Microalloyed steel

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

Ultra Grain Refinement During the Simulated Thermomechanical-processing of Low Carbon Steel

Abstract: Grain re Þ nement is a useful method to improve the strength and toughness of steels without changing their chemical composition. In this study, critical temperatures for the thermomechanical treatment of niobium microalloyed steel were determined experimentally and through thermodynamic data. Simulations of conventional and controlled thermomechanical processing and a thermomechanical treatment to obtain ultraÞ ne-grained microstructures were conducted using torsion tests. The Þ nal microstructures displayed signiÞ cant grain size reÞ nement. Conventional processing produced grains with an average size of 12 .m, while controlled processing led to an average grain size of 4.9 .m and severe plastic deformation at warm temperatures resulted in a grain size of 1.3 .m. The ultra reÞ nement of ferrite grains was associated with strain-induced dynamic phase transformation and dynamic recrystallization of as-transformed ferrite.

Study of the kinetics of the recrystallization of cold-rolled low-carbon steel

Abstract: The suitability of the differential scanning calorimetry (DSC) technique for the quantitative study of the recrystallization process in cold-rolled steels is demonstrated, the recrystallization kinetics was described with a semiempirical model, and the transformation of a cold-rolled steel submitted to an industrial-batch thermal cycle was appropriately simulated. It was found that the activation energy for the recrystallization process depends on the heating-rate range, having values of 522 ± 13 and 259 ± 12 kJ/mole for low and high heating rates, respectively. It was concluded that the smallest value corresponds to the recrystallization process alone, while the largest one contains an additional contribution from the aluminium nitride precipitation. It is also shown that the X-ray diffraction (XRD) line-width measurement is a useful complementary method to determine the temperature regions where recovery and recrystallization occur.

Characterization of Grain-boundary Precipitates after Hot-ductility Tests of Microalloyed Steels

Abstract: The hot ductility of microalloyed steels was investigated by interrupted tensile tests at the temperatures of 850 and 950°C. Analyses of microstructural damage during plastic straining of the steels were performed using an experimental setup that allowed rapidly quenching the tensile specimens after straining to a predefined level. Microstructural investigations on the materials were carried out on longitudinally sectioned samples. Further analyses on crack surfaces were performed by fracturing the strained specimens in liquid nitrogen and by analyzing the surfaces formed by high-temperature decohesion through conventional and field emission SEM. It was demonstrated that AlN and Nb(C,N) precipitates, in isolated or combined form, affected the prior-austenite grain boundaries. Differences in hot cracking sensitivity among the steels was accounted for by modifications of the precipitate size and volume fraction.

Failure mode analysis and a mechanism for hot-ductility improvement in the Nb-microalloyed steel

Abstract: Loss of hot ductility at the straightening stage of the continuous casting of high-strength low-alloy steel is attributed to different microalloying elements, in particular, Nb. However, such elements are essential for the desired mechanical characteristics of the final product. Since the chemistry cannot be altered to alleviate the problem, thermomechanical processing was studied in order to improve the hot ductility. Two Nb-microalloyed steels, one also containing B, were examined. The thermal history occurring in the continuous casting process was taken into account as well. First, it was noticed that the steel with B has a higher hot ductility than the other after being subjected to in-situ melting followed by the thermal schedule. Grain boundary sliding was recognized as the failure mechanism. Then, the effect of deformation applied in the vicinity of the δ→γ transformation, while the thermal schedule was being executed, was investigated. Such deformation appeared to improve the hot ductility remarkably. Finally, the mechanism of such improvement in the hot ductility was elaborated.

Effect of Cooling Rate on the Precipitation Behavior of Carbonitride in Microalloyed Steel Slab

Abstract: In the continuous casting of the microalloyed steel, the slab surface transversal cracking could be prevented through the control of the slab surface microstructure, which correlates with the precipitation behavior of carbonitrides in the microalloyed steel. Therefore, the cooling rate is the key factor to determine the precipitation behavior of carbonitrides. This article used confocal laser scanning microscopy to study the effect of different cooling rates on the precipitation behavior of the carbonitrides in the microalloyed steel slab. When the cooling rate is less than 3 K·s−1, the precipitates in the steel are coarse, growing out along the austenite grain boundaries, and form a chain-like distribution. These precipitates seriously reduced the hot ductility of slab. Quantitative study between the cooling rate and the precipitation behavior of carbonitrides in microalloyed steel also has been developed. The results of this study could be used to improve the understanding of the slab surface microstructure controlling to enhance the hot ductility of the slab and avoid the surface crack of the slab.