Papers by Author: Evgueni I. Poliak

Abstract: Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) at a prestrain of ε* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain ε* can exceed the peak strain if the DRX grain refinement ratio g = D0/DDRX > 4. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at ε*, the time for half softening t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at ε* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.

Abstract: Industrial thermomechanical processing of hot rolled steel strip products with rolling operations in the austenite + ferrite (γ + α) range offers, on one hand, unique possibilities for more precise control of microstructure and mechanical properties of as hot rolled products after accelerated cooling. On the other hand, there are significant technical problems related to the rolling stability induced by steel grade dependent non-monotonous variations in deformation resistance and its sensitivities to temperature and strain rate within the γ + α range. Based on laboratory results, the deformation behavior of low carbon (up to 0.2 %) steels alloyed with Mn, Si, Al and their combinations, as well as microalloyed with Nb at the intercritical temperatures is discussed.

Abstract: The problems associated with the use of conventional rolling mill models are described. These include the unavoidable variations in temperature and strain rate (rolling speed) during rolling. They are exacerbated by the wide variety of mill types and configurations found in industry and their correspondingly broad ranges of interpass time. Finally, a major limitation arises from the approach currently employed to model the “strain accumulation” attributable to incomplete softening between passes, particularly during the processing of microalloyed steels.

Abstract: The hot strip rolling of advanced microalloyed high strength steels still represents a new task to many mills due to the lack of data on the hot deformation resistance. With the aid of processing data from the Ispat-Inland hot strip mill, the “measured mean flow stresses” are calculated from the mill force using the Sims analysis and taking into account roll flattening, slip ratio and the redundant strain. A modification of the Misaka mean flow stress equation is proposed for C – Mn – Si – Al steels microalloyed with up to 0.02 % Nb. The effects of alloying and microalloying are then estimated. A new fitting parameter shows excellent agreement with the mean flow stress data from industrial processing of advanced high strength microalloyed steels. However, during the second half of the rolling schedule (lower temperature region), indications of austeniteto- ferrite transformation were found.

Abstract: It is shown that the kinetics of softening between mill passes can be modeled more simply when the normalized strain (reduction) per pass is employed rather than the conventional strain. This method requires a second important input, namely the strain hardening rate at the end of preloading. Using this approach, the number of input parameters and experiments required for their determination are drastically reduced. The use of the Law of Mixtures to describe the behaviors of the recrystallized and unrecrystallized volume fractions is then illustrated. Finally, the approach required for quantifying the precipitation kinetics (in microalloyed steels) is described.