<p> The automotive industry is using advanced high strength steels (AHSS) to improve the fuel efficiency of passenger vehicles by lightweighting strategy. The higher strength of AHSS allows vehicle manufacturers to implement thinner and lighter components while still meet the safety requirements. Press hardened steels (PHS) exhibit the highest tensile strength among AHSS and are widely used for manufacturing crash relevant automotive parts. Boron-containing steel with enhanced hardenability is the most commonly used grade of steel for press hardening. The addition of a small amount of boron, 0.002 – 0.005 wt.%, can effectively increase hardenability. However, the boron addition also causes problems in commercially production of steel slabs by continuous casting. Defects including transverse corner cracks, surface cracks, and internal halfway cracks are sometimes found in continuously-cast boron steel slabs during or after the final cooling process. These problems can arise during the post-casting cooling process because boron addition changes the phase transformation behavior of steel.</p> <p>The cooling of slabs during and after continuous casting is a multiphysics process including coupled heat transfer, solidification, solid-solid phase transformations, and deformation. Numerical models are helpful for a better understanding of the cooling process and the interaction of different physical phenomena in it. In this work, a 3-D thermomechanical finite volume model (FVM) with coupled heat transfer, stress, and phase transformation calculations is developed to investigate the temperature history, phase evolution, and stress development during cooling.</p> <p>The model is used to simulate the cooling process of continuous cast steel slabs at different post-casting stages. The effect of boron addition on stress development and phase evolution during cooling of a single slab is investigated via simulation of both boron-containing and non-boron steels. The results show the slab with boron consists of mostly bainite, in contrast to the non-boron grade which is mostly ferrite and pearlite after cooling. Higher tensile stresses, both peak and residual, and plastic strains, which could lead to cracking, are observed at the edge of slab in the boron-containing grade. The effect of slowing the cooling rate by using a radiation shield is studied for the boron-containing steel. The reduced thermal gradient and the increased ferrite formation reduce the stresses in the slab. The cooling process of a stack of multiple slabs is also simulated to study the influence of slabs stacking on cooling rate and slab deformation. A slower cooling rate can be achieved in stacked slabs and the compressive load provided by slabs above the slab can prevent large deformation and flatten the slab during cooling. The combination of slab stacking and radiation shield is modeled to study the stress development under a slow cooling rate that is feasible in practice. Boron addition also affects the water quenching process of steel strips on the runout table after hot rolling. Simulations of strips with and without boron show different cooling curves, residual stress and phase distributions as austenite decomposition does not occur for boron-containing steel due to the fast cooling rate. Therefore, the cooling strategy on the runout table should be adjusted accordingly to control the coiling temperature and improve strip quality.</p>

.L’objectif de cette étude était, en premier lieu, de mettre en place en Thaïlande un testd’adhésion par traction-écaillage sur une machine de traction classique (test« macroscopique »), de le comparer au test « microscopique » Grenoblois fonctionnant dansla chambre du MEB et de l’utiliser pour évaluer l’adhérence des calamines de process sur desaciers industriels. Deux paramètres ont été étudiés, la vitesse de déformation et la teneur desaciers en silicium. Il apparaît que l’écaillage des calamines au cours du test augmente quandaugmente la vitesse de déformation. Une vitesse de déformation élevée entraîne unedéformation au premier écaillage plus faible, donc une adhérence mesurée plus faible. Ceteffet est lié aux phénomènes de relaxation. On a pu alors montrer que la présence d’oxyde(s)contenant Si, situé(s) à l’interface avec le métal, augmentait l’adhérence. Les étudesd’oxydation dans la vapeur d’eau qui ont aussi été réalisées ont révélé que la présence desilicium réduisait la vitesse d’oxydation. En augmentant la teneur en Si, les couches defayalite et de wüstite s’épaississent ; par contre, les couches externes s’amincissent. Pour lesaciers contenant du cuivre, la vitesse d’oxydation est réduite quand la teneur en Cu estaugmentée. De la même façon, les couches internes sont plus épaisses et on observe uneaugmentation du nombre de précipités de Cu quand la teneur en cet élément augmente. / The purpose of this study was first to develop in Thailand a “macroscopic” adhesion testusing a conventional tensile machine, to compare it to the micro-tensile test used in Grenobleand sitting in the SEM chamber, and to use it for measuring adhesion of scales grown duringprocessing on industrial steels. Parameters affecting the test, i.e. strain rate and Si content ofsteels were investigated. The results showed that spallation of scales during strainingincreased with increasing tensile strain rate. A higher strain rate resulted in lower straininitiating the first spallation and lower mechanical adhesion of scales, which could beexplained by a relaxation effect. Oxide containing Si existed at the steel-scale interface andpromoted adhesion of scales. Oxidation studies were also performed, and the behaviour inwater vapour of steels with different contents of Si and Cu was investigated. Increasing Sicontent tended to decrease oxidation rate. It also resulted in the thickening of the wüstite andfayalite layers which formed by internal oxidation. When Si in steel increased, theintermediate (FeO + Fe3O4) and outermost (Fe2O3 sitting on Fe3O4) layers formed by externaloxidation were thinner. For Cu containing steel, increasing Cu content tended to decrease theoxidation rate. It also decreased the innermost and intermediate layers and resulted in moreCu precipitates along steel-scale interface.