Materials Science Forum Vols. 500-501

Abstract: The CSP thin-slab technology (Compact Strip Production) provides a highly developed technical process for hot strip grades at low production costs. CSP combines the process stages of casting and rolling, which are discontinuous in the conventional process. This allows advantages to be achieved in terms of investments, energy consumption and operating costs. This paper describes the development work in progress, using the example of API steel grades. Key aspects of hot strip production such as alloying strategy, casting conditions, pass schedule design and the course of cooling are discussed and the resulting range of material properties is outlined. It can be demonstrated that CSP-produced hot strips feature very good toughness at low temperatures and may even allow applications which call for sour gas resistance.

Abstract: In the production with thin slab casting a serious problem of insufficient grain refinement and microstructural inhomogeneity has been found. In this paper attention has been paid to the austenite grain size changes in thin slab of a high strength Nb microalloyed steel under as-cast conditions and after first rolling pass. For comparison, the conventional thick slab was also investigated. It was found that although as-cast thin slab shows a smaller average austenite grain size than that of as cast thick slab; the latter after reheating shows a much finer average austenite grain size. The first rolling pass at high temperature and with heavy reduction causes rapid recrystallization that contributes to austenite grain refinement.

Abstract: Production of thick-walled low C Nb-Ti-V microalloyed linepipe steel has recently commenced in South Africa. Generally, the DWTT failures occurred when the average ferrite grain size was larger than 5µm and the pearlite colonies were coarse. This work examined the hot strip rolling behaviour of Nb-Ti-V microalloyed steel and quantified the most important parameters influencing final grain size and, hence, toughness. A Gleeble 1500 was used to both simulate hot rolling of relatively thick strip and also determine the no-recrystallisation temperature. For a fixed composition, final grain size was mainly controlled by the amount of strain applied below the norecrystallisation temperature, which was determined by the roughing mill exit temperature.

Abstract: An experimental study of Fe-C-Mn-Cr low-carbon steels with varying Cr content is presented. Pronounced bay and a near-stasis behavior were shown in the alloy with 1.1% Cr. Isothermal transformation at temperatures roughly between 500 and 600°C reveals two pronounced stages. SEM examination of the microstructures showed that the rapid first stage is controlled by carbide-free bainite formation while the slow second stage is controlled by a eutectoid-type reaction. Some microstructure features of the transformation were noted and discussed. Based on experimental observations, a model of austenite decomposition was developed to account for the main features of transformation kinetics in the alloys with varying Cr content.

Abstract: The paper examines the effect of boron (B) on the dynamic recrystallization and continuous cooling transformation (CCT) behavior in Nb-Ti microalloyed high strength interstitial free (IF) steels. For this purpose, two Nb-Ti microalloyed IF steels containing 0.003wt.% and 0.0005wt.% B, respectively, and one IF steel without B were chosen. The dynamic recrystallization behavior was investigated using hot compression testing. The character of the austenite to ferrite transformation during continuous cooling was studied by dilatometry test and CCT diagrams for the IF steels have been constructed. It was found that the initiation of dynamic recrystallization is delayed as the amount of boron increases. Addition of B retards the austenite to ferrite transformation as well. Under cooling rates of 0.5 and 1oC s-1, which correspond to slow cooling rates in the hot strip mill, the addition of B leads to the development of acicular ferrite and bainite phases. On the other hand, at similar cooling conditions the B free IF steel was observed to have a polygonal ferrite microstructure.

Abstract: Maraging steels show an excellent combination of high strength and ductility, which makes them very attractive in a large variety of potential applications. The present work is concerned with the main factors influencing the stability of metastable austenite in such a steel. At subzero temperatures a large variation in the isothermal transformation behaviour of austenite to martensite has been observed. Factors such as the austenite grain size and the interstitial content in solid solution are known to influence austenite stability and, therefore, the martensitic transformation. In this steel, the addition of titanium results in carbonitride precipitation. These precipitates play an indirect but important role in the stability of austenite by means of removing interstitials from the solid solution and by inhibiting an austenite grain growth. The combination of techniques such as X-ray diffraction, magnetisation measurements, three-dimensional neutron depolarisation, and internal friction measurements enables a complete characterisation of the transformation. A step towards understanding the factors responsible for the variation in the behaviour observed is the main contribution of this work.

Abstract: There is an increasing emphasis to develop novel hot-rolled high strength steels with fine and ultra fine grain sizes for structural and other applications. Traditionally the concept of microalloying has been employed to refine microstructures thereby obtaining increased strength levels. For example, employing an alloying strategy with Nb, Ti and Mo is promising to attain yield strength levels of 700MPa and beyond. In the present study, the transformation behaviour is investigated for a HSLA steel containing 0.05wt%C-1.65wt%Mn-0.20wt%Mo-0.07wt%Nb- 0.02wt%Ti. The ferrite formation from work-hardened austenite has been studied for simulated run-out table cooling conditions employing a Gleeble 3500 thermomechanical simulator equipped with a dilatometer. The effects of cooling rate and initial austenite microstructure, i.e. austenite grain size and degree of work hardening, on the austenite decomposition kinetics and resulting ferrite grain size have been quantified. Based on the experimental results, a phenomenological transformation and ferrite grain size model is proposed for run-out table cooling conditions. The transformation model includes submodels for transformation start and ferrite growth. The latter is described using a Johnson-Mehl-Avrami-Kolmogorov approach. The degree of work hardening is incorporated by introducing an effective austenite grain size as a function of the strain applied under no-recrystallization condition. The ferrite grain size can be predicted as a function of the transformation start temperature. Increasing both cooling rate and amount of work hardening can optimize ferrite grain refinement. In the present steel, ferrite grain sizes of as low as 2µm have been obtained in this way. The results observed for the present steel are compared to the transformation behaviour in previously studied Nb-Ti HSLA steels of similar strength levels.

Abstract: Nb is added to C-Mn steels in order to use the solute drag and/or strain induced precipitation as a useful tool to condition the austenite in the hot rolling mill and produce during the subsequent cooling a refined ferrite grain size. The highest degree of refinement is obtained in conventional rolling mills by accumulating the deformation in austenite during the last passes, followed by early cooling in the run out table to produce a high density of nucleated ferrite grains. However, the maximum refinement is to a certain extent attenuated due to the ferrite grain coarsening taking place during the transformation. The present work analyses the different aspects limiting the final achievable ferrite grain refinement.

Abstract: The dynamic strain-induced transformation (DSIT) of ferrite from austenite in intense deformation at temperatures close to Ar3 were applied to one C-Mn steel and several Nb and Nb-Ti microalloy steels to obtain an ultrafine ferrite grain size. As another route the static recrystallisation of severely cold-rolled martensite (SRM) was utilized. It was found that in the DSIT route a fine prior austenite grain size was crucial to form ferrite with the grain size of 1-3 µm with a considerable fraction of a secondary phase, carbide aggregate/pearlite or martensite. Grain sizes achieved were somewhat finer in steels with a higher microalloying content. In the SRM route, the ferrite grain size of 1-1.5 µm was obtained by using the cold rolling of 80-90% reduction. Thermal stability of the ultrafine-grained structures, especially those from the DSIT route, was found to be excellent. In electron or laser beam welding of 1-2.5 mm sheets neither any coarse-grained zone existed in the heat-affected zone, nor did form any softened zone.