The effect of solute drag on recovery and recrystallization during hot deformation of Nb microalloyed steels has been modeled using a newly developed microstructure model. The model is based on dislocation theory and the calculated dislocation density determines the driving force for recrystallization. Subgrains act as nuclei for recrystallization and have to reach a critical size and configuration in order for recrystallization to start. In the model, the solute drag effect of Nb in solution is described. Nb retards both dislocation and grain boundary movement giving retardation in both recovery and recrystallization. Calculations were compared to experimental results from axisymmetric compression tests combined with stress relaxation. In order to model the effect of solute drag, the experiments were carried out at temperatures where precipitation of Nb(C, N) should not occur. The calculated flow stresses for the compression tests show good fit with experimental data. Also, the calculated results of the relaxation tests show good agreement with experimental data.

The paper summarizes the creation of a robust online grain size gauge for a hot strip mill. A method and algorithm for calculating the grain size from the measured ultrasonic attenuation is presented. This new method is self-calibrating, does not rely on a geometrical reference sample and can cope with the effects of diffraction on the attenuation. The model is based on 52 quenched samples measured with more than 23,000 laser ultrasonics shots and has a correlation coefficient R2 of 0.8. Typical online laser ultrasonic measurements from the hot strip mill and the calculated grain size versus length are presented for a couple of steel strips.

Hot-rolled, high-strength low-alloy (HSLA) structural steel features excellent engineering properties, such as bending, cutting, and welding. Typical applications include a wide range of fabricated components and steel structures. HSLA steels are micro-alloyed with strong carbide, nitride, and/or carbonitride forming elements, such as Nb and Ti, and the precipitates contribute to grain refinement and strength. [1] Hot rolling of HSLA strip steels is typically done in a hot strip mill consisting of reheating furnace, roughing mill, finishing mill, cooling section, and downcoiler. During hot rolling, the process parameters are controlled to optimize the properties of the final product. [2] Precipitation of Nb(C, N) and (Ti, Nb)(C, N) can occur at high temperatures during rolling (strain-induced precipitation), where recrystallization and precipitation are competing processes, both dependent on the amount of strain. [3,4] Particles formed at high temperatures grow rapidly and usually do not contribute to a significant strength increase in the final product due to their large size. Nb and Ti remaining in solid solution after hot rolling can subsequently form carbides and/or carbonitrides during or after cooling. These nanosized precipitates that form during phase transformation (interphase precipitation) and after phase transformation give the main contribution to precipitation strengthening in the final product. More insight into the precipitation kinetics and the interaction with recrystallization and phase transformation will

In this study, we demonstrate the significance of austenite annealing twin boundaries when calibrating laser ultrasonic measurements for gauging austenite grain size in situ during the thermomechanical processing of high-strength low-alloy steels. Simple calculations show how differences in twinning density can lead to errors in grain size measurements if twins are disregarded during calibration and the method is used for a broad range of steels. Conversely, when calibration is performed using alloys with a metastable austenite microstructure at room temperature, the same calibration is suitable for a broad range of HSLA steels, provided that annealing twins are taken into account. Since light optical microscopy does not allow the characterization of annealing twins in low-alloy steel, the verification of the laser ultrasonic results was conducted using the novel approach of comparing the twinned grain sizes obtained using the ultrasonic method in low-alloy steels with the austenite grain maps reconstructed from martensite orientation maps measured using electron backscatter diffraction. Finally, we show how differences in twinning density occur even for alloys with a roughly similar stacking fault energy, further highlighting the importance of annealing twins in the calibration of laser ultrasonic measurements for industrial use.