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The dramatic increase in use of ferritic stainless steel(FSS)products has been primarily driven by the economic benefits over traditional austenitic stainless steels(ASS)but also the now well-established corrosion resistance.FSS grades however possess a relatively high ductile to brittle transition temperature(DBTT)especially when the plate thickness increases beyond 5 mm.Further economic benefits therefore depend on enhancing the base level toughness of this classic body centered cubic grade.Microalloying is the main factor that determines the final mechanical properties in most steel grades.For FSS grades,it is essential that the detrimental effects of interstitials(C,N and O)be controlled by the addition of stabilising elements,such as Nb,Ti,V.It is the Cr and Mo balance in 18Cr-2Mo FSS,together with stabilising additions,that provide the enhanced corrosion resistance.The benefits of Nb in terms of effective stabilization and control of grains size,essential for both strength/toughness,avoids the detrimental effect of Ti on surface quality and the limitation of V in terms of effective stabilization and so offers a unique solution to increasing the product range of FSS plate.Based on commercial Thermo-Calc software,a range of potential alloy designs was produced to evaluate the mechanical properties and corrosion resistance for a new thick plate FSS grade.Four alloy designs based on a Nb an Nb+Ti stabilised 18Cr-2Mo design were evaluated in terms of microstructure,viz grain size and precipitate species,tensile strength and Charpy impact toughness.Optical microscopy(OM),scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),and also transmission electron microscopy(TEM)was utilized to characterize and correlate the performance of each alloy design.It was demonstrated that Nb provided both effective stabilization and grain refinement,through the formation of a fine dispersion of Nb rich carbonitrides,to increase the DBTT.It was also revealed that the grain misorientation distribution and grain boundary character distribution(GBCD)are not the main factors related to ultimate mechanical properties and will be the subject of this complete paper submission.