Precipitation In Microalloyed Steels Research Articles

Precipitation in Microalloyed Steel by Model Alloy Experiments and Thermodynamic Calculations

Precipitation in microalloyed steel has been studied by applying thermodynamic calculations based on a description of the Gibbs energies of the individual phases over the full multicomponent composition range. To validate and improve the thermodynamic description, new experimental investigations of the phase separation in the cubic carbides/nitrides/carbonitrides in alloys containing Nb, V, Mo, and Cr, have been performed. Model alloys were designed to obtain equilibrium carbides/carbonitrides that are sufficiently large for measurements of compositions, making it possible to study the partitioning of the elements into different precipitates, showing distinctly different composition sets. The reliability of the calculations, when applied to multicomponent alloys, was tested by comparing with published experimental studies of precipitation in microalloyed steel. It is shown that thermodynamic calculations accurately describe the observed precipitation sequences. Further, they can reproduce several important features of precipitation processes in microalloyed steel such as the partitioning of Mo between matrix and precipitates and the variation of precipitate compositions depending on precipitation temperature.

Equilibrium Model of Precipitation in Microalloyed Steels

The formation of precipitates during thermal processing of microalloyed steels greatly influences their mechanical properties. Precipitation behavior varies with steel composition and temperature history and can lead to beneficial grain refinement or detrimental transverse surface cracks. This work presents an efficient computational model of equilibrium precipitation of oxides, sulfides, nitrides, and carbides in steels, based on satisfying solubility limits including Wagner interaction between elements, mutual solubility between precipitates, and mass conservation of alloying elements. The model predicts the compositions and amounts of stable precipitates for multicomponent microalloyed steels in liquid, ferrite, and austenite phases at any temperature. The model is first validated by comparing with analytical solutions of simple cases, predictions using the commercial package JMat-PRO, and previous experimental observations. Then it is applied to track the evolution of precipitate amounts during continuous casting of two commercial steels (1004 LCAK and 1006Nb HSLA) at two different casting speeds. This model is easy to modify to incorporate other precipitates, or new thermodynamic data, and is a useful tool for equilibrium precipitation analysis.

The Effects of Finish Rolling Temperature and Cooling Interrupt Conditions on Precipitation in Microalloyed Steels Using Small Angle Neutron Scattering

Small angle neutron scattering (SANS) was used to quantify the precipitate characteristics (i.e., mean precipitate size, number of precipitates, and distribution broadening) in X-70 and X-80 pipeline steel and in grades 80 and 100 microalloyed steel plate. The precipitate distributions measured for the different steels were correlated with the finish rolling temperature (FRT) and cooling interrupt temperature (CT) as a means of identifying processing conditions that may enhance fine precipitate evolution. It was observed that for some combinations of processing conditions two distinct precipitation events—based on size of the precipitates—were occurring. The first precipitation event (larger size) was strongly associated with the FRT, where a decrease in the mean precipitate radius with decreasing FRT was observed. The second (finer size) precipitation event was affected by both the CT and the FRT. Both the size and volume of the second precipitation event was observed to decrease with decreasing CT. The precipitate distribution predicted from the SANS data for grade 100 steel compared favorably to precipitation data obtained from particle counting analysis conducted with a transmission electron microscopy (TEM).

Niobium Carbide Precipitation in Microalloyed Steel

The precipitation of niobium carbo‐nitrides in the austenite phase, interphase and ferrite phase of microalloyed steel was assessed by a critical literature review and a round table discussion. This work analyses the contribution of niobium carbide precipitates formed in ferrite in the precipitation hardening of commercially hot rolled strip. Thermodynamics and kinetics of niobium carbo‐nitride precipitation as well as the effect of deformation and temperature on the precipitation kinetics are discussed in various examples to determine the amount of niobium in solid solution that will be available for precipitation hardening after thermomechanical rolling in the austenite phase and successive phase transformation.

Determination of precipitation–time–temperature (PTT) diagrams for Nb, Ti or V micro-alloyed steels

A method is described, developed in CENIM-CSIC, for studying the kinetics of strain induced precipitation in micro-alloyed steels. Using torsion tests, the statically recrystallized fraction has been determined for three Nb, V and Ti micro-alloyed steels at different temperatures and strains. When precipitation starts, the recrystallized fraction deviates from Avrami’s equation, making it possible to identify the moment at which precipitation starts (Ps) and finishes (Pf). In this way precipitation–time–temperature(PTT) curves can be drawn, showing the precipitation kinetics in graph form.

Quantitative Microanalysis of Fine Precipitates in Microalloyed Steels

ABSTRACTMatrix and grain boundary precipitation in austenite has been studied in two Nb/V microalloyed steels with different V levels, which were given a number of different heat treatments after hot deformation. The steel with a higher V level was found to have a greater number of fine carbonitride precipitates in the matrix. In both steels, the precipitates became more Nb-rich as the aging temperature was increased from 854°C to 1066°C. For a given steel and heat treatment, the matrix precipitates were generally more Nb-rich than the grain boundary precipitates.