The dynamic recrystallization (DRX) behavior of Nb and Nb–Ti microalloyed steels has been investigated. It has been observed that the initial austenite grain size, the amount of microalloying elements in solid solution, and the deformation conditions (temperature and strain rate), affect dynamic recrystallization kinetics. To characterize the dynamic recrystallization behavior of the microalloyed austenite, continuous torsion tests were carried out after reheating the specimens at different temperatures between 1000 and 1420 °C, leading to a wide range of initial grain sizes, from 16 to 805 μm. It has been observed that decreasing the values of the Zener–Hollomon parameter and the initial grain size, promotes dynamic recrystallization. Microalloying elements in solid solution produce a retardation of dynamic recrystallization, resulting in higher values of the characteristic critical, e c , and peak, e p , strains. A corrective factor has been applied to quantify the retardation produced by the increase in the amount of Nb and Ti dissolved as the reheating temperature increases. In this manner, it has been possible to propose a unique relationship to predict the e p peak strain for both steels.

Dynamic recrystallization behavior covering a wide austenite grain size range in Nb and Nb–Ti microalloyed steels

The constitutive equations for the flow behavior of a commercial 0.34 pct C-1.5 pct Mn-0.7 pct Si-0.083 pct V-0.018 pct Ti microalloyed steel were determined. For this purpose, uniaxial hot compression tests were carried out over a wide range of strain rates (10−4 to 10 s−1) and temperatures (1123 to 1423 K). In combination with models developed in the literature, the experimental results permit the flow stress of the present steel to be predicted within ± 5 pct. It is shown that the classical constitutive equations must be modified to take the grain size into account, particularly when the latter is below 30 µm.

Modeling the Flow Behavior of a Medium Carbon Microalloyed Steel under Hot Working Conditions

Prediction of austenite grain growth during austenitization of low alloy steels

The classical kinetics models of dynamic recrystallization (DRX) in the form of Avrami function describe the development of DRX process to a large extent; however, because of the characteristics of exponent function, the conventional models cannot exactly exhibit the development speed of DRX process. Based on this analysis, a new kinetics model of DRX was proposed, which represents the ‘slow–rapid–slow’ property of DRX development. According to the new model, the development process of DRX can be divided into three phases: slow-beginning phase, rapid-increasing phase and slow-rising-to-balance phase. Because the turning point between the second phase and the third one corresponds to the inflexion from the faster velocity of DRX development to the slower one, the strain at this moment can be considered as the most appropriate and economic strain that guarantees fine grains and saves energy consumption. Take a typical metal characterized by DRX magnesium alloy AZ31B for instance, the Gleeble-1500 thermomechanical simulation compression tests were conducted together with microscopic examination, according to which the model parameters were determined. Statistics shows that the experimental results are in good agreement with the predicted values, which validates the accuracy of the new kinetics model.

A new kinetics model of dynamic recrystallization for magnesium alloy AZ31B

There are many empirical equations for predicting martensite start temperature (Ms) and the kinetics models of martensitic transformation of plain carbon and low alloy steels. The Ms temperature equations are only dependent upon the chemistry, while the martensite transformation kinetics models are based on the degree of undercooling below Ms temperature. However, the prior austenite grain size (AGS) is also expected to influence both Ms temperature and martensite transformation kinetics as it does in diffusive transformations. In this study, herefore, both Ms temperature and martensite transformation kinetics of a low alloy steel with different austenite grain sizes were investigated using a dilatometer. The new Ms equation and martensite transformation kinetics model including the AGS effect are proposed.

Effect of Austenite Grain Size on Martensitic Transformation of a Low Alloy Steel

A set of integrated mathematical models for simulating hot rolling and controlled cooling of wire rods and bars has been developed through extensive laboratory research work and validation against carefully monitored results from industrial mills.Experimental tests have been carried out on C-Mn and eutectoid steels selected as representative of the various applications of wire rods and bars.Static and dynamic recrystallization of austenite, fraction of transformed austenite, final microstructures and mechanical properties are all calculated by modelling physical phenomena and using quantitative relationships between the microstructural and kinetic parameters and the process variables, i.e. strain, strain rate, temperature and time.The models have been applied to predict the microstructure evolution during hot rolling and to investigate the effect of working conditions and recrystallization mechanisms on the formation of heterogeneous austenitic microstructures.The effects of the cooling pattern on the temperature profile and the austenite phase transformation have also been studied to prevent: coarse pearlite and martensite formation at the centre of wire rods which have cores enriched in C and Mn; surface hardening of bars when water tube cooling systems are used to control the temperature at the cooling beds.The models provide an important insight into the process that is beneficial to enhance the quality of long products.

Ettore Anelli

Papers

Application of Mathematical Modelling to Hot Rolling and Controlled Cooling of Wire Rods and Bars