Thermomechanical processing of steels

The combination of hot working technologies with a thermal path, under controlled conditions (i.e., thermomechanical processing) provides opportunities to achieve required mechanical properties at lower costs. The replacement of conventional rolling plus post-rolling heat treatments by integrated controlled forming and cooling strategies implies important reductions in energy consumption, increases in productivity and more compact facilities in the steel industry. The metallurgical challenges that this integration implies, though, are relevant and impressive developments that have been achieved over the last 40 years. The development of new steel grades and processing technologies devoted to thermomechanically-processed products is increasing and their implementation is being expended to higher value added products and applications

Production of a non-stoichiometric Nb-Ti HSLA steel by thermomechanical processing on a steckel mill

Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM (R) software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525-550 degrees C/550-600 degrees C/600-650 degrees C/650-700 degrees C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600-650 degrees C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values

Effect of deformation temperature on microstructure and mechanical behaviour of warm working vanadium microalloyed steels

Plane strain compression tests of two V microalloyed steels and one plain C-Mn steel have been done to analyse the influence of the deformation temperature, in the warm working range, on the final microstructure and subsequent mechanical behaviour. In the case of V microalloyed steels, the reheating temperature has an effect on the amount of vanadium in solution prior to deformation. This factor influences the austenite evolution during warm deformation and the transformation during cooling. As a consequence, in the microalloyed steels complex multiphase microstructures are obtained that lead to a wide range of strength-toughness combinations. In contrast, in the case of the plain C-Mn steel minor effects are observed in the deformation range from 800 to 870 °C. © 2011 Springer Science+Business Media, LLC.Peer Reviewe

Thermomechanical processing of steels

The combination of hot working technologies with a thermal path, under controlled conditions (i.e., thermomechanical processing) provides opportunities to achieve required mechanical properties at lower costs. The replacement of conventional rolling plus post-rolling heat treatments by integrated controlled forming and cooling strategies implies important reductions in energy consumption, increases in productivity and more compact facilities in the steel industry. The metallurgical challenges that this integration implies, though, are relevant and impressive developments that have been achieved over the last 40 years. The development of new steel grades and processing technologies devoted to thermomechanically-processed products is increasing and their implementation is being expended to higher value added products and applications

Evolución del estado macroscópico, microestructural y cristalográfico durante el conformado en caliente de aceros resulfurados.

Una calidad superficial alta de los aceros es fundamental en los procesos de obtención del acero durante la colada continua con el fin de evitar rechazos del material lo cual supone un aumento del coste de la producción. En este sentido, los aceros con maquinabilidad mejorada o de fácil mecanizado como los aceros resulfurados presentan algunas singularidades que pueden afectar a la calidad de los aceros y junto con el problema que presentan de su baja ductilidad en caliente, son susceptibles a la formación de grietas tanto en la superficie como en el interior. La posible aparición de las mismas resulta crítica durante las operaciones de enderezado y las primeras pasadas de laminación. La baja ductilidad en caliente de estos aceros es debido por un lado. a la alta fracción en volumen de inclusiones de sulfuro de manganeso que durante los procesos de deformación en caliente actúan como puntos de nucleación de cavidades por decohesión de la intercara matriz-inclusión, facilitando la propagación de grietas. Este problema se agudiza además con la inevitable presencia de segregaciones. Por otro lado, la presencia de compuestos, tales como el FeS, de bajo punto de fusión (<1000ºC) causan el “quemado del acero” con la consiguiente pérdida de ductilidad por la aparición de grietas intergranulares. En el contexto de la deformación del acero los procesos de recristalización dinámica existentes, que varían la naturaleza de la matriz, también influencian la deformabilidad de las inclusiones. En este sentido, es importante la resistencia relativa existente entre la matriz y la inclusión. Además la naturaleza cristalográfica de laspropias inclusiones junto a las relaciones de orientacion de las mismas con respecto a la matriz también determinan el grado de deformabilidad de las inclusiones. Este trabajo se ha centrado en los factores, tanto micro y macroestructurales así como cristalográficos, que pueden afectar al proceso de laminación y a la aparición de grietas superficiales. Para la determinación de estos factores se han tenido en cuenta todos los aspectos que forman parte en el proceso de colada continua, en el horno de recalentamiento y en las primeras pasadas de la laminación. Para ello, se han realizado ensayos de torsión, compresión en caliente monótonos, es decir, incluyendo diferentes modos de deformación y a diferentes temperaturas y velocidades de deformación, con el objetivo de simular algunas de las condiciones que se dan durante las primeras pasadas de la laminación (desbaste). La evolución del daño se ha estudiado mediante la realización de ensayos monótonos interrumpidos. Se ha realizado un estudio microestructural de la superficie del material bruto de colada y del mismo sometido a diferentes procesos de oxidación, simulando las condiciones que se dan en el horno de recalentamiento previo a la laminación. La presencia de marcas de oscilación, oxidación, grietas, alineación de inclusiones y otro tipo de defectos pueden facilitar la aparición o propagación de grietas ya existentes durante el desbaste. También se ha realizado un detallado estudio de la distribución y deformabilidad de las inclusiones de sulfuro de manganeso en diferentes zonas del material. La presencia de aglomeraciones y alineaciones de inclusiones afectan negativamente en la ductilidad puesto que favorece la formación y propagación de grietas. Por otro lado, factores como el tamaño, forma, fracción en volumen, composición química, plasticidad y naturaleza cristalográfica de las inclusiones influyen de forma intrínseca en su deformabilidad. La realización de ensayos mecánicos monótonos interrumpidos ha permitido el estudio de la influencia de las inclusiones y su deformación en los mecanismos de nucleación, crecimiento y coalescencia de cavidades

Interaction between microalloying additions and phase transformation during intercritical deformation in low carbon steels

Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends, and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, the specific mechanisms that act when different microalloying elements are added remain unclear. To investigate this further, laboratory thermomechanical simulations reproducing intercritical rolling conditions were performed in plain low carbon and NbV-microalloyed steels. Based on a previously developed procedure using electron backscattered diffraction (EBSD), the discretization between intercritically deformed ferrite and new ferrite grains formed after deformation was extended to microalloyed steels. The austenite conditioning before intercritical deformation in the Nb-bearing steel affects the balance of final precipitates by modifying the size distributions and origin of the Nb (C, N). This fact could modify the substructure in the intercritically deformed grains. A simple transformation model is proposed to predict average grain sizes under intercritical deformation conditions

Interaction between microalloying additions and phase transformation during intercritical deformation in low carbon steels

Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends, and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, the specific mechanisms that act when different microalloying elements are added remain unclear. To investigate this further, laboratory thermomechanical simulations reproducing intercritical rolling conditions were performed in plain low carbon and NbV-microalloyed steels. Based on a previously developed procedure using electron backscattered diffraction (EBSD), the discretization between intercritically deformed ferrite and new ferrite grains formed after deformation was extended to microalloyed steels. The austenite conditioning before intercritical deformation in the Nb-bearing steel affects the balance of final precipitates by modifying the size distributions and origin of the Nb (C, N). This fact could modify the substructure in the intercritically deformed grains. A simple transformation model is proposed to predict average grain sizes under intercritical deformation conditions