The 'published literature on strength, structural and thermal aspects of hot working has been reviewed with particular reference to their application to the hot rolling of stainless steel. Mild and stainless steel slabs. have been hot rolled on a mill instrumentated to measure load, torque and speed. Internal temperatures have been measured during rolling using embedded thermocouples whose outputs have been recorded on a U.V. recorder coupled with an automatic back-off system. As-rolled stainless steel slabs recrystallised very ~ slowly relative to the rate found by previous workers for simpler austenitic stainless steels. Additionally, slabs exhibited retarded recrystallisation at the surface compared with the centre whereas theoretical analysis predicts the reverse. These phenomena have been explained by rapid precipitation at temperatures ~950oC and limited electron microscopical evidence has been obtained to support this. Precipitation also increased the flow stresses at lower rolling temperatures and masked any effects that different substructures were expected to have on the hot strength. A finite difference computer programme has been developed to predict the temperature changes that occur through a two dimensional transverse section of a slab during a hot rolling SChedule. The air cooling and roll contact heat transfer coefficients have been derived by comparison of measured and computed temperature-time curves. This programme enabled a mean pass temperature to be derived from measured temperatures which then resulted in good agreement between hot strengths derived from rolling and torsion tests on stainless steel. The programme has been simplified to a one dimensional model for simulating temperature changes over central regions of wide sections during industrial hot rolling. Data have been presented to enable mean temperatures to be derived from measured surface temperatures and to illustrate the effects of various parameters on the heat loss during a pass.

A doctoral thesis submitted in partial fulfillment of the requirements for the award of PhD of Loughborough University.

In this thesis, the influence of various dilute Nb additions (0.005–0.02 wt%) on austenite microstructure evolution along thermomechanical processing, in terms of austenite recrystallisation and grain growth, were investigated using steels with three carbon content levels. At the homogenisation temperature of 1250°C, all dilute Nb additions were dissolved in low carbon steels whereas the Nb dissolution limits for 0.4 wt% and 0.6 wt% C steels were 0.012 wt% and 0.08 wt%, respectively. Dilute Nb additions did not show significant influence on homogenised austenite grain size. The dramatic increase in C contents was more influential, which decreased the homogenised austenite grain size. The influence of dilute Nb additions and C contents on dynamic recrystallisation behaviour was studied by rough rolling at high deformation temperature with low strain rate. The dilute Nb additions were found to increase the critical strain of dynamic recrystallisation whereas C contents showed no influence on the dynamic recrystallisation behaviour. The austenite recrystallisation behaviour after finish rolling and after the holding period between finishing passes were studied by means of interrupted plane strain compression (PSC) tests and double hit PSC tests. It was found that with low Nb supersaturation, recrystallisation happened prior to Nb precipitation. Solute Nb in austenite delayed the onset of austenite recrystallisation through solute drag effect. With high Nb supersaturation, Nb precipitation occurred before the onset of recrystallisation which completely retarded austenite recrystallisation. The austenite grain growth during the holding period between finish rolling passes was studied by the evolution of prior-austenite grain size before and after the 20s holding period at the highest finish rolling temperature (1050°C). There was no Nb precipitation found and the difference in austenite grain growth behaviour was attributed to the solute drag effect from both Nb and C in solution. It was found that Nb in solution suppressed austenite grain growth. However, the effectiveness of solute Nb in suppressing austenite grain growth was affected by the C content.

No description available.

No description available.

Lors d’un recuit inter-critique d’un acier dit « Medium Manganèse », dont la teneur en Mn est située entre 4 et 12 %, avec une microstructure initiale complètement martensitique, la formation de l’austénite obéit à un mécanisme spécifique qui porte le nom d'ART - « Austenite Reverted Transformation » (transformation inverse de l’austénite). L’objectif de ce travail de thèse était d’étudier et de modéliser les évolutions microstructurales en lien avec les propriétés mécaniques lors d’un recuit ART. Il a été déterminé que la microstructure finale se compose de phases de nature (ferrite, austénite résiduelle et martensite de trempe) et morphologie (en forme d’aiguille et polygonale) différentes. Une attention particulière a été accordée aux cinétiques de dissolution des carbures et de formation de l’austénite. Une vision complète de ces processus a été construite. En outre, le mécanisme de stabilisation de l’austénite résiduelle à la température ambiante a été étudié et discuté. Enfin, des essais de traction ont été réalisés afin d’évaluer le comportement mécanique de l’acier après différents recuits ART et établir le lien avec la microstructure. Une analyse plus détaillée du comportement de chaque constituant de la microstructure a été effectuée. A l'issue de cette thèse, un modèle complet est disponible pour calculer les courbes de contrainte vraie - déformation vraie d’un acier Medium Mn / During the intercritical annealing of fully martensitic Medium Mn steel, containing from 4 to 12 wt.% Mn, the formation of austenite happens through the so-called “Austenite Reverted Transformation” (ART) mechanism. In this PhD work, the evolution of both microstructure and tensile properties was studied as a function of holding time in the intercritical domain. The microstructure evolution was studied using a double experimental and modeling approach. The final microstructure contained phases of different natures (ferrite (annealed martensite), retained austenite and fresh martensite) and of different morphologies (lath-like and polygonal). A particular attention was paid to the kinetics of austenite formation in connection with cementite dissolution and to the morphology of the phases. A mechanism was proposed to describe the formation of such microstructure. The critical factors controlling thermal austenite stability, including both chemical and size effects, were determined and discussed, based on the analysis of the retained austenite time-evolution. At last, tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Advanced analysis of the individual behavior of the three major constituents was performed. As a final output of this work, a complete model for predicting the true-stress versus true-strain curves of medium Mn steels was proposed