Simulation of Microstructural Evolution in Rod Rolling of a Medium C-Mn Steel

An ‘Expert System’ is proposed in this work to conduct computational exploration of the deformation and restoration behavior of a medium C-Mn steel under high strain rate conditions, at elevated temperatures and complex strain paths that occur in rod rolling process. The expert system computes appropriate thermomechanical parameters necessary for describing rod rolling process in detail and then utilizes these parameters in mathematical models to determine microstructure evolution during a typical industrial-scale rod rolling process. Microstructure simulation in rod rolling is a challenging problem due to the fact that several softening mechanisms may operate sequentially or concurrently during each deformation step. Different softening mechanisms have very different impact on microstructure development and therefore it is important to investigate the particular combinations of processing conditions under which transition of operating softening mechanisms occurs. In the present work, the transition from dynamic to metadynamic recrystallization is studied in detail based on the criteria of critical strain, austenite grain size and Zener-Hollomon parameter when the interpass (interdeformation) time is very short of the order of few milliseconds during the later stages of rod rolling. Computational results are subsequently validated by comparing the program output to in-plant measured microstructure data. The proposed expert system is designed as an off-line simulation tool to examine and assess the various options for thermomechanical process optimization. Introduction Optimization of the industrial rod rolling process presents a formidable challenge as this process is characterized by continuous multi-pass deformation (up to 30 deformation passes) at high strain rate in the range 0.4 – 3000s, at elevated temperatures in the range 1173 – 1373 K, and very short interpass times of the order 0.015 – 1.0s. These processing conditions make it virtually impossible to study experimentally the microstructural evolution during the intermediate stages of hot rolling. On the other hand, knowledge of the in-process microstructural evolution is important for both the optimization of the process schedule and to adjust the properties of the hot rolled product [1,2]. For example, a fine austenite grain size is desirable at the end of rod rolling to decreases its hardenability to obtain a fine ferrite + pearlite structure via controlled cooling, to eliminate or reduce the necessity of post-rolling annealing treatment, and to improve the mechanical properties of the as-rolled products. Previous efforts [3-5] to simulate microstructural evolution in wire rod rolling have concentrated mainly on calculating evolution of the mean austenite grain size in medium C-Mn steels. In the present work, the focus is on the fundamental aspects of microstructure development mechanisms such as static, dynamic and metadynamic recrystallization (SRX, DRX and MDRX respectively) and their kinetics, how to resolve the boundary conditions when they operate concurrently and finally their impact on microstructure evolution in continuous processing. Expert System Development The flow chart for the expert system is given in Figure 1 located at the end of the text. The initial (i.e. as-reheated) microstructure and expected rolling schedule are the basic inputs to the system. The program then calculates the deformation conditions such critical and peak strains, and Zener – Hollomon parameter based on initial grain size, pass strain, strain rate and temperature. The program subsequently computes the evolution of microstructure by computation of the recrystallized grain size, fraction recrystallized and recrystallization kinetics for a given rolling pass based on the mathematical models listed in Table 1. Table 1: Mathematical models describing kinetics of relevant softening mechanisms. Parameter DRX MDRX SRX Fraction Recrystallized (Fx) [6-8] ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − − k