Roney Lino

Strain induced transformation (SIT) of austenite into ferrite has been frequently used as a powerful ferrite grain refinement mechanism. Ordinarily ferrite grain sizes of the order of 1-3μm are achieved via mechanical testing such as compression and torsion. Nonetheless, most of the work done so far employed continuous deformation in the range of 0.8 for compression experiments and in excess of this for torsion. SIT is a promising technique which may be used during actual hot rolling processing. However, in this case, not only deformations are applied with time interrupts between them but also the amount of total deformation allowable is relatively low, in order to attend to flatness and final gauges requirements. This work explores the consequences on SIT microstructure of deformation given in multiple passes as opposite to the usual continuous deformation presented in the literature. Multiple pass deformation at high temperature led to partial dynamic recrystallization and to a mi...

Z ė exp(Qdef/RT) A(sinh as) .................(1) Here, Qdef is the activation energy for hot deformation, A, a and n are constants, the other symbols have their usual meaning. This expression gives a straight line in a log Z versus log[sinh(as)] plot irrespective of the level of the stresses, whereas the power and exponential laws loose linearity at high and low stresses respectively. The peak stress resulting from the occurrence of dynamic recrystallization, DRX, and also the steady state stress associated with the completion of first wave DRX, have been successfully modeled by the above expression. Qdef can also be estimated for a series of C–Mn and microalloyed steels as a function of chemical composition, as published in the literature. Modeling stress strain curve requires, in addition, the use of an evolution equation such as forwarded by Sellars. The purpose of this paper is twofold; first, to simulate isothermal stress strain curves during hot deformation of double stabilize...

Wire rods are supplied for end users manufactures of products with increasing complexity and mechanical properties requirements. Demanding end users field applications must provide high yield stress, ductility and, in certain cases, toughness. These properties can be improved simultaneously if appropriate rolling technique and alloy design are used. It has been known for decades in the flat rolling industry that microalloyed steels can be produced achieving requirements just mentioned. The technology, however, is less often seen applied at the long products industry. In order to obtain metallurgical sound products, steelmakers need to run industry scale trials. These, however, are costly, time consuming and results are not always easy to analyze given the number of variables involved. Mathematical models are frequently used in order to reduce plant trials reducing costs. There models already published in the literature suitable for wire rod applications. They are however more concer...

Abstract Improving the quality of steel and the steelmaking process has been a matter of routine for metallurgical engineers and steelmaking companies in a demanding market for quality products at highly competitive price. The chemical and temperature adjustment are made during the secondary refining process, as well as the inclusion modification required to product quality, and also the demand for castability accuracy. Continuous casting process is the most used solidification casting process, in which the flow of pouring liquid metal through the submerged entry nozzle is assured by the correct temperature and the formation of liquid inclusion in the casting temperature. Thermocalc and CEQCSI were the software used in this work to assess the effect of carbon, silicon and sulphur in the castability window of the aluminium vs calcium phase diagrams. They have proved to be highly suitable and effective and the results showed that the chemical elements used directly affected the position of the castability window of carbon steel. An analysis of a 0.2%C billet sample using Scanning Electron Microscopy showed that there is a great heterogeneity of inclusions in aluminium-killed and calcium-treated steel.

Abstract Constitutive equations are employed in several algorithms to predict load during hot working. The peak stress ( σ p ) can be correlated to the temperature and strain rate at which the metal was deformed using the known expression: Z  =  A (sinh( ασ p )) n , where, Z is the Zener–Hollomon parameter, given by Z = e ˙ exp ( Q d e f / R T ) . The activation energy for hot deformation, Q def , magnifies the dependence of the stress on temperature in an exponential way requiring accurate measurement of this quantity. Empirical expressions published in the literature can be used to evaluate Q def . Unfortunately, these equations were derived for alloys with relatively high contents of C and N and when applied to the case of IF austenite, they yield unreasonable values. This is so because IF steels have, simultaneously, very low C and N contents and substantial amounts of microalloying elements leading to over prediction of values of Q def . This paper investigates, therefore, how the chemical composition of IF alloys can influence the values of the activation energy for hot deformation allowing suitable predictions to be made. Measurements of Q def were carried out for IF steels containing Ti and a combination of Ti-Nb additions. Predicted and measured values of Q def were compared to those calculated from published equations and a corrected expression was derived in order to better fit the experimental results here presented.

Industry hot deformation processes such as hot rolling are complex in nature. Setting up a rolling mill requires precise knowledge of the loads needed to shape the metal. This in turn, demands the ability to predict the strength of the material when deformed to a value of strain and strain rate at a given temperature. On and off-line models need, however, to be fed with constitutive equations relating the stresses required to deform a certain metal under the usual process variables. This paper shows how a set of stress-strain curves can be modeled so that both hardening and softening mechanisms commonly present during hot deformation are taken into account. The model predictions are compared to a set of literature data in order to be validated. Reasonable agreement between published results and predicted values were obtained indicating how efficiently the model can assess values of stresses under hot working conditions.

Dynamic recrystallization, DRX, has become an increasingly important softening mechanism both from fundamental and industrial points of view. During finishing rolling of strips or wire rods, strain is accumulated from pass to pass so that DRX can be triggered. The time need for 50% of material to recrystallize, t50DRX, is strongly dependent on temperature and to a lesser extent on strain rate at which deformation occurs. Few studies report results on the kinetics of DRX and how this softening mechanism can be predicted for a given set of hot deformation conditions, namely strain, strain rate and pass temperature. The purpose of this paper was to investigate how the chemical composition of IF austenite can affect the kinetics of DRX by measuring the apparent activation energy for DRX, QDRX, for alloys with additions of Ti and a combination of Ti-Nb contents. Predicted and measured values of t50DRX, were compared and an empirical expression was proposed to model measured values.