Řízené válcování vybraných značek oceli v podmínkách kontijemné válcovny Třineckých železáren, a.s.

Abstract

The main purpose of controlled forming is to achieve better material properties than in case of conventional rolling. The basic definition of controlled forming processes is process control in terms of forming temperatures, rolling speed determining the strain rate, cooling method determining the phase conversions and creation of final microstructure being the prime parameter for compliance with requirements for mechanical, or in other words end use, material properties. The continuous light section mill of Třinecké železárny, a.s. is designed to allow bar rolling using both conventional and some other controlled forming methods. Although the general conditions for controlled forming are known they usually cannot be simply applied to any rolling process and any rolled steel. A theoretical analysis of thermomechanical forming notions and principles as well as controlled forming methods focused on bar rolling was made. The round bar rolling process on the continuous light section mill is characterized by oval-circle calibration. Computer simulations of strain over the cross section of rolled material during rolling reflecting the continuous light section mill operating conditions for defined conditions were developed. Special attention was paid to the strain, strain rate and temperature waveforms. The results confirmed the occurrence of deformation parameter inhomogenity over cross section which is due to the applied production process. Series of continuous and discontinuous tests were carried out using SETARAM torsion plastometer in order to identify the deformation behaviour parameters of vanadium microalloyed medium-carbon steels. Mathematical dependence of strain intensity to peak and formula defining peak stress value on temperature and strain rate intensity was determined for the analyzed steel. Mathematical dependence for description of stress-deformation curve in relation to temperature, strain rate intensity and strain intensity was determined. Activation energy values of dynamic and static recrystallization were determined. Mathematical description of statically recrystallized volume fraciton of the analyzed steel on time was made. Experiments simulating the potential offered by selected production processes in the preparation for rolling on continuous light section mill in case of vanadium microalloyed medium-carbon steels were carried out on GLEEBLE plastometer. Hardening increases recorded on SETARAM and GLEEBLE plastometers during the test confirmed the tested steel grades can be processed under the controlled forming operating conditions and temperature range. Results of practical experiments made on selected steel grades under the continuous light section mill conditions based both on the findings from plastometric experiments and theoretical assumptions regarding the effect of lower deformation temperature on the resulting material properties are presented. It was proved the magnitude of change in the mechanical properties, phase ratio and grain size achieved depend on the chemical composition of formed steel.