Stephanie Vervynckt

Modern HSLA steels and role of non-recrystallisation temperature

Abstract The use of heavy gauge steel sheets for structural applications often requires a combination of high yield strength and adequate toughness. The most cost effective way to achieve high yield strength and high ductility in low alloyed steels is through grain refinement. In industrial practice, such refinement is commonly obtained by thermomechanical controlled processing (TMCP). This approach comprises slab reheating to well defined temperatures, a large amount of hot deformation below the non-recrystallisation temperature Tnr and accelerated cooling. In practice, the Tnr is generally raised by the addition of microalloying elements such as Nb and Ti. As these elements contribute substantially to the alloying costs, optimisation of their use allows for a decrease in production cost. Better understanding of the Tnr assists in tuning the rolling process so that optimum mechanical properties can be produced. One area of importance is to recognise that the concept of the Tnr was originally developed for reversing mills and the production of plate steels. Methods of defining and determining it must be modified if it is to be applied to strip mills and their associated short interpass times. The main goal of this review is to provide a concise and complete overview of the current understanding of the fundamental mechanisms that control the Tnr and to address the different methods that can be used to determine it.

Effect of niobium on the microstructure and mechanical properties of hot rolled microalloyed steels after recrystallization-controlled rolling

Microalloyed steels with increased strength and ductility are of considerable interest for use in the ‘as-hotrolled’ condition. However, there is a lack of information on their microstructural characteristics and mechanical properties. Seven different microalloyed steels with variable Nb and C content were evaluated in this work. First, characterization of the microstructure by optical and scanning and transmission electron microscopy was performed. Different microstructural constituents and grain size distributions were observed, and three different groups of precipitates were identified. For all steels, tensile tests were performed and ductile-to-brittle transition temperatures were determined. Finally, the complex interplay between microstructural features and mechanical properties was analyzed to determine structure-property relations for the steels under evaluation.

Control of the Austenite Recrystallization in Niobium Microalloyed Steels

The use of heavy gauge steel sheets for structural applications very often requires a combination of high yield strength and adequate toughness. The most cost effective way to realize a high yield strength and a high ductility in a low alloyed steel is grain refinement. In industrial practice, this refinement is realized by controlled processing. This process consists of controlling the slab reheating temperature, applying a large amount of hot deformation below the non-recrystallization temperature (Tnr) and accelerated cooling. A better knowledge of Tnr could optimize the process and the best mechanical properties could be reached against the lowest cost. Tnr can be raised by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures either by solute drag or by precipitation pinning. In this study, the recrystallization behavior of five Nb-microalloyed model alloys with various Nb contents, was evaluated by double hit compression tests. Further, the precipitation state of the materials was investigated experimentally by Inductively Couples Mass Spectroscopy and X-ray Diffraction. The construction of recrystallization-time-temperature diagrams and precipitation-time-temperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other.

Empirical Relationships for the Impact of Nb and C Content on the Mechanical Properties of Hot Rolled Microalloyed Steels

There currently is a strong interest in using ‘as-hot rolled’ steels instead of ‘heat treated’ steels within the field of microalloyed steels with superior strength and improved ductility. However, mechanical and microstructural characterization of these steels is less elaborated. The present work is an effort in that direction, focusing on the evaluation of the impact of a variable Nb and C content on the microstructure and mechanical properties for seven hot rolled microalloyed steels and deriving an empirical relation between the mechanical properties and the Nb/C ratio.

Model Alloys to Study the Solute Drag and Precipitation Effect on the Recrystallization Kinetics of Nb-Microalloyed Steels

A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures by either a solute drag or by a precipitation pinning (when bonded to C or N) effect. Both mechanisms result in improved mechanical properties due to grain size refinement of the transformed ferrite. In this study, 3 Nb-microalloyed model alloys were designed to investigate the solute drag and the precipitation effect separately. The first alloy, containing a stoechiometric ratio of Nb and C, was designed to study the retarding effect of NbC on the recrystallization behavior. A second alloy, containing Nb and only few ppm C, was casted in order to study the effect of Nb in solid solution. The two alloys were compared with a C-Mn reference alloy. The recrystallization behavior of the three alloys were compared by multi-hit torsion tests and double hit compression tests. The Nb-C and the Nb-very low C showed small differences in recrystallization behavior. These results show that Nb delays the recrystallization by a solute drag effect or by the formation of a very small amount of precipitates.