The microstructure evolution of 55VNb microalloyed steel during warm deformation via single pass uniaxial compression was researched, and the effect of deformation conditions on dynamic spheroidisation of cementite lamellae and ferrite conditioning for a range of deformation temperatures (600 degrees C to 700 degrees C) and strain rates (1 to 10 s(-1)) analysed. Cementite lamellae appear to subdivide irrespective of deformation temperature with the ferrite phase penetrating the pattern formed by the cementite crystallites, in turn confirming that the dissolution of this phase during deformation is an important mechanism leading to the break-up of plates and subsequent globulisation. EBSD measurements allowed orientation gradients leading to the final subdivision of the cementite to be determined. Ferrite softening during heavy warm deformation is attributed to dynamic recovery and continuous dynamic recrystallisation, although the evolution of this phase depends, to a great extent, on the region subject to study, as confirmed by local EBSD studies. Misorientation profiles obtained in different regions of ferrite and pearlite enabled the different stages of the microstructural evolution to be monitored for each phase, this being developed via a variety of mechanisms under the same deformation conditions. Finally, the increase in low angle boundary density correlates with the Zenner-Hollomon parameter, and a linear relation between the density of low angle boundaries and steady-state stress estimated for pearlite and ferrite was found, indicating that new boundaries would have been formed dynamically during deformation. High angle boundary density also increases with deformation, although this is almost irrespective of the temperature and strain rate applied.

The paper presents the design and implementation of a computer system dedicated to the optimization of a hot strip rolling process. The software system proposed here involves the flexible integration of virtual models of various devices used in the process: furnace, descalers, rolling stands, accelerated cooling systems, and coiler. The user can configure an arbitrary sequence of operations and perform simulations for this sequence. The main idea of the system and its implementation details are described in the paper. Besides the computer science part, the material models describing the rolling parameters, microstructure evolution, phase transformations, and product properties are also presented. Effect of precipitation was accounted for various stages of the rolling cycle. Experimental tests were performed to generate data for identification of the models. These include plastometric tests, two-step compression tests, and dilatometric tests. Following this, physical simulations of rolling cycles were performed on Gleeble 3800 to supply data for the verification and validation of the models. Finally, case studies of modern industrial processes were performed, and the selected results are presented.

The paper presents the design and implementation of a computer system dedicated to the optimization of a hot strip rolling process. The software system proposed here involves the flexible integration of virtual models of various devices used in the process: furnace, descalers, rolling stands, accelerated cooling systems, and coiler. The user can configure an arbitrary sequence of operations and perform simulations for this sequence. The main idea of the system and its implementation details are described in the paper. Besides the computer science part, the material models describing the rolling parameters, microstructure evolution, phase transformations, and product properties are also presented. Effect of precipitation was accounted for various stages of the rolling cycle. Experimental tests were performed to generate data for identification of the models. These include plastometric tests, two-step compression tests, and dilatometric tests. Following this, physical simulations of rolling cycles were performed on Gleeble 3800 to supply data for the verification and validation of the models. Finally, case studies of modern industrial processes were performed, and the selected results are presented.

Carbide-free bainitic (CFB) steels belong to the family of advanced high strength steels (AHSS) that are struggling to become part of the third-generation steels to be marketed for the automotive industry. The combined effects of the bainitic matrix and the retained austenite confers a significant strength with a remarkable ductility to these steels. However, CFB steels usually show much more complex microstructures that also contain MA (Martensite-Austenite) phase and auto-tempered martensite (ATM). These phases may compromise the ductility of CFB steels. The present work analyzes the substructure evolution during tensile tests in the necking zone, and deepens into the void and crack formation mechanisms and their relationship with the local microstructure. The combination of FEG-SEM imaging, EBSD, and X-ray diffraction has been necessary to characterize the substructure development and damage initiation. The bainite matrix has shown great ductility through the generation of high angle grain boundaries and/or large orientation gradients around voids, which are usually found close to the bainite and MA/auto-tempered martensite interfaces or fragmenting the MA phase. Special attention has been paid to the stability of the retained austenite (RA) during the test, which may eventually be transformed into martensite (Transformation Induced Plasticity, or TRIP effect).

Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name "back annealing" or "recovery annealing". In the present work, various annealing treatments were applied to a Nbmicroalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350-450 degrees C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550 degrees C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.

The paper describes the material database, which was developed and included in the VirtRoll computer system dedicated to the design of optimal hot strip rolling technologies. The structure and functionalities of the database are described in the first part of the paper. The integration between the database and the system through the Scalarm platform is described next. Following chapters are dedicated to generation of material data, which are included in the database. These data are coefficients in material models, which include flow stress models, microstructure evolution models, phase transformation models and mechanical properties models. Several models of various complexity and various predictive capabilities were chosen for each mentioned phenomenon. All are mean field models to allow fast simulation of the whole manufacturing chain. Modern steel grades were selected as the case studies. Experimental tests performed to generate the data composed plastometric tests, stress relaxation tests and dilatometric tests. Inverse analysis was applied to determine the coefficients in the model. Discussion of results focused on validation and on new aspects of models recapitulates the paper.

Very often Nb contributes to the strength of a microalloyed steel beyond the expected level due to the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in the solution after hot rolling. The first of them is the increase of the hardenability of the steel due to Nb, and the second one is the fine precipitation of NbC in ferrite. The contribution of the precipitates to the work hardening of two thermally and thermomechanically processed microalloyed steels is addressed in this work and this contribution has been integrated into previously developed models by the authors for ferrite-pearlite microstructures. An L (eff) is considered through the effective spacing associated to the different obstacles and their interactions with the moving dislocations. The model obtained shows good agreement with the experimental tensile curves from the end of yield point elongation to the onset of necking. (C) The Minerals, Metals & Materials Society and ASM International 2017

Boron is added to steels to increase hardenability, substituting of more expensive elements. Moreover, B acts as a recrystallization delaying element when it is in solid solution. However, B can interact with N and/or C to form nitrides and carbides at high temperatures, limiting its effect on both phase transformation and recrystallization. On the other hand, other elements like Nb and Ti are added due to the retarding effect that they exert on the austenite softening processes, which results in pancaked austenite grains and refined room microstructures. In B steels, Nb and Ti are also used to prevent B precipitation. However, the complex interaction between these elements and its effect on the austenite microstructure evolution during hot working has not been investigated in detail. The present work is focused on the effect the B exerts on recrystallization when added to microalloyed steels. Although B on its own leads to retarded static recrystallization kinetics, when Nb is added a large delay in the static recrystallization times is observed in the 1273 K to 1373 K (1000 A degrees C to 1100 A degrees C) temperature range. The effect is larger than that predicted by a model developed for Nb-microalloyed steels, which is attributed to a synergistic effect of both elements. However, this effect is not so prominent for Nb-Ti-B steels. The complex effect of the composition on recrystallization kinetics is explained as a competition between the solute drag and precipitation pinning phenomena. The effect of the microalloying elements is quantified, and a new model for the predictions of recrystallization kinetics that accounts for the B and Nb+B synergetic effects is proposed.

Final Report RFSR-CT-2013-00007