Modern HSLA steels and role of non-recrystallisation temperature

Abstract

The use of heavy gauge steel sheets for structural applications often requires a combination of high yield strength and adequate toughness. The most cost effective way to achieve high yield strength and high ductility in low alloyed steels is through grain refinement. In industrial practice, such refinement is commonly obtained by thermomechanical controlled processing (TMCP). This approach comprises slab reheating to well defined temperatures, a large amount of hot deformation below the non-recrystallisation temperature T-nr and accelerated cooling. In practice, the T-nr is generally raised by the addition of microalloying elements such as Nb and Ti. As these elements contribute substantially to the alloying costs, optimisation of their use allows for a decrease in production cost. Better understanding of the T-nr assists in tuning the rolling process so that optimum mechanical properties can be produced. One area of importance is to recognise that the concept of the T-nr was originally developed for reversing mills and the production of plate steels. Methods of defining and determining it must be modified if it is to be applied to strip mills and their associated short interpass times. The main goal of this review is to provide a concise and complete overview of the current understanding of the fundamental mechanisms that control the T-nr and to address the different methods that can be used to determine it

From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

“…The shape of the recrystallised fraction versus time curves were similar for the microalloyed steels used, it being observed that some curves display a plateau caused by the formation of precipitates which momentarily inhibit the progress of recrystallisation [34][35][36][37]. The plateau is caused by strain-induced precipitation, as occurs in all microalloyed steels, and the start and end of the plateau are identified approximately with the start and end of strain-induced precipitation, respectively.…”

“…After the plateau, the superiority of driving forces for recrystallisation over pinning forces is about two orders of magnitude [6]. In Nb-microalloyed steels the Nb atoms/NbCN precipitates retard the recrystallisation of deformed austenite [7][8][9][10]. However, V-microalloyed steels also exhibit grain refinement through intragranular nucleation of ferrite on VN precipitates partly due to low lattice mismatch of VN with ferrite [11,12].…”

Development of Microstructure and Crystallographic Texture in a Double-Sided Friction Stir Welded Microalloyed Steel

“…This may involve both dynamic and static recrystallization and also strain induced precipitation of carbonitrides of niobium and vanadium and possibly titanium carbide, all which influence the mechanical and toughness properties. When they are fine (10-25nm), these precipitates provide dispersion hardening and when they are larger (20-50nm), depending on their volume fraction, grain refinement through grain boundary pinning [15][16][17]. Some or all of these processes might be expected may take place during the FSW of microalloyed steel, as has been discussed in a recent report [18].…”

Interaction between recrystallization and strain-induced precipitation in a high Nb- and N-bearing austenitic stainless steel: Influence of the interpass time

“…Independently of the employed processing, grain refinement and strain accumulation are the keys for achievement good properties of the rolled and forged materials [7]. It is well known that the reduction of grain size will introduce homogeneity in the deformation [8,9] and, in addition, the grain refinement increases the strength of an alloy, according to the Hall-Petch's relationship, leading to a better mechanical behavior after processing. In fact, reduction of grain size is the unique mechanism that increases both strength and toughness [10,11].…”