Microalloyed steel

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

Static recrystallization kinetics in warm worked vanadium microalloyed steels

Abstract: The effect of vanadium on static recrystallization kinetics of vanadium microalloyed carbon steels after simulating warm working conditions has been determined using the stress relaxation method in plane strain compression tests. In the warm working regime, undissolved fine V(C,N) precipitates promote a fine austenite grain size during reheating and interact with the recrystallization process after working, leading to longer recrystallization times in comparison with plain C–Mn steels. The interaction between precipitates and recrystallization is different to that observed for hot working conditions, retarding the total recrystallization process and thus resulting in a lower value of the Avrami exponent and a longer t 0.5 time.

Influence of the chemical composition on transformation behaviour of low carbon microalloyed steels

Abstract: In order to design thermomechanical schedules for processing low carbon microalloyed steels, the various critical transformation temperatures, i.e. the start and finish of the austenite transformation (Ar3, Ar1) and the non-recrystallization temperature (Tnr), must be determined. Continuous cooling torsion and compression testing are useful ways to measure these values. In this study six low carbon microalloyed steels with different additions (Nb, Cu, Si and Mo) were examined using these techniques. Moreover, the equilibrium phase diagrams for each alloy were calculated using FactSage. The comparison of the thermomechanical testing results with the thermodynamic calculations leads to a better understanding of the effect of the different elements on the transformation behaviour of pipeline steels. Regarding transformation temperatures, Cu in residual contents showed a strong effect on decreasing both Ar3 and Ar1, which indicates a hardenability effect of this element. On the other hand, increasing Nb contents increased Tnr by accelerating Nb(C,N) precipitation. However, when Si was added to a Nb-microalloyed steel, the Tnr decreased.

Carbonitride precipitate growth in titanium/niobium microalloyed steels

Abstract: Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used to investigate the morphology, distribution, composition, particle size distributions, and growth kinetics of carbonitride precipitates in steels containing low levels of Ti, Nb, C, and N. During the aging, only the complex carbonitride precipitates of the form (TixNb1−x) (CyN1-y) were found in the newly nucleated and growing particles. The youngest of these particles which approach the size of critical nuclei tends to be Ti-rich. Almost all of these particles are nearly spherical. The initial growth of the precipitates, which is very rapid, lasts less than 30 seconds followed by slow ripening. A model predicting the growth kinetics of carbonitrides and composition variation within the precipitates for the initial stage (before coarsening) has been developed based on equilibrium thermodynamics with the inclusion of capillarity and multicomponent diffusional kinetics. Satisfactory agreement with the experimental results has been demonstrated.

Role of microstructure on sulfide stress cracking of oil and gas pipeline steels

Abstract: Sulfide stress cracking (SSC) behavior of three microstructures, i.e., ferritic-pearlitic microstructure, ultrafine ferrite microstructure, and acicular ferrite dominated microstructure, was investigated using the bent-beam test in aqueous hydrogen sulfide (H2S) environments. The critical stress (Sc) values of these three microstructures were determined experimentally to be 1008, 1190, and more than 1260 MPa, respectively. As a result, the acicular ferrite-dominated microstructure possessed the best SSC resistance, the ultrafine ferrite microstructure was in a second position, and the ferritic-pearlitic microstructure was relatively the worst. It was analyzed that hydrogen embrittlement (HE) was the main failure mechanism in SSC cracking for high-strength pipeline steels, and preferential hydrogen accumulation within the plastic zone of the main crack tip accounted for the exhibited embrittlement. It was remarkable that the strength values of pipeline steels were not the only factor to determine their SSC susceptibilities. Microstructure played an important role in the SSC initiation and propagation of pipeline steels. In particular, both the fine dispersed precipitations of carbonitrides and the high-density tangled dislocations in acicular ferrite, which behaved as the hydrogen traps, should be attributed to the optimal SSC resistance of pipeline steels.

New Microalloyed Steel Applications for the Automotive Sector

Abstract: Developments related to the use of microalloy additions, primarily of Ti, Nb, and V, and controlled processing are reviewed to illustrate how steels with tailored microstructures and properties are produced from either bar or sheet steels for new automotive components. Microalloying additions are shown to control the necessary strengthening mechanisms to produce high strength materials with the desired toughness or formability for a specific application. Selected examples of direct cooled forging steels, microalloyed carburizing steels, and advanced high strength sheet (AHSS) steels are discussed.