API grades toughness analysis for thermo-mechanical rolling in QSP Thin Slab Rolling plants

A methodology for estimating the potentiality of the thin slab two step thermo-mechanical rolling technologyfor the API 5L grades production is described in this paper. The requirements on Ductile to Brittle TransitionTemperature (DBTT) are considered the most important limiting factor for API 5L strip grades produced by ThinSlab technology because of the difficulties in achieving a refined and homogeneous microstructure. The resultsobtained by Danieli QSP layout, which are -60°C DBTT for X70M - 12.7 mm thick strips in OMK Vyksa mini-mill,are very promising. The limitation to thicknesses up to 12.7 mm is due to the product mix defined by the customerand not a limitation of the QSP layout. The relaxation in the requirement on DBTT is in fact potentially convertiblein higher strip thickness and the quantification of such a conversion is the first modeling of the proposedapproach. The limitations in the refinement achievable by thin slab rolling is considered in a second modelingwhich quantifies the effect of finishing rolling reductions on the Effective Grain Size of the final microstructure.The proposed methodology allows in particular defining the limiting thicknesses producible by a given QSP layoutfor each API 5L grade. It can be used as well for defining the slab size to be chosen for a given product mix