J. KLIBER, R. FABÍK, I. VITEZ, K. DROZD HOT FORMING RECRYSTALLIZATION KINETICS IN STEEL Received – Prispjelo: 2008-09-09 Accepted – Prihva}eno: 2009-01-25 Review Paper – Pregladni rad INTRODUCTION The main purpose of forming processes (in steels) is to produce fine-grained uniform microstructure result- ing from phase transformation. Rolling of steel sheets above the T nr temperature (the highest temperature, at which recrystallization can take place) without carbonitride precipitation is typical with long interpass delays, which provide time for complete static recrystallization (SRX). Rolling of sheets below T nr flattens austenite grains, as the strain-induced precipita- tion inhibits the SRX. Neither SRX nor precipitation take place during bar and wire rolling due to short interpass delays. Conversely, dynamic recrystallization (DRX) fol- lowed by metadynamic recrystallization (MDRX) takes place during sheet rolling due to strain accumulation. With regard to its interpass delays, strip rolling may be categorized somewhere between the above mentioned processes. In initial passes, where interpass delays are fairly long, the metallurgical response of the material re- sembles the sheet rolling process. Hot forming leads to both work hardening and restauration. The entire scope of this problem is too complex to be covered in a single paper. Therefore, this study focuses on a narrow area com- prising the actual restauration kinetics or, more pre- cisely recrystallization. An effort was made to summa- rize long-year research activity and offer an overview of equations describing the static recrystallization kinetics in terms of time required for restauration of a proportion of the microstructure [1-7]. KINETICS OF STATIC RECRYSTALLIZATION This process applies to materials characterized with , where T ³ 05 , T m is the melting temperature. From the viewpoint of physics, it follows the static recovery, which involves disclocation climb and cross-slip [6]. The process is assumed to take place in two stages: nu- cleation of new grains and their growth. Static restauration of microstructure can be divided into recovery and recrystallization. Basic models of sta- tic recrystallization kinetics are based on the Johnson- Mehl-Avrami-Kolmogorov equations describing the proportion of recrystallized structure X; and, using the mathematic probability theory, they result in the equa- tion [1-7] X A t t x n =- æ è ç ç ö ø ÷ ÷ é ë ê ê ù û ú ú 1 exp (1) As the curve described by this equation has no math- ematical expression for 0 and 100 % , the time is nor- mally specified as t 0 05 , SRX or t 0 95 , SRX or sometimes generally as t x. The general form of the equation is then written as follows [4,7]: t B D Z Q RT p q r s 05 , & exp SRX rs = × × × × × × æ è ç ç ö ø ÷ ÷ e e (2) where e represents strain, D is the grain size prior to deformation, Z is the Zener-Hollomon parameter, & e is strain rate listed in s -1 , Q rs is the activation energy, R is the gas constant and T is the absolute temperature [8-11]. METALURGIJA 49 (2010) 1, 67-71 67 The theory of kinetics of static recrystallization of steel during hot forming links the phenomenon to certain cri- tical strain, grain size, strain rate, activation energy and temperature. The basic description is provided by the Avrami equation. An overview of equations used was compiled and comments on selected parameters prepa- red. Key words: recrystallization kinetics, steel, hot forming Kinetika rekristalizacije tijekom vru}eg oblikovanja ~elika. Teorija kinetike stati~ke rekristalizacije ~eli- ka tijekom vru}eg oblikovanja ovu pojavu povezuje s kriti~nom deformacijom, veli~inom zrna, brzinom defor- macije, aktivacijskom energijom te s temperaturom. Osnovni opis pru`a Avrami jednad`ba. Rad pru`a pregled kori{tenih jednad`bi i raspravu pojedinih parametara. Klju~ne rije~i: kinetika rekristalizacje, ~elik, vru}e oblikovanje ISSN 0543-5846 METABK 49(1) 67-71 (2010) UDC-UDK 669.14:548.53 = 111 J. Kliber, R. Fabík, K. Drozd, Faculty of Metallurgy and Materials Engi- neering, V[B-TU Ostrava, Czech Republic I.Vitez, Faculty of Mechanical Engineering, University of Osijek, Slavonski Brod, Croatia