Elena V. Pereloma

Mechanical properities of an HSLA bainitic steel subjected to controlled rolling with accelerated cooling

Controlled rolling followed by accelerated cooling was utilised in laboratory simulations to study the microstructure and mechanical properties of an HSLA low carbon bainitic steel. The effects of processing parameters, such as cooling start temperature and cooling rates, on the final microstructure and mechanical properties were studied. Optical microscopy and transmission electron microscopy were used to evaluate the complex microstructures consisting of polygonal ferrite, pearlite, bainite and martensite/retained austenite constituent. The use of the multiple regression analysis allowed establishment of the relationships between mechanical properties and accelerated cooling variables: cooling rates and cooling start temperatures.

Transformation behaviour in thermomechanically processed C-Mn-Si TRIP steels with and without Nb

Abstract Two 0.2wt.% C–1.55wt.% Mn–1.5wt.% Si steels with and without the addition of 0.039 wt.% Nb were studied by laboratory simulations of controlled thermomechanical processing in a quench deformation dilatometer. The microstructures were characterised using optical metallography, image analysis and scanning electron microscopy techniques. The effects of recrystallised and non-recrystallised austenite and discontinuous paths on the final microstructure were studied. The results have shown that the highest volume fraction of retained austenite is associated with a 400°C isothermal bainite transformation temperature and the presence of approximately 50% polygonal ferrite and acicular ferrite as a dominant second phase.

Strain-induced precipitation behaviour in hot rolled strip steel

The strain-induced precipitation of Nb(C,N) into the austenite in a Nb-microalloyed steel was investigated both experimentally and using a predictive model. The precipitation of Nb(C,N) was measured indirectly from the hardness at room temperature after thermomechanical treatment. The predictive model combined the precipitation start model of Dutta and Sellars with the Avrami equation and the additivity principle to allow prediction of the volume fraction of Nb(C,N) precipitated. The effects of several thermomechanical schedules were studied. These were (i) the effect of isothermal hold temperature and duration; (ii) the effect of deformation temperature at high and low cooling rates; (iii) the effect of cooling rate prior to the austenite to ferrite transformation; and (iv) the effect of multiple pancaking deformations. The fit between the experimental data and calculated results was found to be good in all cases with the exception of the slow cooling rate results of schedule (ii). It was concluded that the model could, once calibrated, successfully predict the hardness and strength of thermomechanically processed Nb-microalloyed steels.

Microstructure and mechanical properties after annealing of equal-channel angular pressed interstitial-free steel

Abstract The evolution of microstructure, microtexture and mechanical properties during isothermal annealing of an ultrafine-grained interstitial-free steel after eight passes of route B C room temperature equal-channel angular pressing (ECAP) was studied. The microstructure and microtexture were characterized by electron back-scattering diffraction, and mechanical properties were assessed by shear punch and uniaxial tensile testing. Homogeneous coarsening via continuous recrystallization of the ECAP microstructure is accompanied by minor changes in the ∼63% high-angle boundary population and a sharpening of the original ECAP texture. This is followed by abnormal growth during the final stages of softening due to local growth advantages. Linear correlations between shear and tensile data were established for yield, ultimate strength and total elongation. After yield, the changes in uniaxial tensile behaviour from geometrical softening after ECAP to load drop, Luders banding and continuous yielding after annealing is attributable to a coarsening of the microstructure.

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

The key evidence for understanding the mechanical behavior of advanced high strength steels was provided by atom probe tomography (APT). Chemical overstabilization of retained austenite (RA) leading to the limited transformation-induced plasticity (TRIP) effect was deemed to be the main factor responsible for the low ductility of nanostructured bainitic steel. Appearance of the yield point on the stress-strain curve of prestrained and bake-hardened transformation-induced plasticity steel is due to the unlocking from weak carbon atmospheres of newly formed during prestraining dislocations.

Microstructure and mechanical properties of C-Si-Mn(-Nb) TRIP steels after simulated thermomechanical processing

Abstract Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.

Microstructural evolution during simulated OLAC processing of a low-carbon microalloyed steel

The microstructural evolution during simulated on-line accelerated cooling (OLAC) of a commercial Grade 80 pipe steel was studied using a quench deformation dilatometer. The transformed matrix microstructure contains various amounts of polygonal ferrite, granular bainite and acicular ferrite, depending mainly on the accelerated-cooling interrupt temperature. The final microstructure is predicted well by drawing the OLAC schedule on the appropriate CCT diagram. Three distinct groups of precipitates are found in the final microstructure, which form during reheat, austenite deformation, and cooling, respectively. The distribution and composition of the precipitates varies widely with steel composition and processing schedule. The microstructure of industrially processed plate agrees well with that of corresponding laboratory simulations.

A correlative approach to segmenting phases and ferrite morphologies in transformation-induced plasticity steel using electron back-scattering diffraction and energy dispersive X-ray spectroscopy.

Using a combination of electron back-scattering diffraction and energy dispersive X-ray spectroscopy data, a segmentation procedure was developed to comprehensively distinguish austenite, martensite, polygonal ferrite, ferrite in granular bainite and bainitic ferrite laths in a thermo-mechanically processed low-Si, high-Al transformation-induced plasticity steel. The efficacy of the ferrite morphologies segmentation procedure was verified by transmission electron microscopy. The variation in carbon content between the ferrite in granular bainite and bainitic ferrite laths was explained on the basis of carbon partitioning during their growth.

Simultaneous prediction of austemperability and processing window for austempered ductile iron

Abstract A model is developed for simultaneous prediction of the processing window and austemperability of austempered ductile iron (ADI). The processing window represents a frame of time and temperature in which ADI satisfies optimum mechanical properties defined by ASTM A897M:1990. Austemperability is the maximum section size of ductile iron that can be austempered without formation of pearlite during the austempering process. The outcome of the model presents the processing window and austemperability as a three dimensional diagram (processing - austemperability window). The processing window boundaries are estimated according to a model for prediction of the time for ausferritic reaction in ADI. The austemperability of ductile iron is predicted according to the estimated pearlite curve of the TTT diagram and a mathematical model that simulates conduction of heat in a solid cylinder. The heat transfer model is calibrated for a ductile iron of composition (wt-%) 3.63C, 2.4Si, 0.39Mn, 0.4Mo, 0.25Cu, 0.04Ni, 0.04Mg. The model for the processing - austemperability window is validated for a ductile iron of composition (wt-%) 3.41C, 2.46Si, 0.36Mn, 0.18Mo, 0.25Cu, 0.036Mg at 285, 380, and 400 ° C austempering temperatures. Results show that the material satisfies ASTM A897M:1990 standard for the chosen experimental points within the processing - austemperability window without formation of pearlite in the microstructure.

Addressing Retained Austenite Stability in Advanced High Strength Steels

Advances in the development of new high strength steels have resulted in microstructures containing significant volume fractions of retained austenite. The transformation of retained austenite to martensite upon straining contributes towards improving the ductility. However, in order to gain from the above beneficial effect, the volume fraction, size, morphology and distribution of the retained austenite need to be controlled. In this regard, it is well known that carbon concentration in the retained austenite is responsible for its chemical stability, whereas its size and morphology determines its mechanical stability. Thus, to achieve the required mechanical properties, control of the processing parameters affecting the microstructure development is essential.