bainite,resulting in the reduced toughness of welded joint and heat affected zone(HAZ).Oxide metallurgy is a method for improving the strength and toughness of welded joint and heat affected zone.It controls the formation of fine,dispersed and high melting point non-metallic inclusions,making it as a second phase and nuclei for heterogeneous nucleation.During the cooling process of welding,via pinning the austenite grain boundaries to refine the austenite grain,fine inclusions can influence the phase transformation behavior of austenite and induce the nucleation of acicular ferrite in the grain.Since Mg has strong affinity with O in steel and is easy to form fine,dispersed and stable MgO inclusion particles,it is usually applied for oxide metallurgy technology.However,few studies reported the effect of Mg on the evolution behavior of inclusions and microstructure during the whole process of steelmaking.In this study,the effects of Mg on the microstructure and mechanical properties in the EH36 ship steel,both as slab and plate have been systematically studied.It is found a large amount of fine inclusions are generated in the steel after Mg addition,which refine the microstructure both of the slab and the plate of EH36 steel,leading to improve the strength and toughness of steel.And austenite phase transition temperature in both of the slab and the plate of EH36 steel is reduced by Mg addition through analysis of DSC.Welding thermal simulation experiments with different heat input for EH36 steel are carried out.It is found that when exercising low heat input,Mg addition inhibits austenite grain growth,and promotes nucleation of acicular ferrite to some extent,resulting in improve toughness of HAZ.However,with the increase of heat input,the effect of Mg on promoting the nucleation of acicular ferrite deteriorates.Possible reason is that the added Mg content is too low.It is still needed to examine the optimal Mg content for outstanding effects.