Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

Evolution of Pearlite Microstructure in Low-Carbon Cast Microalloyed Steel Due to the Addition of La and Ce

Abstract: The effects of rare earth elements (RE) addition on the pearlite microstructure in low-carbon microalloyed steels have been investigated under two heat treatment conditions: (1) a normalizing treatment (as a conventional heat treatment used industrially to obtain the final mechanical properties of such steels), and (2) an isothermal treatment at 650 °C. This research reports the following effects due to the addition of RE: (i) refinement of the nodule and colony size of pearlite along with the ferrite grain size in the normalized condition, without a significant change in the volume fraction of pearlite. This microstructural refinement observed at room temperature is a consequence of the refinement of cast and austenitic microstructures formed during cooling in the presence of RE; (ii) the interlamellar spacing of pearlite isothermally transformed at 650 °C, as observed by SEM and TEM, is effectively reduced in the RE-added steel. This is likely due to two different effects combined: (i) direct influence of RE on atom carbon diffusion; and (ii) pearlite growth being boundary diffusion controlled by RE partitioning.

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

Abstract: 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).

An integrated computer model with applications for austenite-to-ferrite transformation during hot deformation of Nb-microalloyed steels

Abstract: This work presents an austenite decomposition model, based on the thermodynamics of the system and diffusion-controlled nucleation theory, to predict the evolution of microstructure during hot working of niobium-microalloyed steels. The differences in microstructural development of hotdeformed microalloyed steel in the single-phase austenite and two-phase (austenite + ferrite) regions have been effectively described using an integrated computer modeling process. The complete model presented here takes into account the kinetics of recrystallization, recrystallized austenite grain size, precipitation, phase transformation, and the resulting ferrite structure. After considering existing austenite decomposition models, we decided that the method adopted in the present work relies on isothermal transformation kinetics and the principle-of-additivity rule. The thermomechanical part of the modeling process was carried out using the finite-element method. Experimental results at different temperatures, strain rates, and strain levels were obtained using a Gleeble thermomechanical simulator. A comparison of results of the model with experiments shows good agreement.

The determining impact of coiling temperature on the microstructure and mechanical properties of a titanium-niobium ultrahigh strength microalloyed steel: Competing effects of precipitation and bainite

Abstract: We elucidate here the influence of coiling temperature on the microstructure and mechanical properties, in an ultrahigh strength titanium-niobium microalloyed steel. The objective was to underscore the impact of coiling temperature on the nature and distribution of microstructural constituents (including different phases, precipitates, and dislocation structure) that significantly contributed to differences in the yield and tensile strength of these steels. Depending on the coiling temperature, the microstructure consisted of either a combination of fine lath-type bainite and polygonal ferrite or polygonal ferrite together with the precipitation of microalloyed carbides of size ~2–10 nm in the matrix and at dislocations. The microstructure of steel coiled at lower temperature predominantly consisted of bainitic ferrite with lower yield strength compared to the steel coiled at higher temperature, and the yield to tensile strength ratio was 0.76. The steel coiled at higher temperature consisted of polygonal ferrite and extensive precipitation of carbides and was characterized by higher yield strength and with yield strength/tensile strength ratio of 0.936. The difference in the tensile strength was insignificant for the two coiling temperatures. The observed microstructure was consistent with the continuous cooling transformation diagram.