Physical simulation of finish rolling of microalloyed steels in isothermal conditions

Abstract

The aim of this work was to establish a temperature of finish rolling stage of Nb/Ti microalloyed steel containing 0.06 wt.% C, 0.77 wt.% Mn, 0.039 wt.% Nb and 0.015 wt.% Ti, using physical simulation. Samples were subjected to laboratory simulation at a twist plastometer at high temperatures, i.e. between 825 and 950 °C. Five pass deformation and interpass times were selected in accordance with a processing parameters at five stand finishing hot strip mill. Restoration (recovery and/or recrystallization) behavior was evaluated by calculation of Fraction Softening (FS) and Area Softening Parameter (ASP) values. At 950 °C all individual pass stress-strain curves, FS and ASP show full recrystallization in all interpass intervals. On the other hand, with a decrease in temperature to the interval of 875-825 °C, the extent of restoration is decreasing, leading to recovery as a sole softening mechanism at 825 °C, which was confirmed by the stress-strain curve shape, and values of FS and ASP. It is assumed that, due to high supersaturation, strain-induced precipitation promoted pinning of grain and subgrain boundaries and suppressed recrystallization. Therefore, the critical temperature for finish rolling was estimated to be 825 °C.

Cilj ovog rada je određivanje temperature završnog valjanja Nb/Ti mikrolegiranog čelika koji sadrži 0,06 % C, 0,77 % Mn, 0,039 % Nb i 0,015 % Ti. Uzorci su ispitivani laboratorijskom simulacijom, na plastomeru - uredjaju za ispitivanje uvijanjem, na temperaturama, između 825 i 950 °C. Režim deformacije u pet provlaka i pauza između provlaka odabrani su u saglasnosti sa parametraima valjanja na petostanskoj završnoj valjačkoj pruzi valjaonice toplovaljanih traka. Udeo obnovljene mikrostrukture (oporavljanja i/ili rekristalizacije) je određivan na osnovu proračuna prekidnog omekšavanja (engl. fraction softening, FS) i parametra omekšavanja na osnovu promene površine ispod krive (engl. area softening parameter, ASP). Na 950 °C krive deformacionog ojačavanja za svaki provlak, FS i ASP pokazuju ponašanje koje odgovara potpuno rekristalisanom uzorku. Udeo obnovljene mikrostrukture se smanjuje sa sniženjem temperature do intervala 875-850°C. Pretpostavlja se da zbog velikog presićenja dolazi do pojave taloženja izazvanog deformacijom, što utiče na smanjenje pokretljivosti granica zrna/subzrna što dovodi do potiskivanja rekristalizacije. Na 825°C obnavljanje se odvija isključivo mehanizmom oporavljanja pa se ova temperatura može uzeti za kritičnu temperaturu završnog valjanja.