Abstract: The slab internal quality and the mechanical properties of linepipe plate with and without in-strand roller type electromagnetic stirring (S-EMS) were studied. The results showed that the centerline segregation and center porosity of slab macroscopic structure were improved with the proper use of S-EMS. The central equiax crystal ratio was increased from 12% to 26%. The impact properties ( -20℃) and the drop weight tear（-15℃）were improved remarkably. The detrimental segregation bandings, coarse Widmannstatten and granular Bainite structures were greatly reduced. The ratio for KV2≥350J of rolled plate samples got to 100%, the qualified rate for the shear fracture area of drop weight tear test was increased 2.65%, and got to 100%.

Key words: S-EMS, linepipe steel grade, slab quality, properties