Microalloyed steel

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

Effect of texture and microstructure on resistance to cracking of high-strength hot-rolled Nb-Ti microalloyed steels

Abstract: Significant texture gradient in the through-thickness direction was observed in high-strength hot-rolled 560 and 770 MPa Nb-Ti microalloyed steels, characterized by polygonal ferrite and ferrite bainite microstructures, respectively. {113}〈110〉 was the most intense deformation texture in the two high-strength grades of Nb-Ti steels and was dominant in the midthickness region compared to 10 and 25 pct depth below the surface. The recrystallization texture of austenite, {100}〈001〉, transformed into {100}〈011〉 component in the ferrite and indicated an increase in the intensity with increase in depth for the Nb-Ti microalloyed steels. The {100}〈011〉 texture has a detrimental effect on the edge formabiity of steels. However, the midthickness plane contained considerable intensity of desired texture, {332}〈113〉, which is expected to offset the undesirable {100}〈011〉 texture resulting in superior edge formability and impact toughness of Nb-Ti steels, consistent with experimental observations.

Complex phase quantification methodology using electron backscatter diffraction (EBSD) on low manganese high temperature processed steel (HTP) microalloyed steel

Abstract: Microalloyed steels are required to have good strength and toughness, properties that are mostly influenced by microstructure and phase distribution in the steel, obtained by controlling the thermomechanical processing. High temperature processing (HTP) Steels, with lower manganese content, has been recently developed in order to reduce segregation, but reduction of Mn decreases the yield strength. In order to understand the relationship between chemical composition, microstructure, mechanical properties and processing parameters, quantitative analysis of the final microstructure is required but identification and quantification of the ferrite microconstituents under optical microscopy or scanning electron microscopy is difficult due to their similarities. The present work presents a methodology for identification and quantification of complex microconstituents on a low manganese HTP steel, subjected to different hot deformation and cooling cycles, by means of electron backscatter diffraction (EBSD). The results showed that the EBSD methodology allowed to identify and quantify different microconstituents in a low manganese HTP steel with similar results as those obtained by point count methodology.

Significant influence of carbon and niobium on the precipitation behavior and microstructural evolution and their consequent impact on mechanical properties in microalloyed steels

Abstract: Carbon and niobium (Nb) play an important role in influencing the ultimate microstructure and mechanical properties. In this regard, we elucidate here the impact of carbon and Nb on the microstructural evolution and precipitation behavior during continuous cooling of industrially processed microalloyed steels with varying carbon and Nb-content. The microstructure and precipitation evolution was studied via electron microscopy and related to the outcome of thermodynamic simulation. The increase of carbon content in steel increased the precipitation temperature of (Nb, Ti)(C, N), which led to relatively larger size (Nb, Ti)(C, N) precipitates. Furthermore, high carbon content contributed to stabilization of austenite and delayed the transformation of ferrite and bainite, such that martensite/austenite content (M/A) was obtained. The M/A islands in high carbon-containing steel contributed to highest strength and intermediate elongation. The high degree of (Nb, Ti)(C, N) precipitation in steel contributed to refinement of prior austenite grain size and strain accumulation, which increased ferrite and bainite start transformation temperature, resulting in higher volume fraction of polygonal ferrite. Polygonal ferrite in steel with high Nb-content was responsible for relatively low strength in comparison with steels with higher carbon or intermediate carbon-Nb contents. Granular bainite and lath bainite in steel with intermediate C and Nb-contents was characterized by best combination of strength and elongation. The outcomes of the thermodynamic simulations were consistent with the experimentally observed microstructure.

Microstructural characteristics and impact fracture behaviors of low-carbon vanadium-microalloyed steel with different nitrogen contents

Abstract: The influence of nitrogen content between 0.0032 and 0.0081 wt % on the microstructures and the impact toughness of low-C V-microalloyed steel was investigated. The isothermal transformation experiments in the medium temperature were performed at Gleeble-3800. The microstructures and impact fracture behaviours were obtained through a series of characterization means and the mechanism were investigated. Results indicated that increasing the nitrogen content created a larger number of finer precipitated (Ti,V) (C,N) particles as well as micro-sized particles. The former decreased the average diameter of prior austenite grains (PAGs) and the latter promoted the nucleation of intragranular acicular ferrite (AF), which led to an increased amount of AF and a reduced mean equivalent diameter (MED) of ferrite grains. Moreover, the increased nitrogen content increased the amount of fine martensite-austenite (M/A) constituents and tended to change the internal substructure of M/A constituent from twin-type to lath-type. Thereby these variations in microstructure resulted in the decrease in 50% fracture appearance transition temperature (50% FATT), and alterations in the fracture behavior in the upper shelf region and lower shelf region. Moreover, the increasing N content led to changes in the mechanism of crack initiation from micro-cleavage to microvoid, and the mechanism of crack propagation from predominantly transgranular cleavage to predominantly intragranular microvoid coalescence at the test temperature of -20 °C. As a result, the initiation and propagation energy increased significantly with the increased nitrogen content.