Solidification Structure And Distribution Of TiN In Ship Plate Steel With Electromagnetic Stirring

With the development of the technology of shipbuilding and marine development in China,the ship plate steel is developing in the direction of high strength,high toughness and large wire energy welding.Along with the increase of the welding heat input,the toughness of the heat affected zone is decreased.Therefore,the welding heat affected zone became the weak area of the ship and bridge.The fine TiN of dispersive distribution can provide more nucleation center for the formation of the needle ferrite,which is beneficial to depress the grain coarsening of grain size in the hot zone and to decrease the effect of the weld heat affected zone on the toughness.Therefore,it has important practical significance and practical value to study the solidification process and the distribution of TiN in ship plate steel with electromagnetic stirring.In this paper,the solidification structure and TiN distribution in ship plate steel with electromagnetic stirring were investigated.The stirrer of rotate magnetic field characteristics,ship plate steel ingot organization as well as the TiN features and regularity were studied and analyzed with Gauss meter,metallographic microscope,SEM,EDS and oxygen nitrogen analyzer and other testing equipment,respectively.The distribution characteristics of electromagnetic field in the rotating electromagnetic stirrer shows that:(1)At a certain current frequency,the stirring speed increases with the current density.While at a certain current density the stirring speed increases with the current frequency.(2)At a certain current frequency,the electromagnetic force is proportional to the square of the current intensity.The electromagnetic force and the current frequency are linear with the current frequency of less than 15Hz.(3)With the stirring frequency increasing,the penetration depth of the magnetic field decreases.The effect of different type of electromagnetic stirring on the central shrinkage,element segregation and ferrite grains of steel ingot in the solidification process of ship steel plate were investigated.The results show that:(1)The centerline pipe and segregation of C and N elements in steel ingot center can be reduced or eliminated with rotating electromagnetic stirring.(2)The coarse TiN particles are mostly located in the grain boundary of the original austenite,and the fine TiN particles are mostly located in the ferrite interior.(3)The ferrite grains are mainly including side plates along the grain boundaries of ferrite lath size and bulk iron large ferrite without electromagnetic stirring applied.With rotating electromagnetic stirring applied,the ferrite grains in the matrix evenly distributed,and mostly are the acicular ferrite element and crystal iron cube voxel based.The number,size and distribution of TiN were statistically analyzed with professional image analysis software.The results show that:(1)The 2D morphology of TiN show that square,triangle or trapezoid with sharp corners.Compound TiN nucleation centers mainly on alumina,titanium oxide and sulfide.TiN is easy to gather near the shrinkage cavity and the large inclusion when the electromagnetic stirring is not applied;while some TiN are distributed like cluster and aggregated in the center region of the ingot when the electromagnetic stirring is applied;meanwhile the aggregation phenomena of TiN is significantly improved due to the use of The intermittent alternating electromagnetic stirring.(2)The application of electromagnetic stirring can promote the removal of non-metallic inclusions,and make the distribution of non-metallic inclusions more even in the steel.(3)The number of TiN particles with more than 3 micron is significantly reduced by the rotating electromagnetic stirring,but the effect of rotating electromagnetic stirring on removal and distribution of TiN particles with less than 3 micron are not obviously.TiN significantly reduced the number of uniformly distributed in each position of the ingot with the intermittent alternating electromagnetic stirring.(4)With the increaseing of Ti content,the number of TiN in ingot is increased and the increase of the number of TiN in center region is more obviously than in the edge region.