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 T-nr and accelerated cooling. In practice, the T-nr 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 T-nr 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 T-nr 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 T-nr and to address the different methods that can be used to determine it

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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 lower temperatures by solute drag or by precipitation pinning. In this study, the recrystallization behavior of four Nb-microalloyed model alloys which were designed to show either extensive or almost no precipitation, was compared by multi-hit torsion tests and double hit compression tests. A good consistency between the different types of tests was found and the results were verified by optical micrographs. Further, by construction of softeningtime-temperature diagrams the recrystallization behavior was linked to the precipitation state of the material which was investigated by thermodynamical equilibrium calculations and by experimental observations from TEM-EDX, Inductively Coupled Plasma Mass Spectroscopy and X-ray Diffraction. Quantitative agreement between the experimental measurements and the calculations for precipitated mass fraction and precipitate composition as a function of temperature and steel composition is demonstrated.KEY WORDS: recrystallization; microalloyed steels; precipitation; solute drag. 911© 2009 ISIJ tions with experimental data from Craven et al. 13)The objective of the present paper is to separate both retarding mechanisms, i.e. solute drag and precipitation pinning, by investigating model alloys designed to show either extensive or almost no precipitation. Moreover, the recrystallization kinetics of four Nb-microalloyed steels during hot deformation is linked to the morphology and composition of the precipitates and the amount of Nb-solutes found in these materials. The recrystallization kinetics were investigated combining different hot deformation testing techniques while information on the precipitation state of the material was obtained from thermodynamic equilibrium calculations and from a combination of experimental observation techniques. With this work, a contribution to the understanding of the fundamental mechanisms responsible for the retardation of austenite recrystallization in Nb-microalloyed steels is achieved. Experimental ProcedureFour model alloys were designed and casted as 100 kg ingots in a Pfeiffer vacuum furnace operated under argon gas atmosphere. The chemical composition of these alloys can be found in Table 1. The C-Mn-reference alloy, without additional Nb, represents a reference steel. The second alloy, a lowC-Mn-Nb alloy containing only a few ppm C, allowed to study the effect of Nb in solid solution. The third alloy (C-Mn-Nb) was designed to study the effect of NbC-precipitates on the recrystallization kinetics. To have the highest fraction of NbC precipitates possible, a stoichiometric Nb/C-ratio of 8/1 was chosen. Finally, the fourth alloy (C-Mn-Nb-N) was designed to study the influence of N on the recrystallization and precipitation behavior. The cast blocks were thermomechanically processed under conditions comparable to those during industrial steel plate rolli...

A high amount of deformation below the non‐recrystallization temperature (Tnr) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of Tnr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental set‐ups and test schedules used to obtain this data. Although it would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques one to another. In this study, different hot rolling simulation techniques and testing schedules were compared, within the experimental constraints of the used equipment, to determine the Tnr temperature of two microalloyed steels. Good agreement was found between the results from different test equipment. Furthermore, the results from the multideformation tests under continuous cooling conditions could be correlated with the results from isothermal double deformation tests.

A high amount of deformation below the non-recrystallization temperature (T nr ) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of T nr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental setups, types of analysis and test schedules that are used to obtain this data. Although this would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques and methods. In this study, different hot rolling simulation techniques, testing schedules and types of analysis were used to determine the recrystallization kinetics of a microalloyed steel. On the one hand, good agreement was found between the results from different test equipment for the double deformations tests. On the other hand, stress relaxation tests showed accelerated kinetics and appeared to be less effective.

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

Abstract. 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 nonrecrystallization temperature (T nr ) and accelerated cooling. A better knowledge of T nr could optimize the process and the best mechanical properties could be reached against the lowest cost. T nr 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-timetemperature diagrams showed that both mechanisms, i.e. recrystallization and precipitation, strongly influence each other.

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